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The highlight for December 2009 is by Andrew R. Delamater who is in the Department of Psychology at Brooklyn College, CUNY. In the interests of full disclosure, I have to say up front that my association with Dr. Delamater goes way back and is such that none of my comments can be viewed as impartial. He began as an undergraduate student in my laboratory (many years ago) and despite this training has become an accomplished learning theorist examining the nature of associative learning. Following a several years’ stint with me, Dr. Delamater went on to work with both Vin Lolordo (Dalhousie) and Bob Rescorla (University of Pennsylvania), and his research efforts and interests clearly reflect this latter training. Specifically, throughout his work, he has applied the basics principles of taste aversion learning to the issue of the nature of learning, more specifically that of sensory-specific flavor nutrient conditioning in flavor preference learning. To date, a number of investigators have used the phenomenon of conditioned taste aversion learning as a tool in the examination of a host of issues, from drug abuse to memorial processing to the neural basis of learning. Dr. Delamater’s approach is unique among all of these applications in that he and his colleagues have argued (and demonstrated convincingly) that the aversion design is actually instrumental in assessing some of the possibly unique characteristics (and the underlying nature) of flavor preference learning. With the taste aversion procedure, he has been able to demonstrate that flavor preference learning reflects a sensory-specific association. These demonstrations utilized classic associative principles, e.g., discriminative conditioning, extinction, latent inhibition, partial reinforcement and reversal learning, as well as taste aversion conditioning (ala US devaluation procedures). Each of these demonstrations was methodologically well thought out and thorough and allowed for a determination of the basic factor responsible for flavor preference conditioning (sensory-specific associations). Dr. Delamater’s most recent work attempting to isolate the neural mechanisms mediating such conditioned effects promises to provide further exciting information regarding the possible uniqueness of such conditioning in relation to other forms of associative control. Dr. Delamater’s summary of his research efforts in taste aversion learning highlight an interesting approach characterized by a sound understanding and application of a myriad of experimental issues and designs.

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Conditioned Taste Aversion Learning as a Tool to Study Sensory-Specific Associations in Pavlovian Conditioning

Andrew R. Delamater, PhD. Brooklyn College - CUNY

Biographical Sketch As an undergraduate student at American University in the early 1980s, I was fortunate to become affiliated with Tony Riley. While working in Tony’s psychopharmacology laboratory I was exposed to a variety of interesting scientific research topics and experimental techniques. Two of these that especially took my fancy were learning theory and conditioned taste aversion learning. Tony had been trained by major learning theorists of the day, Vin LoLordo and Bob Bolles, and he conveyed an excitement about the investigation of Pavlovian conditioning and demonstrated to me how this interesting learning process might be studied using the conditioned taste aversion paradigm. In addition, Tony’s enthusiasm for all things intellectual played a critically important role in my deciding to become an academic, an inspiration that has gone way beyond my keen interest in learning theory and taste aversion learning. Some of my early work in Tony’s lab focused on certain fundamental problems in learning theory that I still grapple with today in one way or another. As an example, one issue we were interested in then was the manner in which rat subjects come to represent flavor solutions consisting of multiple taste components (e.g., saccharin and sodium chloride). Does the rat decompose the taste compound into its constituent parts or does it represent the compound as a rather unique configural unit? This basic issue, indeed, lies at the heart of major theoretical approaches to Pavlovian conditioning, and the major alternative approaches differ in their fundamental assumptions concerning this very issue (e.g., Pearce, 1994; 2002; Wagner, 2008). With Tony’s direction, I later went to do graduate work with Vin LoLordo at Dalhousie in the mid to late 1980s where my area of specialization was the study of associative learning. While at Dalhousie, I became especially interested in the topic of the structure of Pavlovian conditioning, or, as is sometimes called, the “what is learned?” question. To answer this question, one needs to understand the nature of unconditioned stimuli (US) and how this factors into the learning process. One particular question I studied concerned the nature of different aversive USs in taste avoidance/aversion learning. At the time, Tony’s work on the US preexposure effect (e.g., Dacanay and Riley, 1982), Linda Parker’s work on taste reactivity responses (TRs) and conditioning with LiCl and amphetamine USs (e.g., Parker, 1982), and Marcia Pelchat’s work on food aversion/avoidance learning with different types of gastrointestinal upset (Pelchat, Grill, Rozin, and Jacobs, 1983), all pointed to the view that different aversive events were not equal in the nature of the learning they produced when paired with taste stimuli. I also became interested in this issue by comparing the effects of an anxiolytic agent (chlordiazepoxide) upon extinction of a flavor avoidance based upon pairing of that flavor with either electric shock or with LiCl (in different groups of rats). We observed that chlordiazepoxide attenuated the flavor avoidance conditioned by shock but not LiCl, consistent with the view that there exist qualitatively distinct types of “aversive” motivation (Delamater and Treit, 1988).

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While also at Dalhousie I became interested in the possibility that, apart from motivational processes, Pavlovian conditioning also entailed the animal learning to associate the conditioned stimulus (CS) with some very specific sensory aspects of the US (e.g., for a review see Delamater and LoLordo, 1991). In one experiment, we gave the rat pairings of a Tone stimulus (of one frequency, T1) with intra-oral infusions of a highly palatable sucrose solution, and a second Tone stimulus (of a different frequency, T2) with intra-oral infusions of a highly unpalatable quinine solution. Largely because of the earlier work (of which Tony made me aware) of Harvey Grill (Grill and Norgren, 1978) and Peter Holland (e.g., 1981) I was interested in assessing the taste reactivity responses rats would display to intra-oral infusions of water, when these were preceded by the signal for sucrose or quinine compared to plain water alone. Holland’s work on “event representation” suggested that the signals would come to evoke sensory-specific representations of the solutions with which they were paired, and this made me think that the rat, in essence, would perceive water as though it were sucrose or quinine itself, depending upon the signal present. This was the result we obtained: the rats displayed more ingestive TRs (e.g., tongue protrusions, paw licks) to water when accompanied by T1 (the signal for sucrose), and they displayed more aversive TRs (e.g., head shakes, forelimb flails, chin rubs) to water when accompanied by T2 (the signal for quinine) (Delamater, LoLordo, and Berridge, 1986). Although this result does not definitively support our view that these effects were mediated by the Tone CSs’ ability to evoke sensory-specific representations of the associated tastes, other more recent research has used this procedure in other ways to support this general claim (e.g., see Holland, Lasseter, & Agarwal, 2008). My interest in the possibility that in Pavlovian conditioning the animal learns to associate the conditioned stimulus with some very specific sensory component of the unconditioned stimulus was strengthened by a post-doctoral fellowship that I spent in Bob Rescorla’s lab in the early 1990s. While working in Bob’s lab I became interested in studying different properties of sensory-specific associations in Pavlovian conditioning, such as their sensitivity to contingency (Delamater, 1995), reinstatement (1997), and extinction (1996) manipulations. One of the major findings is that although extinction may have some relatively durable effects on conditioned responding, it appears to leave sensory-specific associations fully preserved (see also, Delamater, 2004; Rescorla, 1996; 2001). It is as though, once the animal has learned, for instance, that a tone signals sucrose, if that tone no longer is paired with sucrose the animal will lose its interest in responding but will still “understand” that the tone was once a good signal for sucrose. There is an important disconnection between overt conditioned behavior, on the one hand, and “knowledge” of the associative relationship on the other. One of the important lessons I gained while working in the Rescorla lab was that in order to appreciate this disconnection, one often needs to devise special techniques to probe underlying knowledge of the associative relationship. This is where I became deeply interested in using the conditioned taste aversion learning (CTA) and Pavlovian-to-instrumental transfer (PIT) techniques to gain access to questions related to the nature of learning in different Pavlovian and instrumental tasks. The rest of this highlight will spend time reviewing some more recent work that we have conducted in my laboratory while at Brooklyn College of the City University of New York, focused, in particular, on our use of the CTA technique to get at questions related to the nature of learning in one particular Pavlovian paradigm – flavor preference conditioning.

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On What is Learned in Flavor Preference Conditioning Just like CTAs are established when a consumable food is paired with aversive post-ingestive consequences, conditioned flavor preferences are established when consumable foods are paired with nutrients that have positive post-ingestive consequences. The work of one of my colleagues, Tony Sclafani, has established that flavor preference conditioning can result when intake of an initially neutral flavor cue (e.g., cherry KoolAid) is paired with a nutrient (e.g., glucose) that is infused directly into the stomach (e.g., Sclafani and Nissenbaum, 1988). These pairings result in dramatic preferences for a flavor paired in this manner over a control flavor (e.g., grape KoolAid) that was previously not paired with nutrient infusions. Other work (e.g., Elizalde and Sclafani, 1988) has also demonstrated that flavor preferences can be established when initially neutral flavors are mixed in solution with or precede the oral presentation of a palatable nutrient (such as sucrose or glucose). Preferences resulting from flavor-nutrient pairings in the oral procedure may be interpreted in terms of either Flavor-Flavor or Flavor-Postingestive conditioning processes (e.g., see Sclafani and Ackroff, 1994). When one adopts a conditioning procedure where the flavor is paired with direct nutrient infusions into the stomach, these two forms of learning can be partially separated. However, when using an oral procedure, it is more difficult to do so. The problem is compounded when one questions the nature of “Flavor-Flavor” conditioning. In particular, the flavor of the nutrient consists of specific sensory properties (e.g., the sweet taste of sucrose) as well as highly positive hedonic properties (i.e., the positive affective response evoked by sucrose). It is possible that when an initially neutral flavor cue (like cherry KoolAid) is mixed into solution with sucrose, that this flavor cue becomes preferred because either (1) it has associated with the sweet taste of sucrose, or (2) it has associated with the positive hedonic response to sucrose. Special procedures are required to distinguish between these possibilities. We have employed the CTA technique for this purpose (Delamater, Campese, LoLordo, and Sclafani, 2006). Our experimental design is indicated below.

In this experiment, we first gave hungry rats opportunities to associate different flavor cues (e.g., grape, cherry, or strawberry KoolAid, counterbalanced) with distinct nutrients (16% Polycose or 16% Casein Hydrolysate). The third flavor cue served as a control stimulus and was presented on other occasions without any nutrient. Based on earlier work (Perez, Lucas, & Sclafani, 1995), we knew that this procedure would result in the two nutrient-paired flavor cues being preferred to the control flavor cue in separate two-bottle choice tests. At issue was whether the nutrient-paired flavor cues had associated with the specific sensory properties of the nutrients with which they were paired or had come to be preferred through associations with some more general aspect of the nutrients (e.g., their hedonic or some general post-ingestive reinforcing aspect). In order to make this determination, different subsets of subjects were given discriminative CTA training where either Polycose or Casein was devalued through separate pairings with LiCl injection. This phase of the experiment lasted 6 days where one nutrient was briefly presented

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on odd-numbered days and the other nutrient presented on even-numbered days. Immediately following consumption of the appropriate nutrient, subjects received an i.p. injection of LiCl. By the end of this selective devaluation phase, subjects avoided the nutrient paired with LiCl and consumed substantially more of the non-devalued nutrient. At issue was determining the effect of this treatment on preference for the two flavor associates of these nutrients. If subjects had associated the flavor cues with some specific sensory feature of the nutrient that, itself, was later devalued by the CTA treatment, then subjects should selectively avoid the flavor associate of the devalued nutrient (even though the flavor itself was never paired with LiCl). On the other hand, if the flavor cues had associated with some non-specific reinforcing component of the nutrients (e.g., their positive hedonic responses or post-ingestive reinforcing signals), then the CTA treatment should have no effect. We observed that subjects selectively decreased their intake of the flavor cue that had earlier been associated with the nutrient that underwent the CTA devaluation treatment (see Delamater et al., 2006). This result suggests that the nutrient had, indeed, become associated with some very specific sensory component of the nutrient, that itself had become devalued during CTA training. Although flavor-nutrient pairings undoubtedly establish preferences through multiple associative routes, in order to establish that sensory-specific flavor-nutrient associations are learned in this paradigm a CTA procedure can be employed. In more recent work, we have used this technique to assess the effects of nonreinforcement of the flavor cue on the relative strength of a sensory-specific flavor-nutrient association. In one procedure, we gave nonreinforced exposures to the flavor cue prior to its being paired with the nutrient, as in a “latent inhibition” procedure. In another task, we interspersed nonreinforced exposures to the flavor cue throughout a training phase in which that flavor cue was also paired with the nutrient, as in a “partial reinforcement” procedure. And in another situation, we presented nonreinforced exposures to the flavor cue following a phase in which this cue had been paired with a nutrient, as in an “extinction” procedure. In all cases, we compared the effects of this treatment on learning to another flavor cue that had been paired with the nutrient the same number of times but never was presented without the nutrient. The experimental designs for each of these tasks are listed below.

In each of these experiments, thirsty rats received the same number of pairings of two flavor cues with sucrose on separate occasions (F1-Sucr, F2-Sucr), but one of these flavor cues (F1) was additionally presented alone either before (latent inhibition), interspersed with (partial

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reinforcement), or after (extinction) flavor-nutrient pairings. In each of these experiments, we asked whether nonreinforced presentations of the flavor cue might effectively weaken control by the sensory-specific flavor-nutrient association (in this case between F1 and the taste of sucrose). To examine this, we devalued sucrose in one group of subjects with the CTA procedure. A control group of subjects received sucrose and LiCl presentations but on separate days during this phase. Finally, all subjects were given a choice test between F1 and F2. We reasoned that if the separate nonreinforcement of F1 effectively either weakened the association between F1 and the taste of sucrose or merely weakened control by this association, then subjects should prefer F2 to F1 if sucrose was still valuable at the time of testing but they should prefer F1 to F2 is sucrose had been devalued by the CTA treatment. In other words, if F1 is less able to activate a specific representation of the sweet taste of sucrose than F2, then when sucrose was devalued F2 would be more strongly avoided but preferred if sucrose was not devalued. The results from each of these experiments are depicted below.

In each experiment we observed that separate nonreinforced presentations of F1 weakened control by the sensory-specific flavor-sucrose association. When sucrose had been devalued (Dev), subjects preferred F1 to F2, but when sucrose was not devalued subjects preferred F2 to F1. The results from the extinction study have been published (Delamater, 2007a), but the results from the latent inhibition and partial reinforcement studies have not yet been published. In another set of studies we were interested in determining whether reversal learning and extinction might involve conceptually similar extinction processes (Scarlet, Campese, and Delamater, 2009). In our first study, we wanted to know if pairing a flavor cue first with one nutrient and then a second might result in extinction of the first association. The design of this study is presented below.

We first paired different flavor cues with different nutrients (F1+Sucrose, F2+Polycose). In a second phase, we then trained subjects on the reversed associations (F1+Polycose, F2+Sucrose). This reversal phase effectively meant that the phase one associations were extinguished, and, indeed, could result in weakened control by such associations (see also Delamater, 1996). We determined if this was the case by employing the selective CTA devaluation treatment described above – half of the subjects received CTA training with Sucrose and the others with Polycose. Finally, during a test phase we gave subjects a choice between the two flavor cues in the absence of their associated nutrients. In this test, we reasoned that if the reversed (phase 2) associations

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were dominant, then subjects should avoid the flavor cue that was associated with the devalued nutrient during phase 2. The results depicted below show that this was the case.

Subjects that had CTA training with sucrose selectively avoided the flavor cue that was paired with sucrose in phase 2 (i.e., Fps), but subjects that had CTA training with Polycose selectively avoided the flavor cue that was paired with Polycose in phase 2 (i.e., Fsp). These results occurred in spite of the fact that both flavor cues had previously been paired with the nutrient that later underwent CTA devaluation. Thus, the results from this experiment strongly support the view that reversal training results in extinction of the initially learned associations. What this experiment does not answer, however, is the question of whether the extinguished phase 1 associations are weakened or merely masked by the reversal phase training. We addressed this in a follow-up experiment in which we examined the effect of imposing a delay between reversal training and the CTA treatment (Scarlet, et al, 2009, Experiment 2). Our reasoning was that if the initially learned associations were actually weakened by reversal training, then imposing a time delay after reversal training should have little effect on the results. However, if some inhibitory process (e.g., see Rescorla, 2004) more temporarily masks the initially learned associations during extinction (and perhaps also during reversal training), then control by this process might be attenuated over a long delay effectively allowing the phase 1 associations to spontaneously recover. Thus, in this experiment we used the same design as depicted above, but one group of subjects received a 3-week delay following reversal training before entering the CTA devaluation phase, whereas a second group received a 1-day delay (replicating the procedure described above). The results from this study demonstrated that imposing the delay had dramatic effects on the results. We replicated our effect in the 1-day group, where these subjects avoided the flavor cue that had been associated with the devalued

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nutrient during the reversal phase, but the important new discovery was that the 3-week delay group avoided the flavor cue that had been associated with the devalued nutrient during the initial phase. In essence, the initially learned associations had spontaneously recovered. These results imply that, like extinction, reversal learning results in a temporary masking of the associations learned initially. However, it remains to be determined what is the exact nature of the recovery process in these two paradigms. In summary, we have used the CTA procedure as an effective means for assessing the nature of learning in a flavor preference paradigm. A variety of associations could mediate learning when an initially neutral flavor cue is paired with a nutrient. The use of the CTA technique allows us to focus our analysis of the learning process to the sensory-specific flavor-nutrient association. Further work will be needed to determine the precise nature of this sensory-specific association. We have been working under the assumption that the flavor cue associates with the taste of the nutrient, but sensory-specific post-ingestive processes could also play a role in mediating this effect. Another issue that is in need of follow up is determining the nature of our extinction and recovery effects as well as whether other procedures involving nonreinforcement (latent inhibition and partial reinforcement) similarly result in masked or truly weakened associations. Moreover, we are also currently investigating the possibility that spontaneous recovery might occur in a simple extinction paradigm. We have assumed that our 3-week delay manipulation works in the reversal paradigm in a manner similar to how this manipulation works in extinction, but spontaneous recovery of control by a sensory-specific flavor-nutrient association has not been examined in flavor preference conditioning. We anticipate that the CTA devaluation technique will continue to be extremely helpful in uncovering the specific associative mechanisms that contribute to flavor preference conditioning. On the Neural Mechanisms of Sensory-Specific Associations in Flavor Preference Conditioning Recently we have begun exploring the role of different neural structures in the learning of sensory-specific flavor-nutrient associations in a flavor preference task (see Delamater, 2007b). Other researchers have demonstrated that the basolateral amygdala (BLA), the orbitofrontal cortex (OFC), the gustatory cortex (GC), and communication between the BLA and GC may be critical for the associative encoding of sensory-specific components of nutrient USs. For instance, in one set of studies it was demonstrated that subjects with pretraining BLA or OFC lesions were insensitive to a US devaluation treatment induced by CTA (Hatfield, et al., 1996; Gallagher, McMahan, & Schoenbaum, 1999). In these studies, a magazine approach conditioning task was used to assess conditioning. In this task, a visual cue is paired with food pellets delivered to a food magazine and conditioned responding is assessed by examining magazine-directed approach responses during the visual cue prior to pellet delivery. Rats steadily increase their interest in the food magazine during the cue over the course of conditioning. These authors then devalued the food pellet US by pairing it with LiCl during sessions in which the pellets were presented without the visual CS. Other subjects served as controls and received pellets and LiCl on separate days. In a test phase, the visual CS is presented under extinction conditions in order to assess the effect of this US devaluation treatment. Normal animals decrease their level of conditioned magazine approach responding if the pellet US had been devalued compared to non-devalued controls. However, animals with

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pretraining BLA or OFC lesions failed to show this devaluation effect. The result implies that these structures are somehow involved in the encoding of sensory-specific associations in this procedure. Killcross and his colleagues (e.g., Blundell, Hall, & Killcross, 2003) similarly examined the role of the BLA in learning sensory-specific associations involving taste cues. In this study, thirsty rats drank one solution consisting of NaCl and HCL and a second solution consisting of Sucrose and Quinine. Following these pairings, HCL and Quinine were differentially treated – one was paired with LiCl and the other was not (counterbalanced). In a final test between Sucrose and NaCl, all subjects (BLA lesioned and shams) avoided the taste cue that had been associated with the taste that was subsequently devalued through CTA. In other words, these authors failed to find any role for the BLA and suggested that the BLA was critical in the associative encoding of sensory-specific features of motivationally significant events, but not relatively neutral events. In one more piece of evidence, Saddoris, et al. (2008, SfN) reported with immediate early gene staining techniques that flavor cues and their nutrient associates both activated a population of cells in the BLA, and that activation of different populations of cells in the BLA by different flavor cues associated with different nutrients depended upon input from the GC. These results suggest that a cell population within the BLA is jointly activated by both a flavor CS and a nutrient US, a result that lends support for the long-held view that CSs and USs both come to activate the very same underlying neural representation. Moreover, the results also suggest that for differential representations of the USs to develop in the BLA, input from the GC is critical. Because of all of these strands of evidence, we became interested in exploring the role of these structures in the learning of sensory-specific flavor-nutrient associations in our flavor preference learning paradigm. Our basic experimental design is indicated below.

Rats are first trained to associate different flavor cues (almond or banana extract) with different nutrients (10% sucrose or 10% Polycose). Typically, they are given 8 pairings of each flavor-nutrient pair. Then, different subsets of rats receive CTA training with either sucrose or Polycose. Then the effect of this reinforcer devaluation upon preference for the two flavor cues in the absence of the nutrients is assessed. One of my students, Janina Scarlet, has been working on this project as part of her PhD dissertation and she has performed various lesions in different experiments. The protocol involves infusing NMDA into select regions to lesion the BLA, OFC, or GC pretraining and then allowing the rats to recover from the surgeries for approximately 2 weeks before placing them on a water deprivation regime to ready them for the experiment. Thus far, and quite surprisingly, we have found that none of these structures impairs the rats’ sensitivity to the US devaluation treatment. In other words, during the final choice test subjects always avoid consuming the flavor associate of the devalued nutrient. The data from two experiments is summarized below. In the first experiment, subjects received pretraining lesions of the BLA or the OFC. In a second study, the subjects received GC or GC-BLA disconnection

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lesions. In both experiments, compared to sham controls the lesioned animals displayed comparable devaluation effects to shams. Although these results are preliminary (histology needs to be completed), it would appear that at least with the training procedures employed here the formation of sensory-specific flavor-nutrient associations does not critically depend upon these structures in isolation. It is not clear why lesions to these structures fail to impair the US devaluation effect in this paradigm, while they are critical in other paradigms. One major difference is that the studies showing the importance of BLA and OFC have used hungry rats performing in magazine approach (or Pavlovian-to-instrumental transfer) tasks, whereas, a flavor preference conditioning paradigm with thirsty rats consuming flavor CSs is used here. On the other hand, immediate early gene staining techniques have shown convergence of flavor and nutrient activation patterns at the cellular level in BLA and GC in thirsty rats. Thus, there is a real puzzle: different paradigms lead to different conclusions regarding the importance of these structures for sensory-specific associations as revealed by CTA US devaluation effects, but more molecular measures reveal that cells within these structures are activated by cues for nutrients as well as the nutrients themselves. We are not sure what to make of these discrepancies, but we are currently examining the role of these structures in thirsty rats trained in magazine approach procedures, and are entertaining the hypothesis that with extensive training the neural representation of the nutrient USs we employ may be quite distributed across many structures and this could lead to insensitivity of lesions to any one of these structures. This suggests that with more limited training sensory-specific flavor-nutrient associations may have a less distributed neural representation, but this remains to be determined. Regardless of what we are to make of the discrepancies between these different datasets, one thing that is quite clear from the above-mentioned research is that identifying the neural mechanisms of sensory-specific associations in flavor preference conditioning is a complex endeavor. Use of the CTA technique has proven indispensable towards this end, however, and future work that ultimately resolves some of the mysteries noted above will very likely continue to take advantage of this technique, without which identification of precise associative structures mediating this form of learning will not be easy. Conclusions In summary, I have tried to indicate how the CTA technique has been extremely useful in helping us uncover some of the properties of sensory-specific flavor-nutrient conditioning in flavor preference learning. By adopting this technique it has been possible to isolate learning of

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associations between the flavor cue and the sensory as opposed to hedonic and general post-ingestive reinforcing components of the nutrient in flavor preference conditioning. We have shown not only that such associations are learned in this preparation, but that these associations are sensitive to extinction, latent inhibition, and partial reinforcement procedures, are temporarily masked during reversal training, but spontaneously recover when a delay is imposed between the reversal phase and test. Further work is necessary to identify the precise associative mechanisms involved in these effects. In addition, we have begun to explore the neural substrates that mediate this form of learning, and while our work on this front is just beginning an interesting difference seems to be emerging. Whereas various brain structures (BLA, OFC) seem to be critical for the sensory-specific encoding of nutrient rewards in experiments that use auditory and visual cues in Pavlovian tasks, thus far we have not been able to show that these structures meaningfully participate in the sensory-specific encoding of the associative relationship involving flavor cues and nutrient rewards. Much more work is needed to explore this discrepancy, and further studies may, indeed, reveal critical conditions whereby flavor-nutrient learning may not be so different. On the other hand, it also seems possible that learning involving flavor versus auditory and visual cues may entail different psychological as well as neural processes. References Blundell, P., Hall, G., & Killcross, S. (2003). Preserved sensitivity to outcome value after lesions of the

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