The chemical senses - Psychological SciencesThe chemical senses Jay A. Gottfried, Dana M. Small, and David H. Zald 6.1. Introduction While the orbitofrontal cortex (OFC) receives information

Chapter 6

The chemical senses

Jay A. Gottfried, Dana M. Small, and David H. Zald

6.1. IntroductionWhile the orbitofrontal cortex (OFC) receives information from all sensory modalities, itis most intimately linked to the chemical senses of smell and taste. Portions of the OFCreceive robust projections directly from the primary olfactory and gustatory cortices,and these areas are often referred to as the secondary olfactory and gustatory cortex.Given the prominence of the OFC’s involvement in chemosensory processing, examina-tion of the manner in which the OFC codes and uses chemosensory information provides a critical avenue for understanding OFC functions more generally. A review ofthe animal literature on the OFC suggests the pervasiveness of chemosensory issues evenin tasks that are usually interpreted primarily in terms of learning and executive functions. Indeed, most of the basic concepts and theories regarding OFC processingoriginate from paradigms that utilize or manipulate aspects of food reward or the contingencies necessary to obtain food reward. For instance, animal studies of the OFC’srole in learning and emotion have almost always relied on chemosensory stimuli as eitherreinforcers or associative cues. Many of the non-chemosensory functions of the OFCmay actually represent evolutionary extensions or exaptions of processes involved incoding chemosensory information (exaption is used here to refer to a trait evolved forone purpose that is later used for another). If this hypothesis is correct, the chemosen-sory functions of the OFC provide a critical context for understanding the broad range oftopics covered in this book. The present chapter reviews data on the primate and humanOFC’s involvement in chemosensory processing and feeding behavior.

6.2. Olfaction

BackgroundFor over 100 years it has been well-recognized that the temporal lobe contributes to thehuman experience of smell (Hughlings-Jackson and Beevor 1890; Hughlings-Jackson andStewart 1899). By historical comparison, a role for the OFC in olfactory processing was slowto emerge. Throughout the 19th and early 20th centuries, anosmia (smell loss) was fre-quently documented as a result of post-traumatic head injury, but the inevitable damage toperipheral olfactory structures and olfactory bulb, along with the scarcity of detailed post-mortem studies, generally confounded efforts to relate these smell impairmentsto frontal lobe pathology (reviewed in Eslinger et al. 1982). Research by Allen

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provided the first demonstration of a critical role for the frontal cortex in olfactory function (Allen 1940; Allen 1943a; Allen 1943b). Bilateral ablation of the frontal lobes indogs caused a delay in learning an olfactory-conditioned reflex (lifting the foreleg inresponse to an odor in order to avoid an electric shock), and interrupted the ability to dis-criminate between positive- and negative-conditioned odors (Allen 1940). In contrast,total ablation of parieto-temporal lobes (sparing piriform areas) or hippocampi had noeffect on these responses, indicating that discrimination learning selectively relied on thestructural integrity of the prefrontal cortex. Parallel experiments revealed that the pre-frontal ablation had no impact on auditory, tactile, or visual conditioning (Allen 1943b),highlighting the olfactory specificity of this effect. In subsequent work, extracellularrecordings in un-operated dogs showed that electrical stimulation of the piriform cortexevoked short-latency spike activity in ventrolateral areas of the prefrontal cortex (Allen1943a), suggesting that this region might have rapid access to olfactory information.These physiological findings were complemented by a series of strychnine neuronographyexperiments in monkeys, emphasizing the presence of reciprocal connections between thepiriform, posterior orbital, and anterior insular areas (Pribram et al. 1950; Pribram andMacLean 1953). However, following these studies, scientific interest in the subject of olfac-tory neocortex waned, and further data did not become available for another two decades.

Olfactory input to the OFC and the localization of secondaryolfactory cortexOdor-evoked responses are initially conducted from the first-order olfactory receptorneurons at the nasal mucosa toward the olfactory bulb, where sensory axons makecontact with the second-order (mitral and tufted cell) dendrites within discrete

glomeruli. Axons of the mitral and tufted cells of each bulb coalesce to form the olfactorytract, one on each side. This structure lies in the olfactory sulcus, just lateral to gyrus rec-tus, and conveys olfactory information ipsilaterally to a wide number of brain areaswithin the posterior orbital surface of the frontal lobe and the dorsomedial surface of thetemporal lobe (Fig. 6.1). Collectively, the areas receiving direct bulbar input are some-times referred to as the “primary olfactory cortex” (POC: Price 1973; de Olmos et al.1978; Turner et al. 1978). These structures (from rostral to caudal) include the anteriorolfactory nucleus, olfactory tubercle, anterior and posterior piriform cortex, amygdala(periamygdaloid region, anterior and posterior cortical nuclei, nucleus of the lateralolfactory tract, and medial nucleus), and rostral entorhinal cortex, all of which are sub-stantially interconnected via associational intracortical fiber systems (Carmichael et al.1994; Haberly 1998). Note that because the piriform cortex is the largest of the centralolfactory areas and receives the densest input from the olfactory bulb, the term POC isfrequently used to designate just the piriform cortex.1

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1 Other regions of the basal forebrain, such as the taenia tecta, induseum griseum, anterior hippocampalcontinuation, and nucleus of the horizontal diagonal band, have been shown to receive direct bulbarinput in animal models (Shipley and Adamek 1984; Carmichael et al. 1994), but whether similar con-nections are preserved in humans is unknown.


Higher-order projections arising from each of these primary structures converge onthe secondary olfactory regions in the OFC (Fig. 6.1), agranular insula, additional amyg-dala subnuclei, hypothalamus, mediodorsal thalamus, and hippocampus. Together thiscomplex network of connections provides the basis for odor-guided regulation of behav-ior, feeding, emotion, autonomic states, and memory. In addition, each region of the primary olfactory cortex (except for the olfactory tubercle) sends feedback projections tothe olfactory bulb (Carmichael et al. 1994), supplying numerous physiological routes forcentral or “top-down” modulation of olfactory processing.

Nonhuman primatesThe first methodical investigation of an olfactory projection area in the primate OFC wascarried out by Tanabe and colleagues using electrophysiological techniques (Tanabe et al.

J.A. GOTTFRIED, D.M. SMALL, AND D.H. ZALD 127

Fig. 6.1. Macroscopic view of the human ventral forebrain and medial temporal lobes, depictingthe olfactory tract, its primary projections, and surrounding non-olfactory structures. The rightmedial temporal lobe has been resected horizontally through the mid-portion of the amygdala(AM) to expose olfactory cortex. The blue area signifies the cytoarchitectural homologues to themonkey olfactory region, whereas the pink area denotes the putative human olfactory region, asbased on neuroimaging findings. AON, anterior olfactory nucleus; CP, cerebral peduncle; EA,entorhinal area; G, gyrus ambiens; L, limen insula; MB, mammillary body; PIR-FR, frontal piriformcortex; OB, olfactory bulb; OpT, optic tract; OT, olfactory tract; Tu, olfactory tubercle; PIR-TP, tem-poral piriform cortex. Figure prepared with the help of Dr. Eileen H. Bigio, Dept. of Pathology,Northwestern University Feinberg School of Medicine, Chicago, IL.



POg

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Hypothalamus

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Fig. 6.2 Localization of olfactory OFC in monkeys. (a) A diagram of the left ventral frontal lobeshows the approximate locations of regions LPOF (hatched lines) and CPOF (stippled area). Note“H-shaped” orbital sulci comprised of medial (M), lateral (L), and transverse (T) segments, and theolfactory bulb/tract (OB) medially. Numbers and letters refer to electrode stimulation sites withinthese regions (data not shown). [Modified from Figure 3 of: Yarita et al. (1980). Used with permission of the American Physiological Society, copyright 1980.] (b) This two-dimensional flat-tened cytoarchitectonic map of monkey OFC highlights the olfactory projection areas in posterioragranular and dysgranular regions of OFC and insula and demonstrates the concentric organiza-tion of these paralimbic structures. The open double arrow indicates approximate border betweenOFC and insula. Iap, agranular-periallocortical insula; Idg, dysgranular insula; Ig, granular insula;ils, inferior limiting sulcus; LOS, lateral orbital sulcus; MOS, medial orbital sulcus; OFap, agranular-periallocortical OFC; OFdg, dysgranular OFC; OFg, granular OFC; ois, orbital insular sulcus; POC,primary olfactory cortex; POap, agranular-periallocortical paraolfactory cortex; POdg, dysgranularparaolfactory cortex; POg, granular paraolfactory cortex; sls, superior limiting sulcus. [Reproducedand modified with permission from 1 of: Morecraft, R.J., Geula, C., and Mesulam, M.M. (1992).Cytoarchitecture and neural afferents of orbitofrontal cortex in the brain of the monkey. Journal



1974; Tanabe, Iino et al. 1975; Tanabe, Yarita et al. 1975). These researchers demonstratedthat in awake, unanesthetized monkeys, electrical stimulation of the olfactory bulb oranterior piriform cortex elicited extracellular potentials in the lateral portions of the posterior OFC, substantiating Allen’s earlier findings in the canine prefrontal cortex(Allen 1943a). This area was named the lateral posterior orbitofrontal area, or LPOF,encompassing the posterior part of Walker’s area 12 (Walker 1940), the lateroposteriorpart of area 13, and more posteriorly, the frontotemporal junction and frontal operculum(Fig. 6.2a).

In the same study, damage to either the anterior piriform cortex or the hypothalamusabolished the bulb-evoked potentials in the LPOF, leading to the conclusion that olfac-tory projections to the OFC were conducted via piriform and hypothalamic relays(Tanabe, Yarita et al. 1975). In contrast, ablation of the mediodorsal thalamus (MD) hadno effect on the evoked potentials in the LPOF, nor did direct MD stimulation elicitpotentials within the LPOF region of the OFC. These findings were a surprise at thetime, as it was generally assumed that the primate MD was an obligatory checkpointthrough which all afferent fibers must pass en route to the orbitofrontal surface (vonBonin and Green 1949; Pribram and MacLean 1953; Nauta 1971). Moreover, the medialsubdivision of the MD was already known to receive direct olfactory projections in bothrodents (Powell et al. 1965) and primates (Benjamin and Jackson 1974), so it seemedplausible that this region should form a node in the projection pathway from the olfac-tory bulb to the LPOF. The heretical idea, that olfactory information in the OFC couldbypass the thalamus, prompted a retrograde tracer study in primates intended to charac-terize the afferents to the LPOF (Potter and Nauta 1979). Neurons were labeled in manyregions, including the MD thalamus and prorhinal (entorhinal) cortex, implying that theLPOF was accessible by dual olfactory fiber systems: a thalamocortical pathway via thepiriform cortex/MD, and a cortico-cortical pathway via the piriform/entorhinal cortex.

The same research group that initially characterized the LPOF later identified a second olfactory projection area in the primate OFC, that was in fact mediated by atransthalamic pathway (Yarita et al. 1980). This region was situated in between the medialand lateral orbital sulci, within Walker’s area 13 (Fig. 6.2a). As this region was positionedmedial and slightly anterior to the LPOF, it was labeled the centroposterior orbitofrontalarea (CPOF). Both extracellular and intracellular recordings indicated that electrical stim-ulation of the olfactory bulb or the CPOF evoked spike potentials in the same recording

of Comparative Neurology, 323, 341–358. Copyright 1992 by Wiley-Liss. Reprinted with permis-sion of Wiley-Liss, a subsidiary of John Wiley and Sons, Inc.] (c) A summary of the primary olfactory inputs to monkey OFC on this ventral view of the macaque brain shows that projectiondensity (indicated by the density of dots) is highest in regions adjacent to POC (e.g., Iam, Iapm)and decreases with distance (e.g., 13 m)

Source: Adapted from Figure 22 of: Carmichael, S.T., Clugnet, M.C., and Price, J.L. (1994).Central olfactory connections in the macaque monkey. Journal of Comparative Neurology, 346,403–434. Copyright 1994 by Wiley-Liss. Reprinted with permission of Wiley-Liss, a subsidiary ofJohn Wiley and Sons, Inc.


sites within the medial (magnocellular) division of the MD thalamus, suggesting directparticipation of this nucleus in an olfactory pathway between the bulb and the OFC.

While the above findings suggested the presence of two discrete olfactory areas (LPOF,CPOF) in the monkey OFC, specific details regarding the underlying projection pathwayremained somewhat uncertain, with conflicting evidence for involvement of the hypo-thalamus, thalamus, entorhinal cortex, substantia innominata, and amygdala (Tanabe,Yarita et al. 1975; Potter and Nauta 1979; Yarita et al. 1980; Naito et al. 1984). However,recent advances in cytoarchitectonic and histochemical techniques have made clear thatthe OFC is no more than two synapses removed from the nasal periphery. Mesulam andMufson first showed that projections to anteroventral insular regions (agranular anddysgranular) on the monkey posterior orbital surface arise directly from the POC(Mesulam and Mufson 1982a; Mesulam and Mufson 1982b; Mufson and Mesulam1982). The agranular region consists of a relatively undifferentiated two- to three-layerperiallocortex, is contiguous laterally with the frontal piriform cortex, and was termedIap (agranular-periallocortical) [(Mesulam and Mufson 1982a)]. In turn, the dysgranu-lar region is a more highly differentiated five- to six-layered cortex, is located rostral anddorsal to the agranular sector, and was termed Idg (Mesulam and Mufson 1982a).Subsequent work has documented analogous olfactory projection patterns in agranular(OFap) and dysgranular (OFdg) sectors of the caudal OFC (Morecraft et al. 1992; Barbas1993), which themselves represent medial continuations of Iap and Idg. Together theseinsular and orbital territories comprise concentric rings emanating from an allocorticalpiriform “root” (Fig. 6.2b), and appear to provide the substrate for the convergence ofolfactory, gustatory, visceral, autonomic, endocrine, and emotional information(Mesulam and Mufson 1982b; Morecraft et al. 1992).

In 1994 Price and colleagues conducted further investigations of the POC and its projections to the OFC using anterograde and retrograde tracers and electrophysiologi-cal recordings (Carmichael et al. 1994). These experiments showed that a total of ninedifferent orbital areas, each with distinct structural characteristics (Carmichael and Price1994), received direct input from virtually all portions of the POC, including the anteriorand the posterior piriform cortex, the anterior olfactory nucleus, the periamygdaloidcortex, and the entorhinal cortex. Olfactory neocortical targets included areas Iam, Iapm,Iapl, Iai, and Ial in the agranular insula, and areas 13a, 13m, 14, and 25 in the posteriorand medial OFC, which broadly overlaps the previously identified representations inIap/OFap and Idg/OFdg (Mufson and Mesulam 1982; Morecraft et al. 1992; Barbas1993). Among the cytoarchitectural subregions identified by Carmichael and Price, areasIam and Iapm received the highest density projections from olfactory regions, followedby area 13a. All three areas showed action potentials in response to electrical stimulationof the olfactory bulb. Area Iam was notable for the presence of rapid (4–10ms latency)action potentials, which were not characteristic of Iapm or 13a responses. In comparingtheir findings to the prior physiology data from the Takagi laboratory (Tanabe, Yaritaet al. 1975; Yarita et al. 1980), Carmichael et al. (1994) suggested that area LPOF roughlycorresponded to their projection sites in Iam, Iai, and Ial, and area CPOF correspondedto 13m and portions of Iam and 13a.



Four general principles emerged out of this work. First, despite the broad pattern of distribution, neocortical projections were most heavily concentrated in the regionsnearest to the POC (in areas Iam and Iapm), and progressively declined with distancefrom the POC (Fig. 6.2c). It was concluded that Iam and Iapm are the principal neocorti-cal sites of olfactory information processing, though the role of Iapm may subsume a more general visceral function, given its substantial input from the ventroposteromedialnucleus of the thalamus (Carmichael and Price 1995). Second, there is overall preserva-tion of olfactory topography, such that more anterior-medial portions of the OFCreceived olfactory inputs from the anterior-medial POC (e.g., anterior olfactory nucleus),whereas posterior-lateral portions of the OFC received inputs from the posterior-lateralPOC (e.g., temporal piriform cortex, periamygdaloid cortex). Third, projections betweenthe POC and Iam/Iapm are typically bidirectional, consistent with the previous findings(Mesulam and Mufson 1982b; Mufson and Mesulam 1982), apart from the olfactorytubercle, which lacks input to the OFC (Carmichael et al. 1994). Finally, most of the olfac-tory areas identified in the agranular insula and the posterior OFC are connected to theregion of the MD thalamus that receives input from the POC (Russchen et al. 1987; Rayand Price 1993), substantiating the presence of both direct and indirect (thalamic) path-ways to the olfactory neocortex.

HumansUntil recently, information regarding the anatomical localization of the human olfac-tory neocortex has lagged behind the primate (and rodent) data due to methodologicallimitations. However, the advent of modern functional neuroimaging techniques haspermitted a more precise determination of the structural underpinnings of humanolfactory processing in vivo. In 1992, Zatorre and colleagues published the first imaging(PET) study of human olfaction (Zatorre et al. 1992). Healthy volunteers were asked toinhale during birhinal presentation of an odorless cotton wand (first scan) or duringpresentation of 8 different odorants (second scan). Comparison of odor to no-odorscans revealed significant neural activity in the left and the right piriform cortex and theright OFC. The right OFC activation was located centrally within the orbital cortex, inbetween the medial and lateral orbital sulci, but more rostral than would be predictedfrom the monkey data. Odor-evoked OFC activity was also observed on the left side,albeit at reduced statistical threshold. The greater right-hemispheric OFC activity,combined with evidence of greater impairments in olfactory function following rightvs. left OFC lesions (discussed below), led to a hypothesis of a right frontal dominancefor olfaction.

Following this groundbreaking work, numerous investigators using both PET and fMRItechniques (and a variety of experimental paradigms) have further examined the locationof an olfactory representation in the OFC. To document the localization of the humanolfactory OFC more definitively, we compiled the OFC voxel coordinates reported in allimaging studies where: (a) the neural characterization of basic olfactory processing was acentral aim; (b) odor-evoked activity was not complicated by the use of aversive odorants;and (c) subjects were not asked to make cognitive olfactory judgements (other than odor




Fig. 6.3 Localization of putative olfactory OFC in humans. The peak activation coordinates (23voxels: red dots) from 12 neuroimaging studies of basic olfactory processing are plotted on acanonical T1-weighted anatomical MRI scan in Talairach space. The yellow circles indicate themean voxel coordinates for right and left hemisphere. For presentation, coordinates are col-lapsed across the z-axis (superior-inferior) in order to depict all voxels on a single image. It maybe noted that separate foci also emerged around y = 6 in three of the studies included in theanalysis. These foci are not displayed because they are so far posterior that if mapped to thecurrent slice they would appear to be located in temporal cortex.”See Plate section in the colourgallery.”

detection) during scanning (Gottfried and Zald, in press). Given the data from monkeys,this analysis was broadly constrained to OFC activations posterior to y � 45 along theanterior-posterior axis (Talairach coordinate space). Data came from the following 13studies (5 PET, 8 fMRI): Zatorre et al. 1992; Small et al. 1997; Sobel et al. 1998; Zald andPardo 1998; Francis et al. 1999; O’Doherty et al. 2000; Savic and Gulyas 2000; Savic et al.2000; Sobel et al. 2000; Poellinger et al. 2001; Gottfried, Deichmann et al. 2002; Gottfriedand Dolan 2003; Kareken et al. 2004. To reduce inconsistencies across studies, MNI coor-dinates were converted to Talairach space. A total of 26 coordinates were identified,including 15 on the right and 11 on the left (Fig. 6.3). Seven of these studies reported sig-nificant bilateral activation, five reported significant right-sided activation only, and onereported significant left-sided activation only. While the data generally confirm a right-hemisphere predominance, it is important to note that left hemisphere activations arosein over 60% of the studies.

Examination of the location of activations reveals that olfactory stimulation consis-tently activates a bilateral area along the medial orbital sulcus (i.e., the medial posteriorlimb of the “H”-shaped sulcus), close to the transverse orbital sulcus (i.e., the horizontallimb of the “H”-shaped sulcus) (Fig. 6.3). Voxel coordinate means (with standard devia-tions) and medians for this putative olfactory region are shown below in Table 6.1.


A comparison of these activations to the monkey (Carmichael and Price 1994) andhuman (Ongur et al. 2003) architectonic maps delineated by Price and colleagues suggests that the putative human olfactory OFC roughly corresponds to the posteriorpart of area 11, a location that is strikingly more anterior than one would predict onthe basis of the animal data discussed above (Yarita et al. 1980; Morecraft et al. 1992;Carmichael et al. 1994). Regardless of specific cytological subdivisions, it is clear thatthe putative human region is situated at least 2 cm rostral to the monkey counterpartof the agranular insula. Indeed, among the 13 studies cited above, only three reportedactivations that could possibly be considered to lie within the agranular insular exten-sion into the posterior OFC (Poellinger et al. 2001; Gottfried, Deichmann et al. 2002;Gottfried and Dolan 2003). We did not include these foci in the above analyses (orTable 6.1 and Figure 6.3), because they were so far posterior (around y � 6) that theywere difficult to distinguish from the frontal piriform cortex, and clearly reflected atopographical area different from the dominant location of foci in the OFC. A com-plete survey of the olfactory imaging literature shows that the agranular insular regionis only rarely activated and, if anything, appears to be preferentially activated duringthe higher-order tasks involving a cross-modal integration rather than during passiveolfactory stimulation (De Araujo et al. 2003; Small et al. 2004).

It is possible that this discrepancy between predictions from animal studies andhuman data are the result of methodological differences (see Gottfried and Zald, inpress). We note, however, that the difference is unlikely to be simply a result of signaldropout in fMRI, since a similar distribution of responses is observed in the PET studiesof basic olfactory processing, which are not susceptible to these technical artifacts (seeStenger, this volume). Another possible source of these differences is that in the humanneuroimaging studies, subjects are often actively attending to the odorants even if theyare not told to make any specific judgements. Such cognitive or attentional factors may fundamentally alter the underlying activation patterns, such that a more centralregion is preferentially activated relative to the area that would be predicted from theprimate literature. Regardless of the source of this displacement, it seems clear that thismore central region plays an important role in olfaction in humans. Given the different


Table 6.1 Functional localization of putative human olfactory OFC averaged across 13 neuroimaging studies

Mean (� S.D.) Median

Coordinatesa x y z x y z

Right OFC 23.9 33.8 �12.1 24.0 33.0 �12.0

(5.4) (5.6) (5.3)

Left OFC �21.2 30.8 �15.5 �21.8 30.0 �17.0

(5.7) (4.2) (4.6)

a Coordinates are in Talairach space in mm, where x � medial/lateral from the midline (� � right), y � posterior/ante-rior from the anterior commissure (� � anterior), and z � inferior/superior from the intercommissural plane(� � superior).


cytological and connectional features of this region, its functional properties are almost certainly different from those seen in the studies of the more posterior OFC regions inmonkeys, and may reflect behavioral differences in the role that the sense of smell playsin these two species. Of particular note in this regard, the agranular OFC receives stronginput from the amygdala, which is almost completely absent in the central OFC (seePrice, this volume).

Response characteristics of secondary olfactory cortex

Nonhuman primates

Stimulus specificity: In their initial characterizations of the LPOF and CPOF, Tanabeand colleagues showed that these olfactory areas exhibited different activity patterns inresponse to odor stimulation (Tanabe et al. 1974; Tanabe, Yarita et al. 1975; Tanabe, Iinoet al. 1975; Yarita et al. 1980) [Fig. 6.4]. Out of 44 neurons recorded in the LPOF, 50%responded to just one of eight different odors, and no cell responded to more than fourodors (Tanabe, Iino et al. 1975). In contrast, out of 12 neurons recorded in the CPOF,


(a) (b)

(c) (d)

Fig. 6.4 Response properties of secondary olfactory cortex in monkeys. Neuronal response his-tograms to eight different kinds of odors depicts the activity profiles in olfactory areas CPOF (a) broadly tuned) and LPOF (b) finely tuned), as well as in olfactory bulb (c) and medial amyg-dala/anterior piriform cortex (d). Bars represent the percentage of cells in each respective areathat responded to a given number of odors, as indicated on the abscissa.

Source: Adapted from Figure 9 of: Yarita et al. (1980). Used with permission of the AmericanPhysiological Society, copyright 1980.


none responded to less than three of the eight odorants (Yarita et al. 1980). Althoughrecordings in the CPOF were limited, the data suggested that odor tuning and discrimi-nation were highly specialized and selective in the LPOF, but much more generalizedin the CPOF.

The specificity of the LPOF responses was also notable in relation to other olfactorystructures (Fig. 6.4). Whereas only 20% of cells in the LPOF responded to three or moreodorants, the majority of recorded cells in olfactory bulb (78%) responded to three ormore odorants, and an intermediate response pattern was observed in the anterior piri-form cortex and the medial amygdala (69%). Tanabe, Iino et al. (1975) additionallynoted that LPOF responses were particularly effective in distinguishing odor pairs withoverlapping perceptual qualities (in this case, camphoraceous smells) relative to otherolfactory regions. In other words, the LPOF was particularly able to resolve subtle differ-ences between odors when they smelled alike, underscoring the idea that thisorbitofrontal area supports highly refined olfactory discriminations.

In a complementary attempt to define the role of the LPOF, Tanabe, Yarita et al. (1975)performed a lesion study of this region. After monkeys were conditioned to associate oneof several odors with an unpleasant (bitter) taste, the LPOF was removed bilaterally. Thisexcision resulted in severe impairments of olfactory discrimination. On the surface, thesefindings accord with their data on odor-evoked responses in the LPOF, as detailed above.However, inspection of Table 1 of Tanabe, Iino et al. (1975) reveals that monkeys with themost complete LPOF excisions (subjects M4 and M5) actually responded well belowchance for the majority of odor discriminations, consistently choosing the wrong odor.This observation suggests that odor discrimination was not abolished per se (whichwould have yielded at-chance performance), but was distorted, raising the possibility of amore complex role for the LPOF. For example, if LPOF activity reflected informationabout conditioned odor value, odor preference, or odor-response linkages, then LPOFablation might have disrupted these mechanisms, yielding an aberrant performance pattern such as that observed, without necessarily implicating this region simply in olfactory discrimination.

In contrast to area LPOF, the characterization of the CPOF was less comprehensive,and no lesion data were obtained. Nevertheless, on the basis of the known fiber projec-tions from the limbic areas to the MD thalamus, Yarita et al.(1980) proposed a synthetic, integrative function for this “transthalamic” olfactory area, perhaps as a site ofconvergence for olfactory and emotional information. However, the suggestion that theolfactory system has two anatomically distinct OFC projection areas, each with differenttuning properties and subserving different functions, was called into question by Rollsand colleagues in a series of single-unit recording studies (Rolls and Baylis 1994;Critchley and Rolls 1996a). In an initial investigation (Rolls and Baylis 1994), neuronalresponses in the caudal two-thirds of the monkey OFC were tested to nine differentolfactory stimuli, representing food and non-food smells, as part of a larger study ofmultimodal processing. Unimodal olfactory-responsive cells (n � 15) were found primarily in medial, but also in lateral, portions of the OFC. Critically, most of thesecells were broadly tuned to the odorant set, though each showed clear preferences across



the different stimuli. In a subsequent study, both broadly tuned and finely tuned olfac-tory-responsive cells were identified throughout the posterior orbital surface (Critchleyand Rolls 1996a). Taken together, these findings cast some doubt on the notion offunctionally heterogeneous olfactory areas in the OFC. However, it is possible that theLPOF may represent a specific functional zone that was under-sampled in the work ofRolls’ group. Tanabe, Iino et al. (1975) identified 44 of 119 (37%) olfactory-responsivecells in the LPOF, indicating a highly odor-sensitive neuronal population, whereas Rollsand Baylis (1994) identified only 15 of 2000 (0.75%) cells that responded to unimodalolfactory stimulation. This leaves open the possibility that there is a unique olfactoryregion in the vicinity of posterior-most OFC and agranular insula. Unfortunately, weknow of no recent investigations that have specifically tested this hypothesis.

Odor identity and hedonic value: Several lines of evidence suggest that both odoridentity and hedonic value are separately represented in the monkey OFC. In a study ofolfactory discrimination learning, monkeys were trained to associate nine different foodand -food odors with either rewarding (sweet) or aversive (saline) tastes (Critchley andRolls 1996a). Single-unit recordings were subsequently collected from 1,580 cellsthroughout the OFC, 48 (3.1%) of which responded to at least one odorant.Characterization of the tuning profiles indicated that the majority of the odorant-responsive cells (n � 34; 71%) responded differentially to the odorants, while theremainder (n � 14; 29%) did not. Among the 34 OFC neurons showing differentialselectivity, the responses in 22 cells (65%) were not based on odor-taste associations.Thus, these neurons appear to encode representations of odor identity, independentlyof the reinforcing value of the olfactory stimuli. In contrast, the responses in 12 cells(35%) reflected the taste-reward association of the odorants, irrespective of odorantidentity. Some neurons preferentially responded to the reward-associated odorants, andothers to the saline-associated odorants. Odor-selective, reinforcer-selective, and non-selective cell types were located throughout the medial and the lateral OFC without evidence for functional segregation. Additional support for the presence of OFC cellsthat respond to odorants on the basis of their reinforcer associations rather than theiridentity comes from work in which the reward and aversive outcomes are reversed(Rolls, Critchley, Mason et al. 1996), described in detail in chapter X by Rolls.

Such findings demonstrate that the hedonic or motivational value of an olfactorystimulus is encoded within the OFC, and can be flexibly updated based on changingreinforcement contingencies. Nonetheless, it is important to keep in mind that codingof odor identity, not reinforcement value, is the rule in the majority of olfactory-responsive neurons in the primate OFC. Computational models suggest that evendespite the relative sparseness and broad tuning observed in the posterior OFC (asopposed to the specificity observed by Tanabe et al.), information regarding odor identity may be robustly supported across local ensembles within the OFC (Rolls,Critchley and Treves 1996). As discussed below, there is also evidence that odor identitymay actually be coded across a network of olfactory regions.



Humans

There have been relatively few neuroimaging investigations of the response characteris-tics of the human olfactory OFC, or human POC for that matter. This is partly due tothe experimental challenges of dissociating the many psychological factors that comprise odor perception. Intensity, pleasantness, quality/character, pungency (trigem-inality), familiarity, edibility, and nameability are all distinctive properties of an olfac-tory stimulus, but it has been difficult to study these elements in isolation, since any one factor frequently influences the perception of another (Distel et al. 1999).

Odor hedonics: A number of human imaging studies have centered on odor valenceusing emotionally salient olfactory stimuli. The first study of this kind was conductedby Zald and Pardo (Zald and Pardo 1997), who determined that highly aversive sulfidesevoked activity in the left posterior lateral OFC (and bilateral amygdala), when com-pared to an odorless control (Fig. 6.5). The OFC activation was situated 10–15 mm lateral to the putative human olfactory area in the OFC, and may therefore represent aregion that is selectively engaged by unpleasant odors. This finding was complementedby a regression analysis showing that the neural activity in the left lateral OFC signifi-cantly correlated with subjective ratings of odor unpleasantness (but not odor inten-sity), further supporting the idea that this region participates in hedonic processing ofolfactory stimuli. Interestingly, during exposure to weaker and less unpleasant odorants,the peak of the subjects’ responses corresponded more closely to the putative humanOFC olfactory region in the left hemisphere.

Since this initial report, several additional studies have supported the idea of left lateral OFC sensitivity to aversive odorants. As part of an event-related fMRI study on


(a) (b)

Fig. 6.5 Activations in the left lateral OFC in response to aversive odorants. (a) Left lateral OFCactivation during exposure to highly aversive sulfides. (b) Left putative olfactory region andweaker left lateral OFC activations in the same subjects during exposure to several more mildlyunpleasant (and less intense) odorants from the University of Pennsylvania Smell IdentificationTest (Doty et al. 1984). The PET data derive from Zald and Pardo (1997). Note, the most poste-rior portions of the OFC are covered by the temporal poles in this surface rendering, but no statistically significant actions were seen in this area.”See Plate section in the color gallery.”


visual-olfactory associative learning, presentation of an unpleasant odor evoked signifi-cant left-sided responses in the lateral posterior OFC, whereas a pleasant odor evoked sig-nificant right-sided responses in the medial posterior OFC (Gottfried, Deichmann et al.2002). Examining the effects of three aversive odorants, Rolls et al. (2003) similarlyobserved specific activation of the left lateral OFC, whereas pleasant odorants did notclearly engage this region. In an elegant study that dissociated intensity and valence withina single fMRI experiment, the right medial OFC was activated to pleasant odor, and theleft lateral OFC was activated to unpleasant odor (Anderson et al. 2003). Finally, similarmedial-lateral dissociations in the human OFC according to valence have been identifiedin olfactory cross-modal paradigms of episodic memory (Gottfried et al. 2004).

To characterize these response patterns more accurately, we plotted the activationpeaks for pleasant and unpleasant odor stimuli from the following six studies: Zald andPardo 1997; Zald and Pardo 1998; Gottfried, Deichmann et al. 2002; Anderson et al.2003; Rolls et al. 2003; and Gottfried et al. 2004 [Fig. 6.6]. Although further data areclearly needed to make definitive statements, some basic patterns nevertheless emerge.First, there is a strong tendency of aversive odorants to engage the left OFC, whereaspleasant odorants have a more bilateral representation. Second, neural responses evokedby either pleasant or unpleasant odors do not localize to discrete regions within OFC,nor do these activations systematically cluster around the putative olfactory OFC area.Third, while aversive odorants tend to activate more lateral areas of OFC than do pleas-ant odorants, it is important to point out that the “aversive” voxels are equally situated inthe medial (areas 11 and 13) and lateral (area 47/12) portions of the OFC. The greaterappearance of more lateral responses in the left OFC appears consistent with a recentanalysis by Kringelbach and Rolls (2004), who emphasize a general medial-positive,lateral-negative distinction in the OFC. However, the presence of several more medially


Fig. 6.6 Olfactory hedonics in human OFC.The peak neural responses to plasant (8voxels; red dots) and unpleasant (9 voxels; green dots) odors were identi-fied from six imaging studies of olfactoryhedonic processing and mapped onto atransverse TI-weighted anatomical scan.Presentation as in Figure 3. For reference,the yellow circles indicate the putativeolfactory OFC areas.”See Plate section inthe color gallery.”


located foci in response to aversive stimuli indicate that the pattern of responses is notfully explained by a medial-positive, lateral-negative model of OFC function.

Neuroimaging studies of explicit hedonic judgements have also been used to explorethe emotional underpinnings of olfactory processing. Royet and colleagues (Royet et al.2001; Royet et al. 2003; Royet and Plailly 2004) have emphasized the importance of theleft OFC in the conscious hedonic evaluation of odorants. This premise arises out oftheir initial finding that hedonic judgements produced left OFC activation (at x � �28,y � 16, z � �10) which exceeded that produced by judgements of familiarity, edibility,intensity, or detection. However, two studies suggest that hedonic judgements are notexclusively lateralized to the left OFC. First, in a study by Zatorre et al. (2000), bothintensity and pleasantness judgements were observed to activate the right OFC, and therewas no preferential activation of the left OFC during pleasantness relative to intensityjudgements. An additional concern may be raised regarding the earlier Royet studies, inthat the hedonic judgement tasks are commonly made in the exclusive presence of emo-tionally salient odors, whereas other judgements were not. Therefore, any task-relatedactivations could be confounded by mere stimulus properties of the odorants them-selves. In an effort to overcome this issue, Royet et al. (2003) conducted an fMRI study inwhich subjects smelled pleasant and unpleasant odors during alternating blocks of eitherhedonic judgements or a control task (random button press). A comparison of hedonicand control tasks (thereby controlling for stimulus effects) revealed significant activity inthe OFC bilaterally, further supporting the possibility of right (as well as left) OFCinvolvement in hedonic coding.

Odor quality: In contrast to the neurophysiology data in monkeys, there has been verylittle research on the neural substrates of odor quality in the human brain. As used here,the term “quality” denotes odor-object identity, i.e., the perceptual character of a smellemanating from an odorous object (in contrast to other odor qualities such as intensityor valence). The characterization of odor quality coding faces particular challenges usingneuroimaging, due to the limited spatial resolution of the technique. Take for example, asone’s hypothetical research aim, the comparison of the smell of “strawberry” vs. the smellof “lavender”. Presuming that quality is represented as a distributed ensemble code in theOFC, “strawberry” will evoke a hemodynamic response that is indistinguishable from“lavender”. Thus, in the absence of differential activity, one cannot conclude whether theOFC codes odor quality. Moreover, the comparison of “strawberry” and “lavender” willbe sensitive not only to differences of quality, but also to differences of intensity, pleas-antness, familiarity, pungency, and even molecular structure, thereby confoundingattempts to delineate the neural components of odor quality.

In preliminary studies, we (Gottfried, Winston, and Dolan, unpublished manuscript)have overcome these problems by combining an olfactory version of fMRI cross-adaptation (Buckner et al. 1998; Grill-Spector and Malach 2001; Kourtzi and Kanwisher2001) with a careful selection of odorant pairs that systematically differ in perceptual qual-ity or physical structure. Critically, this experimental design was fully balanced, controlling



for the possibility that the findings could be confounded by variations in intensity, hedo-nics, or other perceptual dimensions, and ensuring that only quality and structure differedacross condition types. Sixteen healthy subjects participated in a task of odor detection(rather than an odor quality judgement task), so that the neural substrates of odor qualitywould not be confounded by explicit cognitive judgements of odor quality. Across thegroup, neural activity in the right OFC, the left posterior piriform cortex, and the lefthippocampus decreased (cross-adapted) in response to pairs of odorants that were similar(vs. different) in quality, irrespective of the effects of molecular structure (Fig. 6.7). Thesedata are consistent with the idea that representations of odor quality may be distributedacross a network of olfactory-related regions, in keeping with prior monkey (Tanabe, Iinoet al. 1975) and human (Eichenbaum et al. 1983; Royet et al. 1999; Savic et al. 2000;


(a) (b)

(d)(c)

Fig. 6.7 Neural representations of odor quality in human OFC. (a, b) A comparison of qualita-tively different vs. qualitatively similar odorants revealed significant activity in right OFC.Functional responses are superimposed on coronal (a) and transverse (b) sections of a T1-weighted anatomical scan. (c) Neural representations of odor quality were also detected inpiriform cortex (“p”) and hippocampus/parahippocampal cortex (“h”) (responses overlaid on atransverse T1 section). (d) Group-averaged plots of the peak fMRI signal from right OFC indicate decreased (cross-habituated) responses to qualitatively similar odorants, in comparisonto qualitatively dissimilar odorants. ”See Plate section in the color gallery.”


Gottfried and Dolan 2003) studies of olfactory discrimination and semantic processing. Wespeculate that each of these areas encodes a distinct qualitative element (e.g., odor-objectidentity, semantic and verbal mnemonic associations, affective and motivational features)that together define an odor percept. It is also possible that the OFC and the hippocampusare directly involved in sculpting odor quality representations in the posterior piriform cor-tex. This idea is supported by our data on olfactory-visual semantics (Gottfried and Dolan2003) showing OFC and hippocampal activation to semantically related odor-picture pairs,suggesting that semantic representations in these regions might shape the expression ofquality-based codes in the piriform cortex.

Olfactory cognitive judgements and the human OFCThe ability to code odor quality is essential for making explicit judgements of odoridentification. Consistent with the evidence of odor quality representations in the OFC(as described above), partial excisions of the prefrontal lobe for the relief of intractableepilepsy have been associated with impairments of odor identification (Jones-Gotmanand Zatorre 1988). In this study, patients were asked to identify common odors usingthe University of Pennsylvania Smell Identification Test (Doty et al. 1984), a “scratch-and-sniff ” test that requires matching each of 40 different odors to different lists of 4verbal descriptors. Compared to a normal control group, only those prefrontal patientswhose excisions involved the OFC were impaired on this task. Interestingly, patientswith temporal lobe excisions (frontal sparing) also performed poorly, though they were relatively less impaired than the OFC group. These effects were generally observed in thepresence of normal detection thresholds, excluding any primary loss of smell, althoughthere was a significant negative correlation between right-nostril odor detection valuesand identification performance in the right frontal patients.

Complementary studies on odor quality discrimination have demonstrated similareffects. Potter and Butters (1980) asked patients with prefrontal damage to rate the similarity of odor quality between successive pairs of stimuli. Performance impairmentappeared to be specific to the olfactory modality, as visual hue discrimination was leftintact. Curiously, odor detection thresholds were lower (more sensitive) in the pre-frontal subgroup than in controls, though the overall younger age of the prefrontalpatients might have accounted for this finding. This same experiment suggested that thesmelling deficit might be specific to the ipsilateral nostril (Potter and Butters 1980),although subsequent work has demonstrated a more complicated picture. In a qualitydiscrimination task, Zatorre and Jones-Gotman (1991) gave monorhinal presentationsof odorant pairs to post-surgical epilepsy patients, who judged odor quality similarityfor each pair of stimuli. Right OFC patients showed discrimination impairments inboth nostrils, whereas right frontal patients with orbital sparing lesions were impairedonly in the ipsilateral nostril. Performance was also markedly reduced in the ipsilateralnostril of left OFC patients, though this was non-significant (possibly due to low samplesize).



Together with the finding that discrimination scores were higher in control subjectswhen odorants were delivered to the right (vs. left) nostril, the data were interpreted tosuggest a right hemisphere (and right OFC) advantage in odor quality discrimination(Zatorre and Jones-Gotman 1991). This conclusion is in keeping with the greater frequency of right sided activations in basic olfactory neuroimaging studies, and thefMRI adaptation data described above (Section I.C.), in which the right (but not the left)OFC was implicated in odor quality coding. However, one concern with this interpreta-tion warrants mention. Inspection of the brain lesion maps delineating extents of surgi-cal excision (cf. Figs. 1 and 2 in Zatorre and Jones-Gotman 1991) suggests that the rightfrontal groups as a whole may have engendered greater damage to olfactory projectionareas in the centroposterior OFC than did the left frontal groups. This might explain whyodor detection thresholds correlated with identification performance in right (but notleft) frontal patients, and also why detection thresholds were lower in the right (but notthe left) nostril of right OFC patients, compared to both left OFC patients and controls(cf. Table 2 in: Zatorre and Jones-Gotman 1991). Thus, although the data are consistentwith a right OFC dominance for olfactory identification and discrimination, it remainsconceivable that differential hemispheric damage to the secondary olfactory cortex couldbe contributing to this apparent asymmetry.

A variety of other explicit cognitive tasks influence neural responses in the OFC(reviewed in: Savic 2002; Royet and Plailly 2004). Judgements of intensity (Zatorre et al.2000), familiarity (Royet et al. 1999; Royet et al. 2001), edibility (Royet et al. 1999; Royetet al. 2001), and pleasantness-unpleasantness (see Section I.C. above), have each beenassociated with activation of the OFC in PET studies. Some of these activations are situated near the purported olfactory projection area in the OFC, while others are foundmore rostrally. As already noted, Royet et al. (2001) have suggested that there is a degreeof specificity in the engagement of the left OFC, such that the activity induced by hedo-nic judgements was greater than that seen for other judgements. Specificity for otherjudgements has been less clear. For instance, in the right OFC, familiarity judgementswere observed to produce greater activation than that produced by odor detection orintensity judgements. However, familiarity judgements did not cause activations beyondthose caused by judgements of hedonicity or edibility, indicating that the involvement ofthe right OFC in explicit judgement tasks is not limited to familiarity. Finally, it is worthnoting that many judgement-related foci lie outside of the OFC altogether: olfactoryjudgement tasks typically evoke responses in large portions of the frontal, temporal, pari-etal, and occipital cortices (Royet et al. 1999, 2000, 2003; Savic et al. 2000). Although thespecific contributions of these regions remain unclear, it appears that several areas thatare tertiary to the olfactory system help mediate a higher-level olfactory decision-making. Together, these findings are consistent with both serial hierarchical and paralleldistributed modes of olfactory information processing that vary according to taskdemands (Savic et al. 2000; Royet and Plailly 2004). However, it remains unclear specifi-cally how these non-olfactory brain areas interact with the OFC and the POC in mediatingthese tasks.



Olfactory memory and learning in the human OFCFollowing from their lesion studies on olfactory identification and discrimination, Jones-Gotman and Zatorre (1993) also showed that patients with postsurgical excisions of theright frontal lobe performed poorly on immediate, short-term (20 min), and long-term(24 hr) tests of odor recognition memory. While these data make it evident that right OFClesions impair performance on odor recognition memory, it remains unclear if this reflectsa specific mnemonic problem, or is secondary to deficits in discrimination and identifica-tion described above (Zatorre and Jones-Gotman 1991). In subsequent PET studies, olfac-tory working memory (Dade et al. 2001) and short- and long-term recognition memory(Savic et al. 2000; Dade et al. 2002) have been associated with enhanced right or bilateralOFC activity. Unfortunately, in some of these studies, the control conditions failed to con-trol for olfactory stimulation, again leaving open to question the extent to which OFCinvolvement in olfactory memory is specific to mnemonic vs. other olfactory processes.

In an effort to characterize odor memory in the absence of odor stimulus confounds,Gottfried et al. (2004) conducted an event-related fMRI study on episodic memoryretrieval of olfactory and emotional contexts. During an initial study phase, subjects weregiven combinations of smells varying in affective valence (pleasant, neutral, and unpleas-ant) and neutral pictures and asked to imagine a link or association between the twostimuli. The aim of this session was to encourage episodic memory encoding ofodor-picture pairs. In a subsequent recognition memory test, subjects had to decidewhether they were viewing a study (old) or novel (new) picture, in the absence of anyodor cues. Comparison of correctly remembered items to correct rejections (“old/new”effect) revealed significant memory-related activity in the piriform cortex (but not in theOFC). Critically, odor absence during the memory session ruled out the possibility that piriform activity was merely being driven by olfactory stimulation. On the other hand,the neural substrates of emotional contextual retrieval were expressed in dissociable subregions of the OFC. The contrast of positive (vs. negative) hits was associated withactivity in the medial OFC, while negative (vs. positive) hits elicited activity in the morelateral OFC regions.

While the above findings do not provide compelling evidence that the OFC is directlyinvolved in explicit forms of odor memory, the related work on implicit memory impli-cates this structure more directly. In fMRI studies of visual-olfactory associative learning(Pavlovian conditioning), a neutral visual stimulus (the conditioned stimulus, or CS�) isrepetitively paired with an emotionally charged odor (the unconditioned stimulus, orUCS). By selecting either a pleasant or unpleasant odor as the UCS, both appetitive(reward-based) and aversive learning can be investigated (Gottfried, O’Doherty et al.2002; Gottfried et al. 2003; Gottfried and Dolan 2004). Presentation of an appetitive oraversive CS�, in the absence of odor, elicits robust neural activity in the OFC (Gottfried,O’Doherty et al. 2002), suggesting that this region participates in the establishment ofpicture-odor contingencies. Moreover, when the current motivational value of an olfac-tory reinforcer (UCS) is either decreased (via selective satiety) or increased (via odor



inflation), cue-evoked OFC responses are modulated in parallel with the behavioralmanipulation (Gottfried et al. 2003; Gottfried and Dolan 2004). The implication is that apredictive cue has direct access to internal representations of value in the OFC, and thatthese representations are flexibly updated according to an individual’s motivational state.

Interestingly, a caudal-rostral pattern of specialization in the human OFC has tenta-tively emerged out of the above imaging data on olfactory memory and learning. Asdetailed above (see Section I.B.), odor-evoked neural activity around the putative olfac-tory region identified in Table 1 is typically associated with low-level aspects of olfactoryprocessing. In contrast, more rostral areas of the OFC (roughly within Walker’s area 13)appears more engaged in higher-order olfactory computations, including working mem-ory (Dade et al. 2001), recognition memory (Dade et al. 2002), and associative learning(Gottfried, O’Doherty et al. 2002; Gottfried et al. 2003; Gottfried and Dolan 2004)


(a)

(b)

Fig. 6.8 Caudal-rostral pattern of functional heterogeneity in human OFC. In an fMRI paradigmof visual-olfactory associative learning, learning-evoked activity was detected across extensiveareas of rostral OFC (a, i, a ,ii), but was also observed in caudal OFC (b,i). In panel (b,ii) thelearning-related responses in caudal OFC (illustrated in red) overlap odor-evoked responses inblue. The learning-related responses more posteriorly may reflect modulation of secondaryolfactory cortex by higher-order (top-down) centers. Conversely, more complex operations per-taining to odor learning would be specifically executed in rostral OFC.

Source: Adapted from Figure 2 of: Gottfried, O’Doherty, and Dolan, (2002). Copyright 2002 bythe Society for Neuroscience. See Plate section in the colour gallery.”


[Fig. 6.8]. In accord with such a distinction, cross-modal integration of vision and olfaction is associated with more anterior activations than when the stimuli are presentedunimodally. (See Part III for more information on multisensory integration involving thechemical senses). Such findings appear consistent with an anatomical hierarchy of theOFC specialization, whereby caudal regions (such as the olfactory OFC) converge onmedial and anterior territories to permit more complex information processing in theservice of feeding and other goal-directed behaviors (Carmichael and Price 1996; Ongurand Price 2000). Thus, with regard to functional complexity, the architecture of the olfac-tory OFC appears to be broadly preserved across humans and nonhuman primates.

6.3. GustationTaste refers to the qualities of sweet, sour, salty, bitter and savory (umami). These sensa-tions occur when molecules interact with receptors on the tongue. What is colloquiallyreferred to as “taste” is actually a combination of the physiologically distinct sensoryexperiences of taste and smell, or flavor. As described below, the OFC plays an importantrole in both taste and flavor processing, as well as a more general role in feeding behaviorand the representation of food reward.

Localization of OFC gustatory areas

Nonhuman primates

In primates, gustatory information travels through cranial nerves XII, IX, and X to thenucleus of the solitary tract (Beckstead et al. 1980). The second-order gustatory fibersleave the nucleus of the solitary tract to join the central tegmental tract and project to theparvocellular division of the ventral posterior medial nucleus (VPM) of the thalamus(Beckstead et al. 1980). Fibers from the VPM project most densely to the dorsal sectionsof the anterior insula and adjacent operculum (Pritchard et al. 1986). This region is oftendesignated as the “primary gustatory cortex”, although it appears to be sensitive to a widerange of oral sensation in addition to taste (Scott and Plata-Salaman 1999). A secondaryprojection exists in a region extending from areas in the precentral opercular area nearthe base of the central sulcus. Both regions are activated by peripheral stimulation of thegustatory nerves (Ogawa et al. 1985). Taste-responsive cells are also found in the adjacentregions of the insula (Ogawa 1994; Scott and Plata-Salaman 1999). Since these additionalareas do not receive inputs from the VPM they can be considered higher-order gustatoryregions (Ogawa 1994).

Neurons that respond unimodally to taste stimulation have also been identified in theOFC. The first gustatory region identified in the OFC is located in a dysgranular,caudolateral portion of the OFC (Rolls et al. 1989). In the macaque, this region is just lat-eral to the caudomedial orbital olfactory area (Price et al. 1991) at the lateral point of thetransition between the ventral agranular insula and the caudal OFC and corresponds toareas 13l and Ial according to Carmichael and Price (Carmichael and Price 1995). Bayliset al. (1995) isolated taste neurons in the gustatory orbital region and injected



horseradish peroxidase into this site. They found substantial labelling in the frontal oper-cular taste area and anterior dorsal insula extending into more ventral regions of theinsula. Since, no labelling occurred in the VPM, Baylis and colleagues proposed that thecaudolateral OFC region represents the secondary taste cortex. However, it has recentlybeen demonstrated that there is at least one additional region in the OFC with promi-nent unimodal gustatory responses. In a recent study, Pritchard and colleagues identifieda taste-responsive region caudal and medial to the so-called secondary taste areadescribed by Rolls and colleagues (Pritchard et al. 2005). This region, which is 12 mm2 inextent, straddles areas 13m, 13a, 13b and Iam, and is remarkable because 20% of the cellsisolated responded to taste stimulation. This represents the highest concentration ofgustatory neurons in any cortical region yet described. Based upon anatomy and thecharacteristics of the evoked taste responses, the authors proposed that it comprises amajor gustatory relay that lies anatomically and functionally between the insular andcaudolateral taste areas. Gustatory responsive neurons have also been identified in amore anterior medial region of the OFC (Thorpe et al. 1983), which is consistent withevidence presented by Barbas (1993), indicating the presence of projections from theinsular operculum gustatory area to an extreme rostral portion of area 13. However, thenature of gustatory processing in this region remains poorly defined.

Humans

The human gustatory pathway is assumed to be equivalent to the monkey’s (Scott andPlata-Salaman 1999), and cytoarchitectural studies (Petrides and Pandya 1994) andfunctional neuroimaging studies (Barry et al. 2001; Cerf-Ducastel et al. 2001; de Araujoet al. 2003; Faurion et al. 1998; Faurion et al. 1999; Francis et al. 1999; Frey and Petrides1999; Kinomura et al. 1994; Kobayakawa et al. 1996; Kobayakawa et al. 1999; O’Doherty,Rolls et al. 2001; O’Doherty et al. 2002; Small et al. 2003b; Small et al. 1997a; Small et al.1999; Zald et al. 2002; Zald et al. 1998) support its location in the anterodorsal insula andthe adjacent frontal opercular cortex in and around the cortex of the horizontal ramus ofthe Sylvian fissure. As in the monkey, there is evidence for some secondary taste represen-tations in the midinsula, the ventral insula, and the parietal and temporal opercula(Faurion et al. 1999; Small et al. 1999).

The OFC is frequently activated in response to gustatory stimulation (Francis et al.1999; Frey and Petrides 1999; O’Doherty, Rolls et al. 2001; O’Doherty et al. 2002; Smallet al. 2003b; Small et al. 1997a; Small et al. 1997b; Zald et al. 2002; Zald et al. 1998).However, the area of activation is often anterior and medial to the orbital taste regiondescribed in monkeys (Small et al. 1999). Small and colleagues used the anatomicaldescriptions of the orbital taste area in the monkey provided by Rolls and colleagues toidentify the anatomically homologous region in the human brain. This is shown inFig. 6.9. Activation foci generated in response to taste stimulation from all published(and some unpublished) PET studies of human gustation were plotted onto a high-resolution MRI image. Eighteen peaks localized to the OFC. However, of these 18 peaks,the authors noted that only 2 were located in the region anatomically homologous to the



monkey caudolateral orbital taste area (Fig. 6.9). The rest were dispersed throughoutareas 11, 13 and 14, according to the terminology employed by Petrides and Pandya(Petrides and Pandya 1994). This raises the possibility of inter-species differences in theprecise location of the orbital gustatory area. The anterior location of OFC tasteresponses converges with the olfactory literature in demonstrating greater activations inmore anterior regions than would be predicted based on a simple examination of cytoar-chitectural homology. As with the olfactory system, one may question whether thisapparent displacement is due to a methodological artifact of the functional imagingtechnology. However, the original analysis by Small and colleagues was based exclusivelyon PET data, where the prominent susceptibility related artifacts and distortion thathamper fMRI imaging of the OFC are not an issue. Alternatively, these anterior activa-tions in both modalities may represent higher-order processing of chemosensory infor-mation. If so, this would imply that humans preferentially activate higher-orderprocessing regions relative to the more formally defined secondary taste and smellregions in the more caudal OFC.


y=23.5

y=23.5ZaldSmall

(b)

Fig. 6.9. The orbitofrontal gustatory region in the human brain (a) Crosses represent peaks fromthree gustatory (Small et al. 1997a; Small et al. 1997b; Zald et al. 1998) studies projected ontoa surface rendering with anterior temporal lobes removed to expose the underlying caudalorbital surface. This was accomplished with a segmentation technique in which the region ofthe temporal lobe at the point where it begins to separate from the insula and OFC (y = 9 inthe left and y – 8 in the right hemisphere) is isolated and removed (indicated by the white dot-ted line). Based on anatomical descriptions in non-human primates (Baylis et al. 1995; Rolls et al. 1990), the proposed gustatory region is at the transition between the ventral agranu-lar insular cortex and the caudal OFC at its lateral point. The two peaks in the small white boxfall within a homologous region of the human brain. These peaks are also show in the coronalplane to illustrate their medial-lateral location. (b) Coronal sections illustrating the transitionfrom agranular insular cortex (at the level where temporal, insular and orbital areas are continu-ous) to the dysgranular cortex in the caudolateral orbitofrontal region. The numbers representthe y-value at which each section is taken. The white arrows point to agranualr cortex in sec-tions 11–17, dysgranular cortex in sections 19–25, and the disappearance of this cortical regionin section 27. The cortical area indicated by the arrows in 19–25 represents an analogousregion to the “secondary gustatory area” described in monkeys.

Source: Reproduced from (Small et al. 1999). See Plate section inthe colour gallery.”


Response characteristics of the orbital taste region

Stimulus identity and specificity

Rolls and colleagues have used single-cell recording techniques to characterize theresponsiveness of taste neurons in the macaque caudolateral OFC region (Rolls andBaylis 1994; Rolls et al. 1989; Rolls et al. 1990). In one study, they isolated 49 gustatoryresponsive neurons in this region. Eighty percent of these neurons responded to just asingle taste, 12% responded to two of the four basic tastes, and only 4% responded tothree or more of the four basic tastes (Rolls et al. 1990). This response profile differs considerably from the profile elicited in the caudomedial orbital area and the insula/opercular gustatory areas, where each neuron responded to more than one of the proto-typical stimuli (Pritchard et al. 2005; Scott et al. 1986a; Yaxley et al. 1988; Yaxley et al.1990). In the gustatory literature, the specificity of individual neural responses to chemi-cal compounds is often described using a breadth of tuning metric developed by Smithand Travers (Smith and Travers 1979). This coefficient can range from 0.0, representing acomplete specificity to one of the stimuli, to 1.0, which indicates an equal response to alltastants. In the primate, peripheral taste nerves have a breadth of tuning coefficient of.54, indicative of a fairly high specificity (Hellekant et al. 1997). Specificity is then lost in


Fig. 6.10 Breadth-of-tuningindex for taste neurons in thebrainstem, insula, operculumand OFC. Breadth-of-tuningindex for taste responsive neurons recorded in thenucleus of the solitary tract(NST), operculum, insula andorbitofrontal cortex (OFC) inthe primate. This coefficientcan range from 0.0, repre-senting a complete specificityto one of the stimuli, to 1.0,which indicates an equalresponse to all tastants (Smithand Travers, 1979). Means,standard error and the num-ber of neurons in the samplefor each area are shown.

Source: Adapted with permis-sion from (Rolls et al. 1990).


the nucleus of the solitary tract (.87) (Scott et al. 1986b) and gradually recovered in the progression from the thalamus (VPM) � .73 (Pritchard et al. 1989) to the frontal operculum � .67 (Scott et al. 1986a) and the insula � .56 (Smith-Swintosky et al. 1991).In the caudomedial OFC the breadth of tuning is. 79, whereas in the caudolateral OFCthe breadth of tuning is. 39, indicating the highest level of response specificity in the gustatory neuroaxis (Fig. 6.10).

This finding resembles the results from the olfactory system, indicating that high stim-ulus specificity is a consistent trait across both smell and taste modalities. However, aswith the olfactory modality, questions remain as to what extent the OFC processingreflects a necessary component in stimulus quality decoding, vs. the OFC being the recip-ient of the results of early processing, or participating in a broader ensemble involved inthe process. Recent animal studies have highlighted the importance of patterns of cellensemble activity in taste quality decoding. (Smith and Scott 2003). For example, Katzand colleagues (Katz et al. 2002) used multiunit recordings in rodents to isolate transientquality-specific cross-correlations in ensembles of taste neurons within the insular tasteregion. This ensemble activity in the insula may act to provide the information necessaryto allow stimulus specific representations to emerge at the level of single cells in the OFC.

Hedonic value: Initial neurophysiological studies of the caudolateral OFC taste regionwere notable in the relative paucity of neurons responding to aversive stimuli (quininehydrochloride and hydrochloride) (Rolls et al. 1990), which led to speculation that theremight be anatomical segregation between areas responsive to pleasant and unpleasanttaste stimuli. Consistent with this possibility, neuroimaging studies in humans frequentlyreport valence-specific gustatory responses in the OFC (O’Doherty, Rolls et al. 2001;O’Doherty et al. 2002; Small et al. 2003a; Zald et al. 2002; Zald et al. 1998). However,although segregated responses are consistently reported within studies, there is a greatdeal of variability across studies. Figure 6.11 shows the location of all activation foci inthe OFC reported in analyses designed to isolate the response to either pleasant orunpleasant taste in eight analyses from five published studies using pure gustatory stim-uli (O’Doherty, Rolls et al. 2001; O’Doherty et al. 2002; Small et al. 2003b; Zald et al.2002; Zald et al. 1998). The only consistent finding is that the pleasant taste appears topreferentially activate the right OFC. This finding is inconsistent with the conclusions ofKringelbach and Rolls (2004), who argued against the presence of hemispheric lateraliza-tion related to valence (based on their meta-analysis of a wide range studies related topositive and negative stimulus exposures in multiple modalities). The present data alsorun against a strict model of medial-positive/lateral-negative segregation of responses(Small et al. 2001; Kringelbach and Rolls 2004). As already described, analysis of hedonicolfactory data provide partial support for this medial-lateral model of segregation, butdo not indicate a complete segregation of responses. These discrepancies lead us to spec-ulate that some of the unique specializations for chemosensory processing in the OFCoverride or obscure the larger trends of hedonic segregation suggested by the Kringelbachand Rolls analysis. Our speculation is further supported by the observation that taste



neurons are equally likely to fire to any one of the taste qualities in the caudomedial tasteregion, whereas there is clearly a dominant response to sweet taste in the caudolateralOFC (Pritchard et al. 2005).

In considering the apparent right lateralization of responses to pleasant tastes, it isuseful to consider that affective value and quality are frequently confounded in tastestudies. Sweet generally represents the pleasant taste, and bitter or intensely salty theunpleasant taste. Thus, an alternative interpretation of hedonic lateralization is that thequality of sweetness, or its nutritive value, is preferentially coded in the right OFC. Ingeneral, the link between specific taste quality, nutritive value and hedonic value is quitestable and can be seen even in infancy (Steiner et al. 2001). This has led some to arguethat the perception of specific tastes is so integrally affective in nature that taste in


Fig. 6.11 Lateralization of responses to pleasant and unpleasant tastes. Top: 25 activation focifrom 8 analyses of pleasant taste (either pleasant taste > a neutral taste or a pleasant taste >an unpleasant taste) and 8 analyses of a unpleasant taste (either unpleasant taste > a neutraltaste or unpleasant taste > a pleasant taste) collected from 5 published studies (O'Doherty et al.2001b; O'Doherty et al. 2002; Small et al. 2003; Zald et al. 2002; Zald et al. 1998) and plottedonto an anatomical image. L = left and R = right. To display peaks in a single plane the averagelevel of z (superior to inferior = –14 in MNI coordinates) was calculated and the peaks wereplotted onto this plane according to their x (medial to lateral) and y (anterior to posterior) coor-dinate values. Peaks from the pleasant analyses are in red and peaks from the unpleasant analy-ses are in green. Bottom: A bar graph showing the number of peaks (y axis) that fell in the leftor right OFC for the pleasant and unpleasant analyses. See Plate section in the colour gallery.”


isolation can be treated as a primary reinforcer. However, the affective value of tastes isnot entirely immutable (Hendricks et al. 2004; Hill et al. 1986; Hill and Przekop 1988;Krimm and Hill 1999; Moskowitz et al. 1975; Pittman and Contreras 2002; Shuler et al.2004) indicating that taste quality and hedonic value can at least partially be dissociated.

Irrespective of whether responses are anatomically segregated according to valence,quality or physiological significance, it appears that OFC responses in humans are relatively independent of stimulus intensity (Small et al. 2003a). Small and colleaguesgave subjects a weak and a strong sweet solution and a weak and a strong bitter solutionwhile recording brain response with fMRI. Importantly, the two sweet solutions wererated as similarly pleasant, the two bitter solutions as similarly unpleasant, the weak solutions as equally weak, and the strong solutions as equally strong. Thus intensity andvalence were effectively dissociated. To isolate the regions sensitive to intensity and notvalence, a conjunction analysis was performed to localize the regions activating to bothstrong intensity solutions compared to the two weak solutions. Activation was observedin the insula/operculum, amygdala, cerebellum and brainstem, but not in the OFC.In contrast, preferential activity in the right caudolateral OFC was observed to the pleasant sweet vs. the unpleasant bitter stimuli, irrespective of the intensity, and a regionof the left anterior medial OFC was responsive to the unpleasant bitter taste irrespective


Fig. 6.12 Dissociation of taste intensity and affective coding. Results from three random effectsanalyses color-coded and superimposed upon an axial section from the averaged anatomicalimage. Green denotes activity related to taste intensity from the analysis {(intense pleasant +intense unpleasant) > (weak pleasant + weak unpleasant)}; pink denotes activity related totaste pleasantness from the analysis {(intense pleasant + weak pleasant > tasteless}; and bluedenotes activity related to taste unpleasantness from the analysis {(intense unpleasant + weakunpleasant) > tasteless}. Note the valence-specific responses and lack of intensity response inthe orbitofrontal cortex. In contrast, activity related to intensity but not valence is seen in theamygdala. The graphs depict the fitted hemodynamic response function from the unpleasant(left = blue box) and the pleasant (right = pink box) responses in the orbitofrontal cortex. Theblue lines represent the unpleasant taste (quinine), the pink lines represent the pleasant taste(sucrose) and the beige line indicates tasteless. Dotted lines represent weak solutions and solidlines indicate strong solutions.

Source: Adapted from (Small et al. 2003).”See Plate section in the colour gallery.”


of intensity (Fig. 6.12). The observation, that the human OFC does not show a generaleffect of intensity, is consistent with the single cell literature in monkeys in that the narrow breadth of tuning and suggestions of regional specializations would make itunlikely that single cells in the OFC would have the ability to code for intensity of multi-ple tastes. Thus, to the extent that single cells in the OFC show concentration responsefunctions that track intensity (Rolls et al. 1990), such responses are likely to be stimulus-specific and indeed may reflect changes in pleasantness that occur with increasing concentrations (Engel 1928). The lack of a general effect of intensity in the OFC contrasts with the response in the amygdala, which appears to reflect a complex functionof taste intensity, valence and familiarity (Small et al. 2003a; Small et al. 1997a; Smallet al. 2001a; Small et al. 2001b; Zald 2003).

Anticipation of taste

Studies in animals on expectancy of reward have typically used tastes as the expectedreinforcer, and demonstrated neurons in the OFC that respond during the anticipationof tastes (see Roesch and Schoenbaum, this volume). In humans, O’Doherty, Rolls et al.(2001) measured BOLD responses while subjects were presented with one of three arbi-trary visual stimuli, which reliably predicted the subsequent delivery of either a pleasantsweet taste, an unpleasant salty taste or a neutral tasteless solution (O’Doherty et al.2002). The right OFC was not only activated more in response to the receipt of a pleasanttaste (compared to either the neutral tasteless solution or the aversive saline solution) butalso to the anticipation of that pleasant taste, compared to either the anticipation of aneutral tasteless solution or to the anticipation of an unpleasant saline solution(O’Doherty et al. 2002). Similarly, anticipation of the unpleasant salty solution activatedthe left lateral OFC compared to anticipation of the neutral tasteless solution. Both antic-ipatory and consummatory responses to taste were attenuated over the course of thestudy, suggesting that processes related to either habituation or satiation influence bothanticipatory and consummatory responses in the OFC (see Part IV below for a moredetailed discussion of satiety issues).

Whether or not a taste or flavor is anticipated may also alter the OFC response tofood reward. As outlined by Schultz (Chapter x) and Montague and Berns (Montagueand Berns 2002), the predictability of a rewarding stimulus is a critical parameter foractivation of reward pathways, and OFC-striatal circuits are thought to be critical forongoing tabulation of reward prediction (Berns et al. 2001; McClure et al. 2003;Montague and Berns 2002; O’Doherty et al. 2003; Schultz et al. 2000). Data from Bernsand colleagues (Berns et al. 2001) indicate that the predictability of a sapid stimulusinfluences the location of the OFC responses. Preferential activation of the medialOFC occurred to the unpredictable arrival of fruit juice, whereas preferential activa-tion of the lateral OFC emerged in response to predictable arrival of the juice.Preliminary findings from one of our laboratories (DMS) suggest that this dissociationalso generalizes to pure tastes.



6.4. Chemosensory integration and the assembly of flavorWe rarely experience smells or tastes in isolation. For example, taste sensations are nor-mally experienced in the context of eating or drinking when they are combined withother sensory experiences into a unitary flavor percept (Small and Prescott 2005;Delwiche 2004). What is colloquially referred to as “taste” is therefore the combination ofa variety of physiologically distinct sensory experiences associated with the flavor offood, such as its smell, texture and temperature. Flavor perception may also be influencedby the sight of a food (Delwiche 2004; Morrot et al. 2001; Zellner and Durlach 2002).

PrimatesIn nonhuman primates, neural representations of flavor in the caudal OFC are not

limited to taste and smell information, but also encode many of the other sensory con-stituents that together comprise flavor (Carmichael et al. 1994; Carmichael and Price1996; Morecraft et al. 1992; Rolls and Baylis 1994; Rolls, Verhagen and Kadohisa 2003).Dispersed throughout this region of the OFC are cells that respond to the sight (Rollsand Baylis 1994), texture (de Araujo et al. 2003; Rolls et al. 1999) and temperature(Kadohisa et al. 2004) of food. Moreover, cells that respond to various combinations ofthese sensory experiences are also found in the OFC (Rolls and Baylis 1994; Kadohisaet al. 2004; Rolls, Verhagen and Kodihisa 2003; Verhagen et al. 2003). In a series of studiesRolls and colleagues examined the response of single cells to multiple sensory inputs(Kadohisa et al. 2004; Rolls and Baylis 1994; Rolls, Verhagen and Kodihisa. 2003; Thorpeet al. 1983; Verhagen et al. 2003). In one study 2000 cells were sampled for their respon-siveness to taste, smell, flavor and visual stimuli (Rolls and Baylis 1994). One hundredand fifty-eight neurons responded to stimulation. Forty-five of these responded in morethan one modality. Although unimodal cells were concentrated in three different areas,considerable overlap occurred, and bimodal and trimodal cells were interspersedthroughout the three areas. Similarly, neurons that respond to different combinations ofviscosity, taste, fat, astringency and temperature have also been described (Kadohisa et al.2004; Rolls, Verhagen and Kadohisa 2003; Verhagen et al. 2003).

One striking feature of many of the multimodal neurons in the primate OFC is theirselective responses for appropriately paired (i.e., congruent) stimuli. For example, a cellresponding optimally to a glucose solution responded optimally to fruit-related odorsrather than to salmon or other odors, that were less appropriate for a sweet tasting solu-tion (Rolls 2001; Rolls and Baylis 1994). Similarly, a bimodal taste/visual cell respondingto glucose also responded to the sight of sweet foods, such as a banana or a syringe, thathad been used to deliver blackcurrant juice, but not to the sight of pliers, which hadnever been associated with a sweet taste. One taste/smell cell responded to neither thesmell nor the taste of any of the palatable food items. Instead, this neuron fired maxi-mally in response to the aversive stimuli of quinine and caprylic acid, suggesting a disso-ciation between neurons that respond to either hedonically positive or negative stimuli(or if considered in terms of ingestive processing, nutritive versus toxic stimuli).



HumansSimilar mechanisms of chemosensory integration have been identified in the humanOFC. Examination of Figures 3, 9 and 11 clearly shows that stimulation with either tastesor smells can produce activation in the overlapping regions of the human OFC. Whilesuch findings do not confirm whether these areas specifically integrate information acrossdifferent modalities, other fMRI studies have addressed this question more directly. Byexamining the neural interaction between smells and tastes (de Araujo et al. 2003; Smallet al. 2004), or between smells and pictures (Gottfried and Dolan 2003), response supra-additivity has been demonstrated in the human OFC. Cross-modal response enhance-ment of this type may reflect the hemodynamic analog of multisensory integration at thecellular level, as originally demonstrated in cat superior collicular neurons exposed tovisual, auditory, and tactile stimuli (Stein and Meredith 1993). Recently, the phenomenonhas been extended to human imaging studies of audio-visual integration (Calvert et al.2000). The present results suggest that supra-additive response patterns are applicable tochemosensory modalities, and may provide a powerful approach to investigating theassembly of flavor representations (and food identity) in the human brain.

As with sensory integration in the primate OFC, the response of the human OFC tomultiple chemosensory inputs is affected by prior experience. In the study by Gottfriedand Dolan (2003), greater activity was observed in the anterior medial OFC to semanti-cally congruent compared to incongruent cross-modal stimuli (e.g., a picture of a double-decker bus and the odor of diesel, versus the picture of cheese and the smell offish), and subjects detected the odor significantly more quickly when it occurred in conjunction with a congruent compared to an incongruent picture. Small and colleaguesidentified a response in the caudal OFC that was significantly greater to a sweet vanilla(congruent) versus a salty vanilla (incongruent) solution. Similarly, de Araujo and colleagues (2003) noted a significant correlation between subjects’ ratings of taste-odorcongruency and activation in the anterior medial OFC. The marked dependence on congruency that characterizes the OFC response indicates that chemosensory integrationin the OFC is heavily influenced by prior experience with both food and non-food stimuli.

A factor unique to taste-odor integration is the route of odorant administration.Ordinarily when odors are perceived with taste, they reach the olfactory epithelium via theretronasal route (in which vapors in the mouth rise through the nasopharynx). In contrast,when subjects sniff, odorants reach the epithelium via the nostrils (orthonasally). Althoughin both cases odorants reach the epithelium, increasing evidence suggests this has a signifi-cant impact on how odorants and tastes are integrated (Burdach et al. 1984; Halpern andSun 2002; Heilmann and Hummel 2001; Heilmann et al. 2002; Small et al. 2004) [but see(Sakai et al. 2001; Pierce and Halpern 1996)]. In the first neuroimaging study to directlyexamine the integration of olfactory and gustatory information, Small et al. (1997a)demonstrated decreased activity in the caudal OFC when an orthonasal smell was presented simultaneous with a taste, relative to when the smell and taste were presented in



isolation. Perhaps in this situation, the orthonasal smell and the taste compete for OFCprocessing. While the specific mechanisms leading to such differences remain to be eluci-dated, the data strongly suggest that the neural integration of taste and smell into a unitaryflavor percept is dependent either upon the two stimuli being perceived as coming from acommon source (i.e., a drink) versus two separate sources (i.e., a cotton wand and filterpaper), or upon the route of olfactory administration (orthonasal versus retronasal).

It is worth noting in this regard that ortho vs. retronasal olfaction differentially map tothe anticipatory vs. consumatory phases of feeding. Berridge (1996) has described theaffective states related to these two processes as “wanting” and “liking”, respectively, andhas proposed that separable neural substrates support each component. In the sensoryrealm, this dichotomy reflects distal vs. contact stimulation. Taste, temperature, textureand trigeminal irritation (as encountered in spicy food) are all contact sensations, signi-fying the receipt of a food reward and are associated with liking. In contrast, the sight oraroma of a food is a distal sensory experience that signifies the availability of a food andare associated with wanting. The olfactory system is unique in that it reflects both distalsensation (orthonasal) and contact sensation (retronasal). This observation led Rozin(1982) to propose that “olfaction is the only dual sensory modality, in that it senses bothobjects in the external world and objects in the body (mouth).”

To study this difference Small et al. (in press) placed two tubes in the nasal cavity usingendoscopic guidance, allowing them to deliver a pulse of vaporized odorant via eitherroute while avoiding simultaneous engagement of oral sensory experiences normallyassociated with retronasal olfaction. Comparison of brain activity evoked to the samefood odor, sensed orthonasally versus retronasally, revealed two different patterns ofOFC activity. Activity in the medial OFC and the subcallosal region responded preferen-tially during retronasal perception, whereas activity in the lateral OFC responded prefer-entially during orthonasal perception (see Fig. 6.13). A similar dissociation was notobserved when comparing orthonasal versus retronasal presentation of three equallypleasant and intense non-food odors (Small et al. under review), suggesting that the con-text in which a sensory stimulus is normally experienced can markedly alter the neuralresponse it evokes. The emergence of this segregation for food odors is remarkable inthat it occurs even in the absence of any current oral sensation.

If the medial response to retronasal olfaction, and the more lateral response toorthonasal olfaction are taken as indices of anticipatory vs. consummatory responses,then these data may be interpreted as suggesting a medial vs. lateral distinction betweenwanting and liking systems within the OFC. While some additional data support thisdistinction (Kringelbach et al. 2003b; Small, Zatorre, Dagher et al. 2001), not all data areconsistent with such a model. Nevertheless, it is clear from work in rodents(Schoenbaum et al. 2003), primates (Schultz et al. 1998; Schultz et al. 2000; Tremblayand Schultz 1999; Tremblay and Schultz 2000), and humans (Berns et al. 2001;O’Doherty et al. 2003; O’Doherty et al. 2002) that the OFC plays a role in both the prediction and anticipation of food reward, making it a key structure for both wantingand liking and their interaction.



6.5. Regulation of feeding behaviorGiven the importance of the OFC in representing reward and flavor, it is useful to con-sider the OFC representation of taste within the context of a more general model of foodreward. For a more in-depth consideration of these topics the reader is referred to (DelParigi, Gautier et al. 2002; Kringelbach and Rolls 2004; Saper et al. 2002).

Relative reward and food selectionThe OFC appears critical in coding the relative reward value of different foods (Aranaet al. 2003; Hinton et al. 2004; Kilgore et al. 2003; Tremblay and Schultz 1999). For


(a)

(b)

Fig. 6.13 Dissociation of orbital regions responding preferentially to orthonasal versus retronasalperception of an appetitive food odor (chocolate). Left: Axial section showing preferentialresponse in the medial orbitofrontal cortex (OFC) to retronasally perceived chocolate odor (CR)compared to orthonasally perceived chocolate odor (CO) as well as to ortho- or retronasal per-ception of three similarly pleasant and intense nonfood odors (LR = lavender retro; BR =butanol retro; FR = farnesol retro; LO = lavender ortho; BO = butanol ortho; FO = farnesolortho). Right: Axial section showing preferential response in the lateral OFC to orthonasal per-ception of chocolate odor versus all other odorants. T-map thresholded at p < 0.001 and acluster threshold of k < 3. Graphs show response (parameter estimates) from each of the eightodorant conditions minus each associated baseline condition at the co-ordinate defined on they-axis. Response is in arbitrary units. Numbers below or above lines joining the bars indicate z-values associated with direct contrasts of CO with each of the 7 other odorant conditions (CR = chocolate retro; LR = lavender retro; BR = butanol retro; FR = farnesol retro; LO = laven-der ortho; BO = butanol ortho; FO = farnesol ortho). * indicates that the z-value is only signifi-cant uncorrected across the small volume of interest. n/s = nonsignificant.


example, in monkeys Tremblay and Schultz have shown that the same OFC neuron willrespond to a preferred apple over a less preferred fruit loop, but when an apple is pittedagainst a more preferred raisin, the neuron switches its responsiveness to the raisin(Tremblay and Schultz 1999). In humans, Arana and colleagues had subjects imaginethat they were in a restaurant looking at different menus while measuring brain activitywith PET (Arana et al. 2003). All of the menus were individually tailored according to thesubject’s preference. In some trials subjects simply looked at the menu while in othersthey were required to select items. They found that the rostral gyrus rectus showed agreater response to high-incentive menus and responded more when a choice wasrequired. In contrast, the lateral OFC responded more when subjects had to suppressresponses to alternative desirable items to select their most preferred. The implication ofthis result is that the OFC not only encodes the relative reward value of the menus butalso contributes to food selection. Studies in both monkeys and humans with OFCimpairment have been observed to show alterations in food selection (Baylis and Gaffan1991; Ikeda et al. 2002). However, it is unclear if these changes reflect actual disruption ofthe evaluation of the relative value of foods, or is simply a consequence of perceptualdeficits or impairments in associative learning. Recent data indicate that monkeys withOFC lesions show stable food preferences in selecting among palatable foods that theyare familiar with (Iqzquierdo, Suda and Murray 2004), indicating that at a minimum,some form of preferential food selection is maintained, even though such animals maylose the ability to adaptively use information about food preferences to guide behavioralchoices, especially when the value of one of the foods changes (Baxter et al. 2000).

Internal stateInternal state markedly influences the affective value of chemosensory stimuli (Berridge1991; Cabanac 1971; Rolls et al. 1983). Specifically, ongoing or recent consumption of afood leads to a temporary reduction in its reward value. Two different processes maycome in to play in this context. First, a general process of alliesthesia may develop inwhich fullness leads to a general reduction in the pleasantness of food (Cabanac 1971).Second, a more narrow satiety for specific foods, but not to other foods, develops duringthe consumption of a given food. This latter phenomenon has been termed sensory-specific satiety (Rolls et al. 1983), because factors, such as similarity of color, shape, flavorand texture, appear more important than nutritive content in directing this reduction inaffective value. There is compelling evidence that the OFC plays a critical role in thisprocess. At the neurophysiological level, unimodal gustatory and olfactory responses tospecific foods are greatly attenuated by satiety (Critchley and Rolls 1996; Nakano et al.1984; Rolls et al. 1989), while leaving responses to other foods intact. Sensory-specificsatiety-related declines have also been seen for visual responses to food items, confirmingthe multimodal nature of the phenomenon (Critchley and Rolls 1996; Nakano et al.1984; Rolls et al. 1989). In contrast to the OFC, single cell recordings of the insular-operculum primary taste region do not show evidence of a sensory-specific satiety effect,which led Rolls and colleagues to conclude that this process first emerges at the level of



the OFC. Data from humans suggest that insular responses to foods do actually decreasewith repetition (Small, Zatorre, Dagher et al. 2001; Gautier 2001), but this may reflect themore general process of alliesthesia, rather than the sensory-specific satiety.

Substantial evidence indicates that the human OFC also changes its response during thesatiation of specific foods. Two studies have reported on the brain response to flavoredfood or drink as subjects ate or drank that substance to satiety or beyond. Responses in themedial OFC to palatable food or drink decreased as that substance was eaten to satiety(and beyond), and these decreases correlated with declining pleasantness ratings(Kringelbach et al. 2003a; Small, Zatorre, Dagher et al. 2001). In contrast, activity in thelateral OFC, extending into the ventrolateral prefrontal cortex, increased with satiety, andcorrelated inversely with pleasantness ratings (Small, Zatorre, Dagher et al. 2001). Thesedata thus suggest that there are both decreases and increases in activity with satiety, andthat a medial/lateral segregation distinguishes these responses. Although no publishedstudies have examined satiety to pure gustatory stimuli in the olfactory domain inhumans, O’Doherty et al. (2000) scanned participants as they smelled either banana orvanilla odor, both before and after eating bananas to satiety. Post-satiety odor-evoked acti-vations to the banana smell decreased in the posterior OFC (right side, 4 subjects; left side,1 subject), without any corresponding OFC signal decline in response to the vanilla smell.O’Doherty et al. did not observe specific areas that became more active with satiety.However, several additional studies examining brain activity in states of hunger and sati-ety confirm that satiety is accompanied by both areas of increased and decreased activity.

Tataranni and colleagues used PET to measure resting brain activity after a 36-hourfast (condition 1: hungry), followed by a 2ml sample of a liquid meal (condition 2: taste)and then a satiating amount of a liquid meal (conditioned 3: satiety) [(Del Parigi, Chenet al. 2002; Gautier et al. 1999; Gautier et al. 2001; Tataranni et al. 1999). Activity in thecaudal OFC was greater during hunger compared to satiety, while activity in the dorsaland ventral prefrontal cortex, extending into the anterior-lateral most part of the OFC,was greater in satiety compared to hunger. The authors have proposed that the prefrontalcortex signals satiation by inhibiting limbic/paralimibic areas. Interestingly, changes inboth caudal and more lateral areas appear amplified in obese subjects (Del Parigi,Gautier et al. 2002; Gautier et al. 2001; Tataranni and Del Parigi 2003).

In order to follow the time course of changes following eating, Lui et al. ( 2000) usedfMRI to assess the change in activity within a single medially placed sagittal slice follow-ing ingestion of 75 g glucose solution. Between 1- and 3-minute post ingestion, activityin the medial OFC extending onto the medial surface began to decrease, with furtherdecreases occurring between 7- and 13-minute post ingestion. The decreasing activitywas located in the same region as the decreasing activity Small and colleagues reported inresponse to eating chocolate to beyond satiety (Small, Zatorre, Dagher et al. 2001) andalso overlapped with the region reported by Tataranni and colleagues as less active in fullversus hungry states (Tataranni et al. 1999).

Additional support for the hypothesis that more anterior-lateral areas play aninhibitory role comes from the menu selection study by Arana and colleagues described



above (Arana et al. 2003). Whereas caudo-medial OFC activity was observed when sub-jects viewed preferred vs nonpreferred menus, lateral OFC activity was observed whenparticipants had to suppress responses to alternative desirable items in order to selecttheir most preferred item (Arana et al. 2003). A follow-up study examined the interac-tion between what the authors termed intrinsic and extrinsic determinants of foodreward (Hinton et al. 2004). Intrinsic factors were defined as contributions by homeosta-tic mechanisms (e.g., hunger, satiety), and extrinsic factors as contributions by variablesrelated to the food, such as food preferences and the associated experience-based incen-tive properties of food. The follow-up study was identical to the earlier experiment,except that subjects performed the task while hungry and while full. Consequently, theywere able to evaluate the effect of intrinsic factors (internal state) upon extrinsic factorsevaluated by their task. In addition to replicating their initial findings, and in accordancewith previous studies comparing hunger and satiety, they found that the lateral OFC wasmore responsive when viewing and choosing items from a menu when full compared tohungry. More importantly, they found that intrinsic factors (i.e., hunger vs. satiety) had afar greater effect upon neural response than extrinsic factors (imagining high versus lowincentive foods). The only region of the brain in which an interaction was observedbetween intrinsic and extrinsic factors was in a region of the anterior orbital gyrus,which was preferentially activated by viewing and choosing food from high versus lowincentive menus when hungry compared to when full (Fig. 6.14).

To summarize, the findings from studies of lean subjects eating food (Kringelbachet al. 2003b; Liu et al. 2000; Small, Zatorre, Dagher et al. 2001), at rest in the state ofhunger or satiety (Del Parigi, Chen et al. 2002; Gautier et al. 1999; Tataranni and DelParigi 2003; Tataranni et al. 1999), and imagining and selecting food items while hungryand full (Arana et al. 2003; Hinton et al. 2004), all point to a role for the medial OFC inguiding feeding behavior while hungry, which declines with fullness, while the anterior-lateral OFC becomes increasingly engaged when individuals are full. These data suggestthat the OFC plays both a general regulatory role, that is sensitive to homeostatic factors(alliesthesia), as well as a sensory-specific role in regulating feeding behavior. The possi-bility that the lateral OFC extending into the ventrolateral prefrontal cortex (areas 47/12)plays an inhibitory role in feeding (Del Parigi, Gautier et al. 2002) is consistent with themore general models of lateral OFC/ventrolateral PFC functions that emphasize itsengagement when a selected action requires the suppression of previously rewardedresponses (Elliott et al. 2000). Additionally, the findings of Hinton and colleagues suggestthat the OFC processing in the anterior orbital gyrus is important in integrating bothintrinsic and extrinsic factors to food reward.

The general differentiation between anterior/lateral and medial OFC in response tosatiety is interesting, in that it appears to at least partially resemble the general medial-pleasant/lateral-unpleasant trends observed for valence (Kringelbach and Rolls 2004).Although we have argued that this distinction does not provide a full explanation ofresponse distributions in the chemosensory domain, the general trend has been observedin many paradigms and modalities, ranging from cross-modal paradigms of episodic



memory (Gottfried et al. 2004) and associative learning (Gottfried, O’Doherty et al.2002) to the processing of faces (O’Doherty et al. 2003) and abstract monetary rein-forcers (O’Doherty, Kringelbach et al. 2001). That stimuli in the hungry state (when theyhave the highest appetitive value), engage more medial areas, while stimuli in the full orsatiated state (when they have lower appetitive value) engage more lateral areas, appearsconsistent with this pattern, though in the satiated state, the food stimuli are often lesspleasant rather than specifically unpleasant.

Finally, it is worth considering how the aforementioned results may contribute to ourunderstanding of eating disorders. Food reward is multifaceted with evidence for separa-ble circuits representing food wanting, food liking, and signals of internal state such ashunger and fullness (Berridge 1996). A remarkable feature of the OFC is that it appearsto encode all aspects of food reward. This contrasts with the representation of foodreward in the amygdala, which appears highly specific to incentive salience encoding(Gottfried et al. 2003; O’Doherty et al. 2002; Small et al. 2005). One implication of thisconfluence of function in the OFC is that an integrated output computed here represents


(a)

(b)

Fig. 6.14 Interaction of incentive value and hunger state in the anterior OFC. The interactionbetween incentive value and hunger state. (A) Sagittal and coronal sections depicting a distinctarea of the left OFC, which showed increased activation in the interaction between hunger stateand incentive value. (B) The pattern of interaction between hunger state and incentive value isshown by this condition-specific plot: the conditions were Hunger/High Incentive, Hunger/LowIncentive, Satiety/High Incentive and Satiety/Low Incentive.

Source: Adapted with permission from (Hinton et al. 2004).


a final common pathway for food reward supporting food selection. Although dysfunc-tion in any one aspect of food reward (e.g., enhanced palatability vs. increased hunger)could lead to aberrant eating behaviors, if the OFC represents a final common pathway,abnormal responses in the OFC are likely to be associated with multiple forms of eatingdisorders. Further, it is possible that the precise location of such responses may be associ-ated with either overeating or undereating. There is indirect support in the literature forsuch a hypothesis. For example, while Uher et al (2004) reported that an abnormal ven-tral medial prefrontal response to symptom-provoking stimuli was a common feature ofboth anorexia and bulimia nervosa the findings of Del Parigi and Tataranni pointtowards abnormal ventrolateral responses in obese subjects (Del Parigi, Chen et al. 2002).

6.6. ConclusionIn their seminal paper on the connections of the OFC, Carmichael and Price wrote:

At first consideration, there appears to be a discrepancy between the suggestion from the lesionresults that the orbital cortex has a behavioral role in stimulus-reward association and the indica-tion from the connectional data that there is a sensory hierarchy related to feeding. On furtherconsideration, however, such apparent discrepancy may be an artifact of experimental methodol-ogy. In fact, the hierarchical processing of chemical and visceral sensations and the formation ofreward may be the same thing. (Carmichael and Price 1996, p. 205).

Although, we would not go so far as to say that the hierarchical processing of chemical vis-ceral sensations is synonymous with the formation of reward, the data reviewed in thischapter make clear that the processing of chemosensory stimuli, the formation of reward,and the regulation of feeding are intimately interwined in the OFC. Human feeding behav-ior is extremely complex, and includes both anticipatory (wanting) and consummatory(liking) factors. Numerous processes must be accomplished just to get to the point of eatinga food. For instance, the acquisition and selection of items for ingestion requires an abilityto identify and associate odors and visual objects with their taste or flavor properties, tolearn how to properly acquire the items and/or make them palatable, to select one potentialfood over another, to learn when one food item is no longer good, to learn when it is nolonger safe or advantageous to acquire a food, and to gauge the value of specific foods andfoods in general, given recent experience and current internal state. While obviously not allof these processes are accomplished by a single brain region, the specificity of its olfactoryand gustatory processing, its ability to integrate these representations with visual sensationsand other oral sensations, its coding of changes in stimulus-food reward contingencies andthe modification of responses by hunger and satiety, ideally position the OFC to implementexecutive-level control in the regulation of feeding.

Although much of this book focuses on the OFC’s role in non-chemosensory behavior,the fundamental processing properties that allow the OFC to flexibly influence foodacquisition and consumption may apply to the OFC’s involvement in other cognitive andbehavioral domains. We have characterized several of these properties, such as high stimulus specificity, separable responses to positive and negatively valenced stimuli,



engagement during judgements about hedonics, multisensory integration, sensitivity toprior experience with stimulus combinations, and differential coding based on internalstate. To the extent that these properties generalize to other behavioral domains, theymay critically influence how the OFC differentially contributes to neurobehavioral functions relative to other brain regions, and may causally direct the phenomenologicalfeatures of neuropsychiatric symptoms associated with OFC dysfunction.

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