Backward cortical projections to primary somatosensory cortex in rats extend long horizontal axons in layer I

Backward Cortical Projections to PrimarySomatosensory Cortex in Rats Extend

Long Horizontal Axons in Layer I

LAWRENCE J. CAULLER,1* BARBARA CLANCY,1 AND BARRY W. CONNORS2

1Cognition and Neuroscience Program, School of Human Development,University of Texas at Dallas, Richardson, Texas 75083-0688

2Department of Neuroscience, Division of Biology and Medicine, Brown University,Providence, Rhode Island 02912

ABSTRACTWe have studied the origin and extent of axons within layer I of the primary somatosen-

sory cortex (SI) of rats by using retrograde and anterograde tracers with an emphasis onreciprocal connections to layer I of SI from ipsilateral cortical areas that are the target of SIprojections. Small crystals of 1,18,dioctadecyl-3,3,38,38-tetramethyl-indocarbocyanine perchlor-ate (DiI) labeled horizontal axons projecting in all directions within layer I, which extended forup to 4 mm with numerous terminal branches. Applications of horseradish peroxidase,Diamidino yellow, or fast blue to the pial surface of SI labeled a characteristic pattern ofneurons below the application site that excluded neurons in layer IV of the barrel fields,unless the dye penetrated deeper than layer II. This provided a control for the effective depthof the layer I dye applications. Retrograde transport from layer I of SI was traced to theprimary motor area, the lateral parietal areas, including the secondary somatosensory (SII)and agranular insular cortex ipsilaterally, as well as the homotopic areas of SI contralaterally.Injections of the anterograde tracer dextran amine at the same site as the SI surfaceapplication labeled dense fiber terminations in middle layers of these same secondary areas inthe primary motor cortex (MI) or SII in the midst of cells labeled by retrograde transport fromlayer I of SI. Injections of dextran amine into these secondary cortical areas labeled fibers thatcoursed through deep layers to SI, where they ascended to layer I. These reciprocal corticocorticalinputs to SI were concentrated in layer I, where they branched and extended horizontally acrossseveral SI barrels. J. Comp. Neurol. 388:297–310, 1998. r 1998 Wiley-Liss, Inc.

Indexing terms: neocortex; barrel field; corticocortical; feedback; dextran amine; fiber tracing

Layer I is an important site of convergence within theneocortex (Vogt, 1991; Cauller, 1995). The diverse sourcesof inputs to layer I include the brainstem and basalforebrain, the thalamus, and the neocortex itself. Themajor targets of these afferents are the apical dendrites ofpyramidal cells whose somata lie in deeper layers, from IIto V. Layer I is about 0.15 mm thick, consists of virtuallyall neuropil, and has a density of presumed excitatorysynapses that is among the highest in the cortex (Jonesand Powell, 1970; Adinolfi, 1972; Vaughan and Peters,1973; Beaulieu and Colonnier, 1985; Braitenberg andSchuz, 1991). Among the many systems of axons terminat-ing within layer I, those from other areas of the cortex mayplay an important role in sensory-motor activity.

The reciprocal nature of corticocortical connections hasbeen recognized as a general principle of cortical organiza-tion (Pandya and Yeterian, 1985; Zeki and Shipp, 1988;Felleman and Van Essen, 1991). Each of the ‘‘forward’’

projections from primary to secondary sensory areas inauditory, somatosensory, and visual systems of monkeys isreciprocated by ‘‘backward’’ projections from those second-ary areas to their corresponding primary areas. Further-more, each of the projections from primary to secondary,from secondary to higher order cortical areas, and so on, tofrontal or paralimbic areas appears to be likewise recipro-cated by backward projections from higher to lower ordercortical areas. The potential theoretical significance of

Grant sponsor: NIH; Grant numbers: NS08376, NS25983; Grant sponsor:Whitehall Foundation; Grant sponsor: ONR; Grant number: N00014-90-J-1701.

*Correspondence to: Larry Cauller, Neuroscience, GR 41, P.O. Box830688, University of Texas at Dallas, Richardson, TX 75083-0688.E-mail: [email protected]

Received 16 December 1995; Revised 14 August 1997; Accepted 15August 1997

THE JOURNAL OF COMPARATIVE NEUROLOGY 390:297–310 (1998)

r 1998 WILEY-LISS, INC.

reciprocal interconnections for cortical function has beenemphasized by several authors (see, e.g., Edelman, 1978;Damasio, 1989; Rolls, 1989; Squire and Zola-Morgan,1991; see also a collection of theoretical papers in Koch andDavis, 1994).

Forward corticocortical projections in cats and monkeysterminate focally in middle cortical layers III and IV,similar to specific thalamocortical projections. In contrast,backward projections characteristically terminate mostheavily in layer I and tend to avoid middle layers. Layer Icontains an extensive network of horizontal axons (Jonesand Powell, 1968; Fleischhauer and Laube, 1977; Szen-tagothai, 1978), and many of these may be backwardcortical projections. Rockland and Virga (1989) tracedsingle axons from secondary visual areas to the primaryvisual cortex. These backward fibers ran horizontallywithin layer I, parallel to the pia, for 1–4 mm.

Despite its intriguing organization, the functions andbehavioral significance of these layer I characteristics areobscure. Evidence from various studies (Sandell andSchiller, 1982; Bullier et al., 1988; Mignard and Malpeli,1991) shows that backward connections influence primaryvisual cortex, although the relative importance of the layerI component of the backward system has not been specifi-cally addressed in these studies. Layer I inputs, evidentlyfrom backward projections, generate a signal in the pri-mary somatosensory cortex (SI) of awake monkeys thatpredicts touch-discrimination behavior (Kulics and Cauller,1986; Cauller and Kulics, 1991). Intracellular studies ofrat SI in vitro have found that the excitatory synapticinfluence mediated by horizontal layer I inputs onto thedistal apical dendrites of pyramidal neurons is surpris-ingly powerful (Cauller and Connors, 1994). Our aim hereis to describe the anatomy of corticocortical axons thattravel horizontally for long distances within layer I of ratSI and to further develop this in preparation for the studyof reciprocal corticocortical connections in particular anddistal synaptic inputs in general. Preliminary reports ofthis work have been presented in abstract form elsewhere(Cauller and Connors, 1989, 1990; Clancy and Cauller,1993, 1994).

MATERIALS AND METHODS

Adult (100–450 g) male and female Long Evans (CharlesRiver, Wilmington, MA) and Sprague Dawley (Zivic-Miller,Allison Park, PA) rats were subjects for all experiments.No systematic experimental differences were observedwithin this population. Prior to all surgical procedures,rats were deeply anesthetized by intraperitoneal injectionof sodium pentobarbital (Nembutal; 50 mg per kg bodyweight) and maintained nonreflexive to tail pinch by 5-mgsupplements every 20 minutes. At all times, rats werehoused, handled, and maintained under the NIH guide-lines for the care and use of laboratory animals.

Several tracers were employed to identify the course andorigin of layer I inputs. To trace the pattern of fibers inlayer I, in vitro slices of superficial cortex (200–350 µmthick at the center) were cut by Vibratome (Pelco, Redding,CA) parallel to the surface (i.e., in the plane tangential tothe dorsolateral cerebral convexity) from the region of themain bifurcation of the middle cerebral artery, whichoverlies the SI barrel field (according to the in vitromethods of Cauller and Connors, 1994). Slices were fixedin 4% paraformaldehyde in phosphate buffer (PB; 0.1 M

Na-K PB, 7.4 pH), and a solid fragment (,0.15 mmdiameter) of 1,18,dioctadecyl-3,3,38,38-tetramethyl-indocar-bocyanine perchlorate (DiI; Molecular Probes, Eugene,OR) was inserted through the pia into layer I (Godemont etal., 1987). Following transport times up to 6 weeks at roomtemperature, slices were wet mounted without sectioningand photographed under epifluorescence.

To identify the origins of inputs to layer I of SI, a pledgetof filter paper (1 mm2) soaked in horseradish peroxidase(HRP; 10–30% Sigma Type VI dissolved in PB; Sigma, St.Louis, MO) or in one of the fluorescent dyes, Diamidinoyellow or fast blue (DY or FB; .20% in distilled H2O;Sigma), was applied in vivo during surgery directly to thepial surface for less than 10 minutes. Application siteswere thoroughly rinsed with buffer, covered with bonewax, wounds were closed with application of local anes-thetic and antibiotic ointment, and rats revived. Forcomparison, in two cases, HRP dissolved with 0.1% dimeth-ylsulfoxide was applied for more than 15 minutes to staindeeper layers. For retrograde studies using fluorescentdyes, DY or FB was either applied to the pial surface, asdescribed for HRP above, or pressure injected into layer IIas a control (see Results) by using brief pulses of nitrogenpressure (General Valves, Fairfield, NJ). Survival periodsof from 48 hours to 7 days were employed for retrogradetransport studies (no differences in the extent of labelingwere observed across this range of survival times). Follow-ing the retrograde survival time, rats were given anoverdose of pentobarbital and were perfused transcardi-ally, first with 0.9% NaCl and 0.5% NaNO3, then withfixative (for HRP, 2% paraformaldehyde/1% glutaralde-hyde in PB; for DY or FB, 4% paraformaldehyde in PB),and finally with 10% sucrose in PB. Following perfusion,brains were removed and sunk in 30% sucrose PB at 4°C.To identify retrograde labeling by using either HRP orfluorescent dyes, frozen sections were cut 50–60 µm thickin the coronal plane, and alternating sections were cellstained by using cresyl violet. Transported HRP waslabeled in floating sections by adding hydrogen peroxide inthe presence of tetramethylbenzidine to form chromagen,which was enhanced with nickel and ammonium molyb-date and intensified with cobalt, according to the Olucha etal. (1985) modification of Mesulam’s (1978) protocol. Forfluorescence microscopy, sections were simply mountedfrom very dilute buffer and desiccated, cleared in xylene,and coverslipped in DPX (Fluka, Buchs, Switzerland) formicrography under epifluorescence.

To trace fibers of the backward corticocortical projec-tions by anterograde transport, biotinylated or tetrameth-ylrhodamine-labeled dextran amine (bDX or rDX, respec-tively; .10% in distilled H2O or PB; 10 kD MW, lysinefixable; Molecular Probes; Schmued and Heimer, 1989)was pressure injected during acute surgery into sitesidentified by retrograde transport to be the origin of layer Iinputs to SI: 1) the lateral parietal cortex (1.5–2.5 mmposterior to Bregma, ,2 mm dorsal to the rhinal vein;tooth bar adjusted for dorsal skull in horizontal plane) and2) the lateral agranular frontal cortex (2.0 mm lateral and2.0 mm anterior to Bregma). To avoid inadvertent spreadof tracer over the pial surface, small dural openings andcraniotomies into the temporal plate were prepared thatdid not extend over SI. In addition, the injection pipettewas inserted to 1.2 mm below the surface, retracted to 0.6mm, and up to 25 small injection pulses (,10 nl per pulse)

298 L.J. CAULLER ET AL.

were applied once every minute while washing the surfacewith PB.

For anterograde studies using DX, rats were given anoverdose of pentobarbital following 5–7 days’ survival,perfused transcardially (same protocol as DY above), andeither the brains were frozen sectioned in the coronalplane (60 µm thick) or the cortex was removed, flattened,frozen in dry ice, and sectioned (30–60 µm thick) parallelto the dorsolateral pial surface (i.e., tangential). To processthe fibers labeled with bDX, frozen sections were firstwashed in several changes of PB overnight to removeresidual paraformaldehyde. Sections were then pretreatedwith H2O2 (0.5%; to reduce background peroxidase activ-ity) and Triton X-100 (1%; to promote membrane penetra-tion) and were processed for biocytin by overnight incuba-tion in avidin-HRP (1:100 with 1% Triton-X in PB; ABCstandard kit, Vector Laboratories, Burlingame, CA). Fi-bers labeled with this biocytin-avidin-HRP complex werethen developed by peroxidase reaction in Tris buffer toproduce cobalt-intensified diaminobenzidine (DAB) chro-magen (protocol available upon request). Initially, fluores-cent fibers labeled with rDX were simply processed forepifluorescence micrography as described above for DY.However, subsequently, we found that background fluores-cence was greatly attenuated by treating free-floatingsections in sodium borohydride (0.5% in PB) for 10–20minutes prior to mounting (Clancy and Cauller, 1995).

All photomicrographs were digitally scanned directlyfrom negatives at 1,200 pixels per inch resolution. Thesedigital scans were optimized for contrast and sharpnesswith PhotoShop software (Adobe, Mountain View, CA),which was also used to organize and label the figures.These figures were submitted in digital form.

To identify the relative location of layer I fibers withrespect to the underlying barrel field that defines SI(Welker, 1976; Wallace, 1987), tangential sections (60 µmthick) from middle layers were first washed in buffer for 48hours to remove residual paraformaldehyde and were thenincubated in 0.03% cytochrome C type III (Sigma), 0.05%DAB, and 4% sucrose in PB for 1–2 hours at 37°C to labelcytochrome oxidase (CO). This identification of the barrelfield was also employed to establish reliable stereotaxiccoordinates for placement of dye applications or injectionsinto SI relative to the major branches of the middlecerebral artery. In addition, the consistent location of theforepaw area of SI (4.0 mm directly lateral from Bregma)was verified by recording field potentials evoked by electro-cutaneous stimulation of the contralateral forepaw duringacute dye-injection surgery.

RESULTS

DiI embedded in layer I of fixed tangential slices labeledhorizontal fibers that extended more than 4 mm from theDiI fragment within layer I, with most fibers endingwithin 2 mm from the DiI (Fig. 1). SI was located withrespect to the middle cerebral artery and underlyingbarrels and was faintly evident in unstained tangentialsections of middle layers. Fibers radiated in layer I in alldirections from the DiI fragment, apparently withoutconsistent bias. Although, for any given slice, the densityof horizontal layer I fibers varied with respect to horizontaldirection, no systematic, preferred direction was detectedamong seven slices from four rats. The majority of large,straight, horizontal layer I fibers gave the overall impres-

sion of an omnidirectional horizontal pattern of straightfibers radiating from the DiI crystal. Upon closer inspec-tion at any point in layer I or in upper layer II, fine fibersextended in all directions, with numerous obtuse arboriza-tions and apparent varicosities spaced every 10–20 µm(Fig. 2).

Applications of HRP or fluorescent dyes directly on thepial surface were used to identify the source of neuronsprojecting to layer I of SI by retrograde transport. Twosites in SI were targeted, both located within the regionidentified by CO staining: the whisker barrels (5.5 mmlateral and 2 mm posterior to Bregma) and the forepawarea (4.0 mm lateral and 0 mm posterior to Bregma),which was verified by recording the field potential re-sponse to contralateral forepaw stimulation. Surface appli-cations of FB, DY (Figs. 3A, 5B), or HRP (Fig. 4) resulted ina characteristic pattern of cell labeling in SI cortex imme-diately below the surface application site. For 10-minuteapplications, the dyes penetrated no deeper than layersI/II, and underlying cells were found only in layers II, III,and V. No labeled cells were found in layer VI or in adiscrete band corresponding to layer IV that was identifiedin alternating cell-stained sections (Fig. 3). In contrast,direct injections of DY into layer II with some dye directlypenetrating upper layer III resulted in cell labelingthroughout layers II–V, including layer IV (Fig. 3C; n 5 6).Similarly, long HRP applications (.15 minutes dissolvedin 0.1% dimethylsulfoxide; n 5 4) penetrated as deep aslayer III and labeled cells in layer IV as well. Therefore,the absence of cell labeling in layer IV provided a directcontrol for the depth of the dye penetration. Accordingly,only those surface applications that excluded cell labelingin layer IV (HRP, n 5 10; DY, n 5 12; FB, n 5 2) wereincluded in the following retrograde analysis of the originsof inputs to layers I/II.

Close examination of the cell-free layer IV below superfi-cial HRP applications revealed vertically oriented labeledfibers as well as the apical dendritic trunks of layer Vpyramidal neurons (Fig. 4A). These dendrites were prob-ably filled directly via their distal apical tufts, becauseretrogradely filled cells in other cortical areas did notinvolve dendritic labeling. Cells were rarely found withinthe surrounding area of SI except immediately below theapplication site.

No fibers from surrounding cortical areas to layer Iprojected directly to SI through horizontal paths in upperlayers I–IV (Figs. 4, 6). Most fibers connecting the applica-tion site to adjacent cortical areas traveled horizontally indeep layers V/VI. Only the most distant ipsilateral orcallosal projections appeared to enter the subcortical whitematter. Anterogradely labeled fibers were traced from thelayer I HRP application site in SI through the internalcapsule as far as the cerebral peduncle, indicating that thecells with layer I dendrites included those that projected tothe brainstem and, perhaps, to the spinal cord. Sectionswere not prepared caudal to the diencephalon.

The majority of cells labeled by retrograde transport ofHRP applied directly to layer I over SI were found in twoipsilateral cortical areas: 1) the lateral agranular frontalcortex (primary motor cortex or area MI; also called Fr1 byZilles, 1985) and 2) the parietal cortex lateral to SI down tothe rhinal sulcus, including the secondary somatosensorycortex (SII; also called Par2 by Zilles, 1985) and theimmediately lateral areas (including agranular insularcortex of Zilles, 1985; roughly corresponding to parietal

HORIZONTAL AXONS IN LAYER I OF NEOCORTEX 299

ventral and parietal rhinal areas defined by Fabri andBurton, 1991). In addition, retrograde transport fromlayer I applications in SI was traced to the homotopic pointin contralateral SI cortex. Because the number of HRP-labeled fibers traced from the SI application site throughthe callosum appeared to be much greater than thenumber of filled cells found in the contralateral cortex, asubstantial anterograde callosal projection from SI wasapparently labeled directly by the layer I HRP application.Presumably, this was a result of filling superficial pyrami-dal neurons with HRP directly through their apical den-drites.

Cells labeled by retrograde transport from layer I of SIwere found in ipsilateral MI cortex scattered throughoutlayers III–V (Figs. 4B, 5A). Most inputs to layer I of SIfrom SII and more lateral parietal areas originated from aband of cells restricted to layer V (Figs. 4B, 5H,I), withmany others scattered through layers III–IV. In contrast,the projection from contralateral SI cortex originated fromcells in supragranular layers. By using HRP, the thalamuswas the only observed source of noncortical inputs to layer

I (Fig. 4B; the brainstem was not prepared), with the mostfocused concentration of cells in the ventromedial nucleus(VM) and numerous cells scattered throughout the ventro-basal complex [especially the medial ventroposteriornucleus (VPm) following whisker barrel applications or thelateral ventroposterior nucleus (VPl) following forepawarea applications] and the posterior nucleus (Po). How-ever, by using fluorescent tracers, other cells were alsoidentified as origins of layer I inputs to SI that were notevident with HRP, including a few cells within the zonaincerta and a widespread, thin band of cells at the borderbetween the dorsolateral cortex and the white matter (Fig.5B) extending from MI to lateral parietal areas.

Anterograde HRP terminal labeling was observed in allcortical areas in which cells labeled by retrograde trans-port from layer I of SI were found. This reciprocity ofinterconnections between SI and its major targets, MI andSII, was studied in detail in six rats by injecting antero-grade rDX tracer into middle layers of the SI forepaw areaimmediately below a surface application of retrograde DYtracer (Fig. 5). Terminal labeling was observed in all layers

Fig. 1. The omnidirectional pattern of horizontal layer I fibers inthe primary somatosensory cortex (SI) labeled with 1,18,dioctadecyl-3,3,38,38-tetramethyl-indocarbocyanine perchlorate (DiI) in fixed tan-gential slices of superficial cortical layers. The center shows a low-power brightfield photograph of the tangential slice with a fragment ofDiI embedded in the crotch of the major branch of the middle cerebralartery (medial is upward, anterior is to the left). Unstained barrels arefaintly visible anterior to the DiI fragment in this relatively thick

section (approximately 0.25 mm). Immediately surrounding the DiIfragment, there is a faint halo resulting from the passive diffusion ofthe dye through membranes in contact with the fragment (themechanism of DiI labeling). The surrounding photomicrographs weretaken under epifluorescence with greater magnification within layer I(,0.15 mm below the pial surface) at the corresponding locationsindicated by the rectangles in the center.


of the retrogradely labeled cortical areas, but especiallydense terminal labeling was found in middle layers III/IVof ipsilateral SII (Fig. 5F,G) and less densely in middlelayers of MI (Fig. 5D). These dense terminations labeled byanterograde projections from SI were used to define thelocations of SII and MI for injections of anterograde tracersto study the reciprocal projections to layer I.

Injections of bDX or rDX (Schmued and Heimer, 1989)into the lateral parietal cortex (i.e., SII and adjacentlateral areas identified above by transport from SI; n 5 12)or the lateral agranular frontal cortex (MI; n 5 6) labelednumerous horizontally oriented fibers in layer I over SI byanterograde transport. In coronal sections, these corticocor-tical inputs to layer I projected to SI mostly through layerVI, and the rest projected through layer V and the subcor-tical white matter (Fig. 6). Upon reaching SI, these deepfibers abruptly turned toward the surface and ascendeddirectly to layers I/II, with some collateralization in layersV and II/III. In all cases, the greatest concentration ofbackward fiber labeling in SI from MI, SII, or other lateralparietal areas terminated in the most superficial layersI/II, with the least amount in layer IV. Upon reaching layerI, these backward corticocortical fibers from lateral pari-etal and frontal areas typically branched abruptly andextended horizontally in layer I parallel to the pial surface.It is this horizontal extension of fibers in layer I that wasresponsible for concentrating the greatest density of fibersin layer I relative to the deeper layers. Numerous short,horizontal fiber segments were observed concentrated inlayer I of these coronal sections (Fig. 6). Although a singlefiber could be traced occasionally through layer I fordistances exceeding 1 mm, it was difficult to follow indi-vidual fibers in coronal sections of layer I, because the fibertrajectories were not restricted to the coronal plane.

In tangential sections through layer I over SI, many ofthese reciprocal fibers could be traced more than 1 mmparallel to the surface, with no obvious preferred direc-tions or orientation with respect to the underlying somato-sensory topography (Figs. 7, 8). These horizontal layer Ifibers from MI and lateral parietal cortex extended acrossall parts of the underlying SI topography, sd identified byCO staining, including the whisker, forelimb, hind limbrepresentations and the intercalated dysgranular zones.The fine detail of their terminal arbors involved numerousobtuse arborizations in all directions, with apparent bou-ton varicosities spaced 10–20 µm apart and resembling thehorizontal layer I pattern labeled by DiI. In tangentialsections from below layer II, thick ascending fibers werecut in cross section as they ascended to layer I (Fig. 8, D1).Although the alignment between the superficial tangentialsections and the underlying barrel field was not suffi-ciently accurate to determine the precise relationshipbetween the layer I fibers and the individual barrels oflayer IV, the density of layer I fibers ensured that indi-vidual fibers extended over both barrel centers and theintercalated septa. The terminal zone of the backwardfibers extended horizontally up to 4 mm within layer I ofSI; however, the density of fibers and the difficulty ofsectioning so near the pial surface made it unfeasible totrace individual fibers more than approximately 1 mm.The straight length of several individual layer I fiberscould be traced horizontally greater than the width of twomajor whisker barrels or four minor barrels (Figs. 7, 8).

DISCUSSION

The major findings of our study of layer I in the rat SIcortex are: 1) a dense plexus of long axons (up to 4 mm)

Fig. 2. Fine detail of superficial horizontal layer I fibers labeled with DiI at a single site in the SI.These fluorescence photomicrographs were taken at a site 2.0 mm anterior and slightly lateral to the DiIfragment show in Figure 1 (middle left rectangle; double magnification). From left to right, photomicro-graphs were focused at greater depths below the pial surface.


extends horizontally throughout layer I of SI with noprominent directional bias; 2) long horizontal branches ofbackward projecting cortical axons originating from cellsin the cortical areas that are targets of SI outputs,including areas MI and SII, extended more than 1 mmwithin layer I of SI; 3) these horizontal branches result inthe dense concentration of the backward corticocorticalprojections within layer I of SI; 4) the backward corticalinput to layer I of SI covered all parts of the somatosensorytopography.

Our findings add to a growing body of evidence (Koraleket al., 1990; Fabri and Burton, 1991) that somatosensorycortical areas are reciprocally interconnected in rats.Similar reciprocal interconnections have also been demon-strated between visual areas in rats (Coogan and Burkhal-ter, 1990; see also Deacon et al., 1989). The findings of thisstudy are more generally in agreement with descriptionsof the ‘‘hierarchical’’ organization of corticocortical connec-tions in monkeys (see, e.g., Felleman and Van Essen, 1991)or cats (see, e.g., Scannell et al., 1995). However, althoughthe infragranular origins in the lateral parietal areas ofmost backward inputs to layer I of SI are characteristic ofthe established backward or ‘‘descending’’ pattern of projec-tions, many backward projections to layer I were alsofound to originate from supragranular cells in this study.Furthermore, cells of all layers in MI were found to projectto layer I of rat SI. The possibility that backward projec-tions which originate from another set of cortical neuronswithin SII or MI terminate only in deeper cortical layersremains to be tested.

In rats, there are few connectional steps between theprimary somatosensory area and the paralimbic areas

that may be considered the highest reaches of the cortical‘‘hierarchy.’’ Although projections from the entorhinal cor-tex avoid SI in rats, these paralimbic projections denselyinnervate layer I of the lateral parietal areas (Swansonand Kohler, 1986), which we find project to layer I of SI.Therefore, the higher order limbic influences upon theprimary somatosensory area in rat are relatively direct,involving only one intermediate synaptic stage in thesecondary somatosensory areas.

Until the backward system of corticocortical projectionswas widely recognized, excitatory inputs to layer I werebelieved to originate either locally from nonpyramidalneurons (see, e.g., Martinotti cells, Szentagothai, 1978) orextrinsically from the thalamus outside the specific sen-sory nuclei (i.e., ‘‘nonspecific’’ projections; Herkenham,1986). It is now clear that axon collaterals of local pyrami-dal neurons reach layer I (Martin, 1984; Clancy andCauller, 1994) and that a subpopulation of thalamic neu-rons in the specific sensory nuclei also project to layer I(Penny et al., 1982; Rausell and Avendano, 1985), whichwas confirmed in this study. In addition, cholinergic (Bearet al., 1985; DeLima and Singer, 1986; Lysakowski et al.,1986) and monoaminergic (Emson and Lindvall, 1979)systems project heavily to layer I. The function of back-ward corticocortical projections must be considered withinthe context of these inputs that converge in layer I uponthe distinct population of pyramidal neurons with distalapical tufts extending to the surface.

The results of this study add to a growing number ofanatomical studies, which have found that fluorescentretrograde tracers label neurons in areas that are notlabeled by HRP. In particular, Lin et al. (1990) identified

Fig. 3. A: The characteristic pattern of labeling immediately belowa surface application of Diamidino yellow (DY) fills neurons in layersII, III, and V but specifically excludes neurons in a middle layer thatcorresponds to layer IV in the same section following cresyl violet

cell-staining (B). C: In contrast, a small injection of DY into layer IIlabels neurons throughout middle layers, including layer IV, asidentified in the same section by cell staining (D).


g-aminobutyric acid (GABA)ergic neurons in zona incertathat were labeled by retrograde transport of fluorescentbeads from SI, indicating a novel subcortical inhibitoryprojection to the cortex, which is not labeled by HRPtransport. This study confirms that the projection fromzona incerta to layer I of SI can be labeled by using otherfluorescent tracers and that HRP does not label these cells.Similarly, Nicolelis et al. (1991) found neurons in a denseband at the border between the dorsolateral cortex and thewhite matter following retrograde transport of fluorescent

tracers from SI. Likewise, we find that these cells projectto layer I of SI with the use of fluorescent tracers but notwith HRP (Clancy and Cauller, 1995). The possibilitiesthat these deep border neurons are also GABAergic andthat HRP is less sensitive for transport in GABAergicneurons remain to be tested.

Well-established descriptions of primary sensory corti-cal cytoarchitecture (see, e.g., Jones, 1981), based primar-ily on Golgi studies, have concluded that the populations ofpyramidal neurons with distal apical dendrites extending

Fig. 4. A: The pattern of labeling immediately below a surfaceapplication of horseradish peroxidase (HRP) shows the characteristicabsence of cell bodies labeled in layers IV and VI, which verifies thatthe application did not penetrate deeper than layer II. This photomicro-graph shows retro- and anterogradely labeled fibers from the applica-tion site, which extended contralaterally through the callosum tohomotypical SI, ipsilaterally to the lateral cortex (including thesecondary somatosensory; SII) and frontal cortex (including theprimary motor cortex; MI), and subcortically to the thalamus andbrainstem. B: Ipsilateral distribution of cells at representative loca-tions labeled by the HRP application shown in A. The upper tracing ofa coronal section 1.2 mm anterior to Bregma shows labeled cells in all

layers of the agranular frontal cortex (including MI). The lower tracingof a coronal section 3.2 mm posterior to Bregma shows labeled cells inlateral parietal areas (including SII), especially in deep layers V/VI.Cells were also found ipsilaterally in the thalamus [shown here in theestimated locations of the ventromedial nucleus (VM), the medialventroposterior nucleus (VPm), and the posterior nucleus (Po)]. Thedorsolateral location of the HRP application in SI is indicated in thelower tracing of a coronal section through caudal SI and SII. The HRPapplication site in the whisker barrel field was centered 1.3 mmanterior to this caudal section. These tracings of characteristic retro-grade HRP findings show all cells observed within two sections (e.g.,120 µm).


Figure 5


to layer I are found in layers II, III, and V, whereas theascending dendrites of neurons in layers IV and VI rarelyextend above layer III. The pattern of cell labeling ob-served below surface applications in this study directlyreveals this select subpopulation of cortical pyramidalneurons with layer I dendrites. Several recent studieshave correlated distal apical dendrites on a subset ofpyramidal neurons in layer V with distinct physiologicaland connectional properties of those neurons. Layer Vpyramidal neurons with distal apical dendrites reaching tolayer I project to the brainstem superior colliculus, pons, orspinal cord, whereas those layer V pyramidal neurons thatproject through the corpus callosum to the contralateralcortex have sparse apical dendrites in layer I or lack themaltogether (Hubener et al., 1990; Larkman and Mason,1990; Miller et al., 1990; Pockberger, 1991). These studiesfurther show that the distal apical dendrites of intrinsi-cally bursting pyramidal neurons of layer V are moreprofusely branched than those of regular spiking layer Vpyramidal neurons (also Chagnac-Amitai et al., 1990).These findings indicate that the backward cortical projec-tions to layer I of SI engage a subpopulation of corticalneurons with unique physiology and connections.

A novel result of this study is that the backwardconnections to SI contribute significantly to the horizontalsystem of fibers in layer I. However, there are two unre-solved questions concerning corticocortical projections fromsecondary to primary areas in rats: 1) How extensive arecollaterals of horizontal layer I fibers below layer I, and 2)do all corticocortical projections from secondary to primaryareas extend horizontally in layer I? A single-fiber recon-struction technique, like that of Rockland and Virga (1989)employed in monkey visual system, will be necessary to

illustrate directly the laminar specificity of these corticalprojections to layer I. The possibility that the other originsof projections to layer I of SI observed in thalamus, zonaincerta, or subcortical white matter extend horizontally inlayer I also remains to be tested.

The extensive omnidirectional pattern of all horizontallayer I fibers labeled by DiI indicates that a barrel in thecenter of SI shares horizontal layer I inputs with many,and possibly all, other points in the SI topography. Thisfinding conforms to the conclusions of Szentagothai (1978),who employed fiber degeneration in cats to demonstratelong horizontal layer I fibers extending 4–6 mm in alldirections. On the other hand, Fleischhauer and Laube(1977), on the basis of fiber silver staining in rabbits,concluded that horizontal layer I fibers prefer a uniformdirection from anteromedial to posterolateral across thedorsolateral cortex. Although this discrepancy may berelated to interspecies differences, it is possible that asubset of all horizontal layer I fibers with affinity forcertain stains or differing degeneration rates do extendthrough layer I in a preferred direction. However, thehorizontal fibers in layer I from backward cortical projec-tions labeled in this study extended in all directions to allparts of the SI topography, and they shared the fine detailobserved in the omnidirectional horizontal layer I fiberslabeled by DiI. Because the majority of cells that werelabeled retrogradely by direct applications to layer Ioriginated in the cortex, it is likely that the horizontallayer I pattern labeled by DiI largely represents thepattern formed by the backward corticocortical inputs toSI.

In a related physiological study (Cauller and Connors,1994), we found that the synapses of long horizontal layer Iaxons can generate strong excitation of the pyramidal cellsin layers II, III, and V of SI that have distal apicaldendrites extending to layer I. We suggest that the conver-gent inputs to layer I can mediate more than a simple,tonic modulatory interaction between cortical areas. In-stead, the backward inputs to layer I mediate directactivation for purposes of associational interactions orsynchronization, as computational models of neocortexhave anticipated (Edelman, 1987; Rolls, 1989; for a collec-tion of theoretical papers, see Koch and Davis, 1994).

This study adds to a growing body of evidence thatsupports a new view of neocortical primary sensory areas.These areas should not be thought of simply as ‘‘receiving’’stations that relay sensory information to the ‘‘associative’’areas required for perception. The backward system ofcortical projections converges upon the specialized pri-mary sensory areas from each of their targets. Thisconnectional architecture makes primary areas the com-mon focus of outputs from all higher order cortical areasengaged in parallel sensory processes. This new view isconsistent with the apparent necessity of primary visualcortex for visual experience, despite evidence of higherorder visual processing in ‘‘blind sight’’ phenomena (Weisk-rantz 1990; see also Farah, 1990). Accordingly, a growingbody of evidence indicates that the same cortical areas thatare required for conscious sensation are also activated bymental processes in the absence of specific sensory inputs(Roland and Friberg, 1985; Farah, 1989; see also Llinasand Pare, 1991). The finding that excitation of SI layer Ipredicts touch-discrimination behavior in monkeys (Caullerand Kulics, 1991) and appears only during wakefulness

Fig. 5. Reciprocity and laminar origin of ipsilateral MI and SIIprojections to layer I of SI. A: Cell bodies in the lateral agranularcortex (MI) were labeled by retrograde transport from an applicationof DY in the forepaw area of SI (shown in B). This location in MI isshown in a coronal section approximately 2 mm anterior to Bregmaand 2 mm lateral to the midsagittal (scales in D and E are shown bythe rectangles in A). B: The application of DY to layer I in the forepawarea of SI (4 mm lateral and 0 mm posterior to Bregma) labeled cellsimmediately below, excluding layer IV (this site in the forepaw areawas verified by recording the response to contralateral forepawstimulation). C: Tetramethylrhodamine dextran amine (rDX) wasinjected into middle layers at the same site as the DY application in SI.Both B and C are at the same site from the same section, and theappropriate filters were used for each dye (scale bar in C also applies toB). Transport in A–I resulted from these SI dye applications. D: Fibersin MI (the same section shown in A and E) were labeled by anterogradetransport from the injection of rDX at the same site as the DYapplication in the forepaw area of SI. The location of these SIterminals in the midst of the MI cells that projected to layer I of SI isshown by the rectangles in A and E. E: Cells in layers II–VI of MI wereretrogradely labeled by the application of DY to layer I of SI. F: Densefiber terminations were labeled in lateral parietal cortex by antero-grade transport from the injection of rDX into the forepaw area of SI.The location of these terminals is indicated by the rectangles in G–I,which were all taken from the same section in the lateral cortex. G:The dense terminations shown in F were in the middle layers of areaSII and were identified as such by these anterograde projections fromthe forepaw site in SI. H: The cells in SII that were retrogradelylabeled by the DY application in layer I of SI surrounded the denseterminations of anterograde SI projections. I: The location of F–H(scales related to rectangles in I) was approximately 2 mm dorsal tothe rhinal fissure shown in the same section of the lateral cortex and1.6 mm posterior from Bregma.


Fig. 6. A: Fibers labeled by an injection of biotinylated dextranamine (bDX) into SII are traced in this coronal section. No fibers fromSII to SI projected between these areas superficial to layer V. B: ThebDX injection site did not penetrate the white matter and wasrestricted to the lateral parietal cortex at the same location identifiedas area SII (approximately 2 mm dorsal to the rhinal fissure, 1.6 mmposterior to Bregma) by anterograde transport from SI (Fig. 5).C: Coronal section through SI at the location framed in A demon-strates that fibers labeled from the SII bDX injection were most dense

in layer I, with less density in layers II/III and V, and minimal labelingin layers IV and VI. D: Coronal section through layer I of SI (samesection shown in C) at higher magnification shows the fibers labeled bythe bDX injection in SII cortex. E: This fluorescence photomicrographof a coronal section through layer I of SI following an injection of rDXinto SII (same coordinates used for C) shows that the concentration ofbackward fiber branches within layer I consisted of short segments inall orientations passing through this coronal plane.


Fig. 7. Backward fibers from a point in SII extended horizontallyin layer I across the SI barrel field. Top: Reconstruction showing theSI barrel field traced from tangential sections through layer IV labeledby cytochrome oxidase reaction. Bottom: Reconstruction showing thelayer I fibers labeled by anterograde transport from an injection ofbDX into the lateral parietal cortex (marked by an asterisk in SII)traced in the most superficial tangential sections. The pinholes

indicated by arrowheads were used to align the layer I fibers with theunderlying layer IV barrel field. The relation of these sections to thesurface landmark of the middle cerebral artery (MCA) is shown in themiddle tracing. The rectangles A–C indicated in the layer IV barrelfields correspond to the approximate locations (0.5 mm) in photomicro-graphs A–C of fibers in overlying layer I (white dots in rectanglescorrespond to orientation of letters in photos).


(Cauller and Kulics, 1988) supports the possibility thatconscious sensation depends upon backward activation ofprimary sensory cortex. For these reasons, the top-downactivation of primary sensory areas via convergent back-ward projections should be considered a plausible mecha-nism for binding together parallel streams of sensoryfeature analysis (Damasio, 1989; cf. Mesulam, 1990).Furthermore, the concentrated convergence of these back-ward inputs upon the distal apical dendrites in layer Iimplies that top-down neocortical influences are involvedin the control of eye and finger movements mediated bycorticobulbar projections from primary sensory areas forinteractive, sensory-oriented behavior (Cauller, 1995).

ACKNOWLEDGMENTS

We thank David Berson for guidance with anatomicaltechniques and Chris Lydon for technical assistance at

Brown University. This work was supported by a postdoc-toral fellowship from the NIH (NS08376) and a grant fromthe Whitehall Foundation to L.J.C and by grants from theNIH (NS25983) and the ONR (N00014-90-J-1701) to B.W.C.

LITERATURE CITED

Adinolfi, A.M. (1972) Morphogenesis of synaptic junctions in layers I and IIof the somatic sensory cortex. Exp. Neurol. 34:372–382.

Bear, M.F., K.M. Carnes, and F.E. Ebner (1985) An investigation ofcholinergic circuitry in cat striate cortex using acetylcholinesterasehistochemistry. J. Comp. Neurol. 234:411–430.

Beaulieu, C., and M. Colonnier (1985) A laminar analysis of the number ofround-asymmetrical and flat-symmetrical synapses on spines dendritictrunks and cell bodies in area 17 of the cat. J. Comp. Neurol.231:180–189.

Braitenberg, V., and A. Schuz (1991) Anatomy of the Cortex. Berlin:Springer-Verlag.

Fig. 8. Horizontal fibers in layer I of SI were labeled by rDXinjected into the lateral agranular frontal cortex at site A in MI (2 mmanterior and 2 mm lateral to Bregma) and into the lateral parietalcortex at sites B–D in SII. In the upper left brightfield photomicro-graph (tangential section through layer IV stained for cytochromeoxidase), the approximate locations (0.5 mm) of the fluorescencephotomicrographs are shown with respect to the underlying somatosen-sory barrel fields. Fibers in A1 and A2 were labeled by injection A.Fibers in B1 and B2 were labeled by injection B. Fibers in C werelabeled by injection C. Fibers in D1 and D2 were labeled by injectionD. Separate rats were used for each injection. The barrel field in this

figure was prepared from the rat of injection C. In each case, fiberswere not restricted to the region of the barrel field shown in examplemicrographs and no obvious topographic relationships between theinjected regions and SI were observed. Each fluorescence micrographshows rDX-labeled fibers within tangential sections (,30 µm thick) oflayer I (,0.15 mm below pia) except D1. The micrograph in D1 is froma tangential section through upper layer III and shows thick fibers cutin near cross section as they ascend to layer I. Horizontal fibers inlayer I could be traced for greater than 1 mm in length across SI, withno obvious orientation as a result of each of these injections.


Cauller, L.J. (1995) Layer I of primary sensory neocortex: Where top-downconverges upon bottom-up. Behav. Brain Res. 71:163–170.

Cauller, L.J., and B.W. Connors (1989) Origin and function of horizontallayer I afferents to rat SI neocortex. Soc. Neurosci. Abstr. 15:281.

Cauller, L.J., and B.W. Connors (1990) Horizontal layer I inputs to primarysomatosensory (barrelfield) neocortex of rat. Soc. Neurosci. Abstr.16:242.

Cauller, L.J., and B.W. Connors (1994) Synaptic physiology of horizontalafferents to layer I in slices of rat SI neocortex. J. Neurosci. 14:751–762.

Cauller, L.J., and A.T. Kulics (1988) A comparison of awake and sleepingcortical states by analysis of the somatosensory-evoked potential inpostcentral area 1 of rhesus monkey. Exp. Brain Res. 72:584–592.

Cauller, L.J., and A.T. Kulics (1991) The neural basis of the behaviorallyrelevant N1 component of the somatosensory-evoked potential in SIcortex of awake monkeys: Evidence that backward cortical projectionssignal conscious touch sensation. Exp. Brain Res. 84:607–619.

Chagnac-Amitai, Y., H.J. Luhmann, and D.A. Prince (1990) Burst generat-ing and regular spiking layer 5 pyramidal neurons of rat neocortex havedifferent morphological features. J. Comp. Neurol. 296:598–613.

Clancy, B., and L.J. Cauller (1993) Cortico-cortical projections betweenprimary and secondary somatosensory areas of rats: Backward projec-tions label layer I most heavily. Soc. Neurosci. Abstr. 19:1211.

Clancy, B., and L.J. Cauller (1994) Neuroanatomical evidence in SIneocortex of rats that widespread backward projections in layer I fromSII and MI converge with layer VII projections upon the distaldendrites of cortico-bulbar neurons. Soc. Neurosci. Abstr. 20:985.

Clancy, B., R. Brown, and L.J. Cauller (1995) Reciprocal ‘‘top-down’’projections to neocortical layer I of rat primary somatosensory andauditory areas converge from higher cortical areas with afferents frominterareal neurons of layer VII upon the distal dendrites of cortico-bulbar neurons. Soc. Neurosci. Abstr. 21:112.

Coogan, T.A., and A. Burkhalter (1990) Conserved patterns of corticocorti-cal connections define areal hierarchy in rat visual cortex. Exp. BrainRes. 80:49–53.

Damasio, A.R. (1989) The brain binds entities and events by multiregionalactivation from convergence zones. Neural Computation 1:123–132.

Deacon, T.W., D.-W. Wang, and A. Carpenter (1989) Similarities anddifferences in the laminar organization of corticocortical connections inrat as compared to monkey. Soc. Neurosci. Abstr. 15:283.

DeLima, A.D., and W. Singer (1986) Cholinergic innervation of the catstriate cortex: A choline acetyltransferase immunocytochemical analy-sis. J. Comp. Neurol. 250:324–338.

Eccles, J.C. (1981) The modular operation of the cerebral neocortexconsidered as the material basis of mental events. Neuroscience6:1839–1856.

Edelman, G.M. (1978) Group selection and phasic reentrant signalling: Atheory of higher brain function. In F.O. Schmitt (ed): The MindfulBrain. Cambridge, MA: MIT Press, pp. 51–100.

Edelman, G.M. (1987) Neural Darwinism. New York: Basic Books.Emson, P.C., and P. Lindvall (1979) Distribution of putative neurotransmit-

ters in the cortex. Neuroscience 4:1–30.Fabri, M., and H. Burton (1991) Ipsilateral cortical connections of primary

somatic sensory cortex in rats. J. Comp. Neurol. 311:405–424.Farah, M.J. (1989) The neural basis of mental imagery. Trends Neurosci.

12:395–399.Farah, M.J. (1990) Visual agnosia: Disorders of object recognition and what

they tell us about normal vision. Cambridge, MA: MIT Press.Felleman, D.J., and D.C. Van Essen (1991) Distributed hierarchical process-

ing in the primate cerebral cortex. Cereb. Cortex 1:1–47.Fleischhauer, K., and A. Laube (1977) A pattern formed by preferential

orientation of tangential fibres in layer I of the rabbit’s cerebral cortex.Anat. Embryol. 151:233–240.

Godement, P., J. Vanselow, S. Thanos, and F. Bonhoeffer (1987) A study indeveloping visual systems with a new method of staining neurones andtheir processes in fixed tissue. Development 101:697–713.

Herkenham, M. (1986) New perspectives on the organization and evolutionof nonspecific thalamocortical projections. In E.G. Jones and A. Peters(eds): Cerebral Cortex, Vol. 5. New York: Plenum Press, pp. 403–445.

Hubener, M., C. Schwarz, and J. Bolz (1990) Morphological types ofprojections neurons in layer 5 of cat visual cortex. J. Comp. Neurol.301:655–674.

Jones, E.G. (1981) Anatomy of cerebral cortex: Columnar input-outputorganization. In F.O. Schmitt, F.G. Worden, G. Adelman, and S.G.

Dennis (eds): The Organization of the Cerebral Cortex. Cambridge, MA:MIT Press, pp. 199–235.

Jones, E.G., and T.P.S. Powell (1968) The ipsilateral cortical connections ofthe somatic sensory areas in the cat. Brain Res. 9:71–94.

Jones, E.G., and T.P.S. Powell (1970) Electron microscopy of the somaticsensory cortex of the cat. II. The fine structure of layer I-II. Phil. Trans.R. Soc. London B 257:13–21.

Koch, C., and J.L. Davis (1994) Large-Scale Neuronal Theories of the Brain.Cambridge, MA: Bradford Books, MIT Press.

Koralek, K.A., J. Olavarria, and H.P. Killackey (1990) Areal and laminarorganization of corticocortical projections in the rat somatosensorycortex. J. Comp. Neurol. 299:133–150.

Kulics, A.T., and Cauller L.J. (1986) Cerebral cortical somatosensory-evoked potentials; multiple unit activity and current source-density:Their interrelationships and significance to somatic sensation as re-vealed by stimulation of the awake monkey’s hand. Exp. Br. Res.62:46–60.

Kulics, A.T., and L.J. Cauller (1988) Somatosensory neural events in awakemonkeys viewed from the multielectrode perspective. In J.S. Lund (ed):Organizing Principles of Sensory Processing: The Brain’s View of theWorld. Cambridge: Oxford University Press.

Larkman, A., and A. Mason (1990) Correlations between morphology andelectrophysiology of pyramidal neurons in slices of rat visual cortex. I.Establishment of cell classes. J. Neurosci. 10:1407–1414.

Lin, C.S., M.A. Nicolelis, J.S. Schneider, and J.K. Chapin (1990) A majordirect GABAergic pathway from zona incerta to neocortex. Science248:1553–1556.

Llinas, R.R., and D. Pare (1991) Of dreaming and wakefulness. Neurosci-ence 44:521–535.

Lysakowski, A., B.H. Wainer, D.B. Rye, G. Bruce, and L.B. Hersh (1986)Cholinergic innervation displays strikingly different laminar prefer-ences in several cortical areas. Neurosci. Lett. 64:102–108.

Martin, K.A.C. (1984) Neuronal circuits in cat striate cortex. In E.G. Jonesand A. Peters (eds): Cerebral Cortex, Vol. 2. New York: Plenum Press,pp. 241–284.

Mesulam, M.-M. (1978) Tetramethyl benzidine for horseradish peroxidaseneurohistochemistry: A noncarcinogenic blue reaction-product withsuperior sensitivity for visualizing neural afferents and efferents. J.Histochem. Cytochem. 26:106–117.

Mesulam, M.-M. (1990) Large-scale neurocognitive networks and distrib-uted processing for attention, language and memory. Ann. Neurol.28:597–613.

Mignard, M., and J.G. Malpeli (1991) Paths of information flow through thevisual cortex. Science 251:1249–1251.

Miller, M.W., N.L. Chiaia, and R.W. Rhodes (1990) Intracellular recordingand injection of corticospinal neurons in rat somatosensory cortex:Effect of prenatal exposure to ethanol. J. Comp. Neurol. 296:1–15.

Nicolelis, M.A., J.K. Chapin, and C.S. Lin (1991) Ontogeny of corticocorticalprojections of the rat somatosensory cortex. Somatosens. Motor Res.8:193–200.

Olucha, F., F. Martinez-Garcia, and C. Lopez-Garcia (1985) A new stabiliz-ing agent for the tetramethyl benzidine (TMB) reaction product in thehistochemical detection of horseradish peroxidase (HRP). J. Neurosci.Methods 13:131–138.

Pandya, D.N., and E.H. Yeterian (1985) Architecture and connections ofcortical association areas. In A. Peters and E.G. Jones (eds): CerebralCortex, Vol. 4. New York: Plenum Press, pp. 3–61.

Penny, G.R., K. Itoh, and I.T. Diamond (1982) Cells of different sizes in theventral nuclei project to different layers of the somatic cortex in the cat.Brain Res. 242:55–65.

Pockberger, H. (1991) Electrophysiological and morphological properties ofrat motor cortex neurons in vivo. Brain Res. 539:181–190.

Rausell, E., and C. Avendano (1985) Thalamocortical neurons projecting tosuperficial and to deep layers in parietal frontal and prefrontal regionsof the cat. Brain Res. 347:159–165.

Rockland, K.S., and A. Virga (1989) Terminal arbors of individual ‘‘feed-back’’ axons projecting from V2 to V1 in the Macaque monkey: A studyusing immunohistochemistry of anterogradely transported Phaseolusvulgaris-leucoagglutinin. J. Comp. Neurol. 285:54–72.

Roland, P.E., and L. Friberg (1985) Localization of cortical areas activatedby thinking. J. Neurophysiol. 53:1219–1243.

Rolls, E.T. (1989) Functions of neuronal networks in the hippocampus andneocortex in memory. In J.H. Byrne and W.O. Berry (eds): NeuralModels of Plasticity. San Diego: Academic Press, pp. 240–265.


Sandell, J.H. and P.H. Schiller (1982) Effect of cooling area 18 on striatecortex cells in the squirrel monkey. J. Neurophysiol. 48:38–48.

Scannell, J.W., C. Blakemore, and M.P. Young (1995) Analysis of connectiv-ity in the cat cerebral cortex. J. Neurosci. 15:1463–1483.

Schmued, L.C., and L. Heimer (1989) Circuitry of the extended amygdala:Introducing the use of fluorescent dextrans for rapid anterograde andretrograde tract tracing. Soc. Neurosci. Abstr. 15:306.

Squire, L.R., and S. Zola-Morgan (1991) The medial temporal lobe memorysystem. Science 253:1380–1386.

Swanson, L.W., and C. Kohler (1986) Anatomical evidence for directprojections from the entorhinal area to the entire cortical mantle in therat. J. Neurosci. 6:3010–3033.

Szentagothai, J. (1978) The neurons network of the cerebral cortex: Afunctional interpretation. Proc. R. Soc. London B 201:219–248.

Vaughan, D.W., and A. Peters (1973) A three dimensional study of layer I ofthe rat parietal cortex. J. Comp. Neurol. 149:355–370.

Vogt, B.A. (1991) The role of layer I in cortical function. In A. Peters (ed):Cerebral Cortex, Vol. 9. New York: Plenum Press, pp. 49–80.

Wallace, M.N. (1987) Histochemical demonstration of sensory maps in therat and mouse cerebral cortex. Brain Res. 418:178–182.

Weiskrantz, L. (1990) The Ferrier lecture 1989: Outlooks for blindsight:Explicit methodologies for implicit processes. Proc. R. Soc. London B239:247–278.

Welker, C. (1976) Receptive fields of barrels in the somatosensory neocortexof the rat. J. Comp. Neurol. 166:173–189.

Zeki, S., and S. Shipp (1988) The functional logic of cortical connections.Nature 353:311–317.

Zilles, K. (1985) The Cortex of the Rat. New York: Springer-Verlag.


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Backward cortical projections to primary somatosensory cortex in rats extend long horizontal axons in layer I