Cortical connections of subdivisions of inferior temporal cortex in squirrel monkeys

A multisensory perspective onto primate pulvinar functions

Perception in ambiguous environments relies on the combination of sensory information from various sources. Most associative and primary sensory cortical areas are involved in this multisensory active integration process. As a result, the entire cortex appears as heavily multisensory. In this review, we focus on the contribution of the pulvinar to multisensory integration. This subcortical thalamic nucleus plays a central role in visual detection and selection at a fast time scale, as well as in the regulation of visual processes, at a much slower time scale. However, the pulvinar is also densely connected to cortical areas involved in multisensory integration. In spite of this, little is known about its multisensory properties and its contribution to multisensory perception. Here, we review the anatomical and functional organization of multisensory input to the pulvinar. We describe how visual, auditory, somatosensory, pain, proprioceptive and olfactory projections are differentially organized across the main subdivisions of the pulvinar and we show that topography is central to the organization of this complex nucleus. We propose that the pulvinar combines multiple sources of sensory information to enhance fast responses to the environment, while also playing the role of a general regulation hub for adaptive and flexible cognition.

Feedforward and feedback connections and their relation to the cytox modules of V2 in Cebus monkeys

To study the circuitry related to the ventral stream of visual information processing and its relation to the cytochrome oxidase (CytOx) modules in visual area V2, we injected anterograde and retrograde cholera toxin subunit B (CTb) tracer into nine sites in area V4 in five Cebus apella monkeys. The injection site locations ranged from 2° to 10° eccentricity in the lower visual field representation of V4. Alternate cortical sections, cut tangentially to the pial surface or in the coronal plane, were stained for CTb immunocytochemistry or for CytOx histochemistry or for Nissl. Our results indicate that the V4-projecting cells and terminal-like labeling were located in interstripes and thin CytOx-rich stripes and avoided the CytOx-rich thick stripes in V2. The feedforward projecting cell bodies in V2 were primarily located in the supragranular layers and sparsely located in the infragranular layers, whereas the feedback projections (i.e., the terminal-like labels) were located in the supra- and infragranular layers. V4 injections of CTb resulted in labeling of the thin stripes and interstripes of V2 and provided an efficient method of distinguishing the V2 modules that were related to the ventral stream from the CytOx-rich thick stripes, related to the dorsal stream. In V2, there was a significant heterogeneity in the distribution of projections: feedforward projections were located in CytOx-rich thin stripes and in the CytOx-poor interstripes, whereas the feedback projections were more abundant in the thin stripes than in the interstripes. J. Comp. Neurol. 522:3091–3105, 2014.

Cortical input to the frontal pole of the marmoset monkey

We used fluorescent tracers to map the pattern of cortical afferents to frontal area 10 in marmosets. Dense projections originated in several subdivisions of orbitofrontal cortex, in the medial frontal cortex (particularly areas 14 and 32), and in the dorsolateral frontal cortex (particularly areas 8Ad and 9). Major projections also stemmed, in variable proportions depending on location of the injection site, from both the inferior and superior temporal sensory association areas, suggesting a degree of audiovisual convergence. Other temporal projections included the superior temporal polysensory cortex, temporal pole, and parabelt auditory cortex. Medial area 10 received additional projections from retrosplenial, rostral calcarine, and parahippocampal areas, while lateral area 10 received small projections from the ventral somatosensory and premotor areas. There were no afferents from posterior parietal or occipital areas. Most frontal connections were balanced in terms of laminar origin, giving few indications of an anatomical hierarchy. The pattern of frontopolar afferents suggests an interface between high-order representations of the sensory world and internally generated states, including working memory, which may subserve ongoing evaluation of the consequences of decisions as well as other cognitive functions. The results also suggest the existence of functional differences between subregions of area 10.

Uncovering the visual "alphabet": advances in our understanding of object perception

The ability to rapidly and accurately recognize visual stimuli represents a significant computational challenge. Yet, despite such complexity, the primate brain manages this task effortlessly. How it does so remains largely a mystery. The study of visual perception and object recognition was once limited to investigations of brain-damaged individuals or lesion experiments in animals. However, in the last 25years, new methodologies, such as functional neuroimaging and advances in electrophysiological approaches, have provided scientists with the opportunity to examine this problem from new perspectives. This review highlights how some of these recent technological advances have contributed to the study of visual processing and where we now stand with respect to our understanding of neural mechanisms underlying object recognition.

Connections of the dorsomedial visual area: pathways for early integration of dorsal and ventral streams in extrastriate cortex

The dorsomedial area (DM), a subdivision of extrastriate cortex characterized by heavy myelination and relative emphasis on peripheral vision, remains the least understood of the main targets of striate cortex (V1) projections in primates. Here we placed retrograde tracer injections encompassing the full extent of this area in marmoset monkeys, and performed quantitative analyses of the numerical strengths and laminar patterns of its afferent connections. We found that feedforward projections from V1 and from the second visual area (V2) account for over half of the inputs to DM, and that the vast majority of the remaining connections come from other topographically organized visual cortices. Extrastriate projections to DM originate in approximately equal proportions from adjacent medial occipitoparietal areas, from the superior temporal motion-sensitive complex centered on the middle temporal area (MT), and from ventral stream-associated areas. Feedback from the posterior parietal cortex and other association areas accounts for <10% of the connections. These results do not support the hypothesis that DM is specifically associated with a medial subcircuit of the dorsal stream, important for visuomotor integration. Instead, they suggest an early-stage visual-processing node capable of contributing across cortical streams, much as V1 and V2 do. Thus, although DM may be important for providing visual inputs for guided body movements (which often depend on information contained in peripheral vision), this area is also likely to participate in other functions that require integration across wide expanses of visual space, such as perception of self-motion and contour completion.

Cortical connections of area V4 in the macaque

To determine the locus, full extent, and topographic organization of cortical connections of area V4 (visual area 4), we injected anterograde and retrograde tracers under electrophysiological guidance into 21 sites in 9 macaques. Injection sites included representations ranging from central to far peripheral eccentricities in the upper and lower fields. Our results indicated that all parts of V4 are connected with occipital areas V2 (visual area 2), V3 (visual area 3), and V3A (visual complex V3, part A), superior temporal areas V4t (V4 transition zone), MT (medial temporal area), and FST (fundus of the superior temporal sulcus [STS] area), inferior temporal areas TEO (cytoarchitectonic area TEO in posterior inferior temporal cortex) and TE (cytoarchitectonic area TE in anterior temporal cortex), and the frontal eye field (FEF). By contrast, mainly peripheral field representations of V4 are connected with occipitoparietal areas DP (dorsal prelunate area), VIP (ventral intraparietal area), LIP (lateral intraparietal area), PIP (posterior intraparietal area), parieto-occipital area, and MST (medial STS area), and parahippocampal area TF (cytoarchitectonic area TF on the parahippocampal gyrus). Based on the distribution of labeled cells and terminals, projections from V4 to V2 and V3 are feedback, those to V3A, V4t, MT, DP, VIP, PIP, and FEF are the intermediate type, and those to FST, MST, LIP, TEO, TE, and TF are feedforward. Peripheral field projections from V4 to parietal areas could provide a direct route for rapid activation of circuits serving spatial vision and spatial attention. By contrast, the predominance of central field projections from V4 to inferior temporal areas is consistent with the need for detailed form analysis for object vision.

Subdivisions of inferior temporal cortex in squirrel monkeys make dissociable contributions to visual learning and memory

Inferior temporal cortex of squirrel monkeys consists of caudal (ITC), intermediate (ITI), and rostral (ITR) subdivisions, possibly homologous to TEO, posterior TE, and anterior TE of macaque monkeys. The present study compared visual learning in squirrel monkeys with ablations of ITC; ITI and ITR (group ITRd); or ITI, ITR, and more ventral cortex, including perirhinal cortex (group ITR+), with visual learning in unoperated controls. The ITC monkeys had significant impairments on pattern discriminations and milder deficits on delayed non-matching to sample (DNMS) of objects. The ITRd monkeys had deficits on some pattern discriminations but not on DNMS. The ITRd monkeys were significantly impaired on DNMS and some pattern discriminations. These results are similar to those found in macaques and support the proposed homologies.

Quantitative Analysis of the Corticocortical Projections to the Middle Temporal Area in the Marmoset Monkey: Evolutionary and Functional Implications

The connections of the middle temporal area (MT) were investigated in the marmoset, one of the smallest primates. Reflecting the predictions of studies that modeled cortical allometric growth and development, we found that in adult marmosets MT is connected to a more extensive network of cortical areas than in larger primates, including consistent connections with retrosplenial, cingulate, and parahippocampal areas and more widespread connections with temporal, frontal, and parietal areas. Quantitative analyses reveal that MT receives the majority of its afferents from other motion-sensitive areas in the temporal lobe and from the occipitoparietal transition areas, each of these regions containing approximately 30% of the projecting cells. Projections from the primary visual area (V1) and the second visual area (V2) account for approximately 20% of projecting neurons, whereas "ventral stream" and higher-order association areas form quantitatively minor projections. A relationship exists between the percentage of supragranular layer neurons forming the projections from different areas and their putative hierarchical rank. However, this relationship is clearer for projections from ventral stream areas than it is for projections from dorsal stream or frontal areas. These results provide the first quantitative data on the connections of MT and extend current understanding of the relationship between cortical anatomy and function in evolution.

Brain maps, great and small: lessons from comparative studies of primate visual cortical organization

In this paper, we review evidence from comparative studies of primate cortical organization, highlighting recent findings and hypotheses that may help us to understand the rules governing evolutionary changes of the cortical map and the process of formation of areas during development. We argue that clear unequivocal views of cortical areas and their homologies are more likely to emerge for "core" fields, including the primary sensory areas, which are specified early in development by precise molecular identification steps. In primates, the middle temporal area is probably one of these primordial cortical fields. Areas that form at progressively later stages of development correspond to progressively more recent evolutionary events, their development being less firmly anchored in molecular specification. The certainty with which areal boundaries can be delimited, and likely homologies can be assigned, becomes increasingly blurred in parallel with this evolutionary/developmental sequence. For example, while current concepts for the definition of cortical areas have been vindicated in allowing a clarification of the organization of the New World monkey "third tier" visual cortex (the third and dorsomedial areas, V3 and DM), our analyses suggest that more flexible mapping criteria may be needed to unravel the organization of higher-order visual association and polysensory areas.

Reappraisal of DL/V4 boundaries based on connectivity patterns of dorsolateral visual cortex in macaques

We placed injections of 3-5 distinguishable tracers in different dorsolateral locations in the visual cortex of four macaque monkeys to help define the extent of the dorsolateral visual complex (DL) commonly known as area V4. Injections well within DL/V4 region labeled neurons in V2, V3, MT, IT, and sometimes V1. In contrast, injections in caudal area 7a dorsal to current descriptions of DL/V4 produced a different pattern of labeled neurons largely involving posterior parietal and adjoining occipital cortex, as well as cortex of the medial wall. Injections placed in the dorsal prelunate cortex (DP), near the expected location of the dorsal border of DL/V4, labeled neurons in a third pattern, including regions of the posterior parietal and occipital cortex, inferior temporal (IT) cortex, and sometimes parts of dorsal area V2, DL/V4 complex and MT. Injections placed near or ventral to previous estimates of the ventral border of the rostral divisions of DL (DLr) and near the expected rostroventral border of V4 with TEO labeled cells in a pattern distinctively different from either central DL/V4 injections or those dorsal to DL/V4. Injections placed rostroventral to DL/V4 labeled neurons over a large extent of the IT cortex, while failing to label neurons in V1, V2 and MT. Injections that partially involved the rostroventral border of DL/V4 produced a similar pattern of labeled neurons, but also labeled a few cells in ventral V1 and V2, as well as many in DL/V4. Dorsal and rostroventral injections also labeled different regions of the prefrontal cortex, but only DL/V4 injections that included area DP labeled neurons in the prefrontal cortex. The results revealed contrasting and transitional connection patterns for four regions of the dorsolateral visual cortex, and they provided evidence for the locations of dorsal and rostroventral borders of the DL/V4 complex.

Pulvinar and other subcortical connections of dorsolateral visual cortex in monkeys

The present study used injections of neuroanatomical tracers to determine the subcortical connections of the caudal and rostral subdivisions of the dorsolateral area (DL) and the middle temporal crescent area (MT(C)) in owl monkeys (Aotus trivirgatus), squirrel monkeys (Saimiri sciureus), and macaque monkeys (Macaca fascicularis and M. radiata). Emphasis was on connections with the pulvinar. Patterns of corticopulvinar connections were related to subdivisions of the inferior pulvinar (PI) defined by histochemical or immunocytochemical architecture. Connections of DL/MT(C) were with the PI subdivisions, PICM, PICL, and PIp; the lateral pulvinar (PL); and, more sparsely, the lateral portion of the medial pulvinar (PM). In squirrel monkeys, there was a tendency for caudal DL to have stronger connections with PICL than PICM and for rostral DL/MT(C) to have stronger connections with PICM than PICL. In all three primates, DL/MT(C) had reciprocal connections with the pulvinar and claustrum; received afferents from the locus coeruleus, dorsal raphe, nucleus annularis, central superior nucleus, pontine reticular formation, lateral geniculate nucleus, paracentral nucleus, central medial nucleus, lateral hypothalamus, basal nucleus of the amygdala, and basal nucleus of Meynert/substantia innominata; and sent efferents to the pons, superior colliculus, reticular nucleus, caudate, and putamen. Projections from DL/MT(C) to the nucleus of the optic tract were also observed in squirrel and owl monkeys. Similarities in the subcortical connections of the dorsolateral region, especially those with the pulvinar, provide further support for the conclusion that the DL regions are homologous in the three primate groups.

Hippocampal tyrosine kinase A receptors are restricted primarily to presynaptic vesicle clusters

Adult septohippocampal cholinergic neurons are dependent on trophic support for normal functioning and survival; these effects are largely mediated by the tyrosine kinase A receptor (TrkA), which binds its ligand, nerve growth factor (NGF), with high affinity. To determine the subcellular localization of TrkA within septohippocampal terminal fields, two rabbit polyclonal antisera to the extracellular domain of TrkA were localized immunocytochemically in rat dentate gyrus by light and electron microscopy. By light microscopy, TrkA immunoreactivity was found mostly in fine, varicose fibers primarily in the hilus and, to a lesser extent, in the granule cell and molecular layers. By electron microscopy, the central and infragranular regions of the hilus contained the highest densities of TrkA-immunoreactive profiles. Most TrkA-labeled profiles were axons (31% of 3,473), axon terminals (20%), and glia (38%); fewer were dendrites (6%), dendritic spines (5%), and granule cell and interneuron somata (<1%). TrkA immunolabeling in axons and axon terminals was discrete, often concentrated in patches of small synaptic vesicles that were adjacent to somatic and dendritic profiles. TrkA-labeled terminals formed both asymmetric and symmetric synapses, primarily with dendritic shafts and spines. TrkA-immunoreactive glial profiles frequently apposed terminals contacting dendritic spines. The findings that presynaptic profiles contain TrkA immunolabeling in sites of vesicle accumulation suggest that NGF binding to TrkA may influence transmitter release. The presence of TrkA immunoreactivity in somata, dendrites, and glia further suggests that cells within the dentate gyrus may take up NGF.

Cortical connections of the dorsomedial visual area in new world owl monkeys (Aotus trivirgatus) and squirrel monkeys (Saimiri sciureus)

The dorsomedial visual area (DM) is an extrastriate area that was originally described in owl monkeys as a complete representation of the visual hemifield in a heavily myelinated wedge of cortex just rostral to dorsomedial visual area V2. More recently, connections of DM in owl monkeys have been described (Krubitzer and Kaas [1993] J. Comp. Neurol 334:497-528). As part of an effort to determine whether DM exists in other primates, we compared the architecture, connections, and visual topography of DM in owl monkeys and the presumptive DM in squirrel monkeys. In both species of New World monkeys, the DM region was more heavily myelinated than adjacent cortex, and this region was connected with the first and second visual areas, the middle temporal area (MT), the medial area, the ventral posterior parietal area, the dorsointermediate area, the dorsolateral area, the ventral posterior and ventral anterior areas, the medial superior temporal area, the fundal area of the superior temporal sulcus, the inferior temporal cortex, and frontal cortex in or near the frontal eye field. In squirrel monkeys, both blob and interblob regions of V1 contributed equally to DM, whereas the blob regions provided most of the projections to V1 in owl monkeys. In squirrel monkeys, connections were also found with cortex on the ventral surface in the ventral occipital temporal sulcus. In owl monkeys and squirrel monkeys, connections were with both the upper and lower visual field representations in V1, V2, and MT, demonstrating that DM contains a complete representation of the visual field. These similarities in architecture, connections, and retinotopy argue that DM is a visual area of both owl and squirrel monkeys.

Convergence of limbic input to the cingulate motor cortex in the rhesus monkey

Limbic system influences on motor behavior seem widespread, and could range from the initiation of action to the motivational pace of motor output. Motor abnormalities are also a common feature of psychiatric illness. Several subcortical limbic-motor entry points have been defined in recent years, but cortical entry points are understood poorly, despite the fact that a part of the limbic lobe, the cingulate motor cortex (area 24c or M3, and area 23c or M4), contributes axons to the corticospinal pathway. Using retrograde and anterograde tracers in rhesus monkeys, we investigated the ipsilateral limbic input to area 24c and adjacent area 23c. Limbic cortical input to areas 24c and 23c arise from cingulate areas 24a, 24b, 23a, 23b, and 32, retrosplenial areas 30 and 29, and temporal areas 35, TF and TH. Areas 24c and 23c were also interconnected strongly. The dysgranular part of the orbitofrontal cortex and insula projects primarily to area 24c while the granular part of the orbitofrontal cortex and insula projects primarily to area 23c. Afferents from cingulate area 25, the retrocalcarine cortex, temporal pole, entorhinal cortex, parasubiculum, and the medial part of area TH target primarily or only area 24c. Our findings indicate that a variety of telencephalic limbic afferents converge on cortex lining the lower bank and fundus of the anterior part of the cingulate sulcus. Because it is known that this cortex gives rise to axons ending in the spinal cord, facial nucleus, pontine gray, red nucleus, putamen, and primary and supplementary motor cortices, we suggest that the cingulate motor cortex forms a strategic cortical entry point for limbic influence on the voluntary motor system.

Collateralized divergent feedback connections that target multiple cortical areas

Nonreciprocal feedback connections from ventromedial areas TE and TF have previously been reported to visual areas V1 and V2 (Kennedy and Bullier, 1985; Rockland and Van Hoesen, 1994). The present report confirms these earlier observations by utilizing anterograde label in conjunction with serial section analysis. Furthermore, it directly demonstrates the divergent configuration and range of these terminal fields. Thirteen axons were analyzed from ventromedial TE (4) or area TF (9) to occipitotemporal areas, and two from area TF to the upper bank of the intraparietal sulcus (IPS). All these axons have narrow, elongated fields that range from 4.0-21.0 mm. Terminations are distributed linearly along the axon or, in some cases, concentrated in irregularly spaced clusters. Most of these axons have terminations concentrated in layer 1. The two axons in the IPS have a bistratified terminal distribution (in layers 1-3 and 6) in their anterior field, but a distinctly different laminar pattern (with terminations concentrated in layer 1) in their distal 2.0 mm. These fields probably correspond to different areas, most likely MIP and PO. Axons projecting from higher order to early visual areas may contribute to extraperceptual, complex processes within area V1, such as activation in response to visual imagery, and are a possible substrate for synchronous linkage of spatially discrete assemblies of neurons. In summary, these results demonstrate 1) that some neurons in ventromedial TE and TF are in direct communication with early visual areas, including V1 and V2, and 2) that some feedback axons target several areas, sometimes with different laminar termination patterns. These results emphasize that cortical areas are interrelated by multiple direct and indirect pathways, not all of which are strictly hierarchical.

Topographic patterns of V2 cortical connections in macaque monkeys

Patterns of connections of dorsal and ventral portions of the second visual area (V2) were used to evaluate and extend current theories of cortical organization and processing streams in macaque monkeys. Injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and up to four different fluorochromes in V2 labeled neurons and terminations in V2 and in 1) caudal (DLc) and rostral (DLr) subdivisions of dorsolateral cortex between V2 and the middle temporal area (MT); 2) regions we define as dorsomedial (DM) and dorsointermediate (DI) areas; 3) MT, medial superior temporal area (MST), and fundal superior temporal area (FST); 4) the dorsal part of inferior temporal (TEO) cortex; and 5) two locations in posterior parietal cortex. The largest extrastriate connection zone was DLc, which occupied the caudal one-third to one-half of the fourth visual area (V4) region of other proposals. Based on the connection pattern, foveal vision in DLc is represented adjacent to foveal vision in V2, with the lower quadrant represented dorsally and the upper quadrant ventrally, as in V2, but within a much less extensive region of cortex. The sparser connections of DLr formed a more compressed but parallel visuotopic pattern. A third visuotopic pattern of connections was located in a moderately myelinated region of cortex just rostral to dorsomedial V2. Whereas the region would include parts of dorsal visual area 3 (V3), V3a, and possibly other areas of other proposals, we interpret the connection pattern as reflecting a dorsomedial visual area, DM, with foveal vision represented caudolaterally and other parts of the lower and upper quadrants represented more medially and rostrally. A fourth pattern of label in dorsointermediate cortex suggested the location and organization of another visual area (DI). Most of a fifth connection pattern with MT was congruent with the known visuotopic organization of MT area, but visuotopically mismatched foci of connections were observed as well. Sparser foci of label in MST suggested a rostrodorsal representation of foveal vision, with paracentral vision represented more caudally. Separate dorsal and ventral foci of label in FST were consistent with previous evidence for dorsal (FSTd) and ventral (FSTv) visual areas. Finally, connections with TEO and posterior parietal cortex were sparse. Our results suggest that much of visual cortex organization is similar in New and Old World monkeys.

Two types of corticopulvinar terminations: round (type 2) and elongate (type 1)

Corticopulvinar axons were anterogradely labeled by Phaseolus vulgaris-leucoagglutinin injections in the occipitotemporal cortex of the macaque to determine quantitative parameters of divergence and convergence, arbor size and shape, and distribution of terminal specializations. Forty individual axons were analyzed by serial section reconstruction and divided into two major groups. The majority of axons have numerous (typically 500-1,000) small, spinous endings (boutons terminaux). These axons have terminal fields that are beam-like or elongated (E, corresponding to classical type 1) and highly divergent (1.0-3.0 mm). These frequently innervate several of the traditionally designated pulvinar subdivisions; namely inferior pulvinar (PI) and the ventral part of interal pulvinar (PL); medial pulvinar (PM) and dorsal PL, and (one axon) PM, dorsal PL, and PI. Some axons, however (R or round, corresponding to classical type 2), have a small number (typically 70-160) of primarily large, beaded endings (boutons en passant), which concentrate in sharply delimited, round arbors (diameters 100-125 microns). R axons appear to be larger caliber than E axons (1.0-1.5 microns vs. 0.5-1.0 micron, respectively). These differences in phenotype are probably associated with distinct types of projection neurons. In visual areas, corticopulvinar terminations are reported to originate from pyramidal cell subpopulations in layer 5. Indirect evidence, presented here, suggests that the more numerous medium-sized neurons give rise to E axons, and the sparser giant pyramids give rise to R corticopulvinar axons. If this is correct, corticopulvinar connectivity may be involved in multiple transformations. Spatially, axons of giant neurons (with basal dendrites that collect intracortically from a disc-like area, about 1.0 mm in diameter) converge onto a small number of pulvinar neurons. Axons of medium neurons (with basal dendrites that occupy a small intracortical disc, about 0.3 mm in diameter) diverge over 1.0-3.0 mm in the pulvinar and may form many contacts. Giant neurons, although numerically few in relation to medium pyramids (1 or 2: 50?), are likely to have distinctive membrane properties (functionally equivalent to bursting neurons?). Their larger boutons and axon caliber may be associated with a faster transmission that compensates for their small numbers. In primates, the E and R duality does not characterize cortical projections to the caudate, lateral geniculate nucleus, pons, or superior colliculus and thus may be essentially linked to pulvinar-specific processes.

Qualitative and quantitative features of axons projecting from caudal to rostral inferior temporal cortex of squirrel monkeys

On the basis of cortical and subcortical connections and architectonics, inferior temporal (IT) cortex of squirrel monkeys consists of a caudal region, ITC, with dorsal (ITCd) and ventral (ITCv) subdivisions; a rostral region, ITR; and possibly a third region intermediate to ITC and ITR, ITI (Weller & Steele, 1992; Steele & Weller, 1993). The present study qualitatively and quantitatively examined the terminal arborizations of 26 axons in ITR and ITI labeled by injections of biocytin or, in one case, horseradish peroxidase, in ITCv. The majority of axons gave rise to a single terminal arbor, with a small number branching into two overlapping or nearby arbors. Presumptive terminal specializations consisted of rounded, bead-like swellings, most often located en passant. All axons terminated in layer 4 of cortex, and most had additional terminations in layers 3 and 5. The total extent of each axon's terminal arbor was 125-750 microns dorsoventrally (mean = 360.6 microns) and 150-725 microns anteroposteriorly (mean = 328.1 microns; all values uncorrected for shrinkage). In most axons, especially those with larger terminal fields, boutons were not uniformly distributed, but formed 2-4 clumps (mean = 2.2), with a mean width of 149 microns, separated by narrower regions of fewer boutons. Based on a cluster analysis of characteristics of the 26 axons, axons projecting from caudal (ITCv) to rostral (ITR or ITI) IT cortex of squirrel monkeys comprised three groups that we called Type I, Type II, and Type III. Type I axons, the smallest in area extent of terminal arbor, terminated predominantly in dorsal ITR. Type III axons, largest in areal extent, and Type II axons, intermediate in areal extent, terminated in ventral ITR and throughout ITI. The three classes of axons may correspond to different types of visual information entering rostral IT cortex. The clumping of boutons suggests that individual axons terminate in limited patches within their terminal fields.

Neurofilament protein defines regional patterns of cortical organization in the macaque monkey visual system: a quantitative immunohistochemical analysis

Visual function in monkeys is subserved at the cortical level by a large number of areas defined by their specific physiological properties and connectivity patterns. For most of these cortical fields, a precise index of their degree of anatomical specialization has not yet been defined, although many regional patterns have been described using Nissl or myelin stains. In the present study, an attempt has been made to elucidate the regional characteristics, and to varying degrees boundaries, of several visual cortical areas in the macaque monkey using an antibody to neurofilament protein (SMI32). This antibody labels a subset of pyramidal neurons with highly specific regional and laminar distribution patterns in the cerebral cortex. Based on the staining patterns and regional quantitative analysis, as many as 28 cortical fields were reliably identified. Each field had a homogeneous distribution of labeled neurons, except area V1, where increases in layer IVB cell and in Meynert cell counts paralleled the increase in the degree of eccentricity in the visual field representation. Within the occipitotemporal pathway, areas V3 and V4 and fields in the inferior temporal cortex were characterized by a distinct population of neurofilament-rich neurons in layers II-IIIa, whereas areas located in the parietal cortex and part of the occipitoparietal pathway had a consistent population of large labeled neurons in layer Va. The mediotemporal areas MT and MST displayed a distinct population of densely labeled neurons in layer VI. Quantitative analysis of the laminar distribution of the labeled neurons demonstrated that the visual cortical areas could be grouped in four hierarchical levels based on the ratio of neuron counts between infragranular and supragranular layers, with the first (areas V1, V2, V3, and V3A) and third (temporal and parietal regions) levels characterized by low ratios and the second (areas MT, MST, and V4) and fourth (frontal regions) levels characterized by high to very high ratios. Such density trends may correspond to differential representation of corticocortically (and corticosubcortically) projecting neurons at several functional steps in the integration of the visual stimuli. In this context, it is possible that neurofilament protein is crucial for the unique capacity of certain subsets of neurons to perform the highly precise mapping functions of the monkey visual system.

Perirhinal and parahippocampal cortices of the macaque monkey: cortical afferents

Neuropsychological studies have recently demonstrated that the macaque monkey perirhinal (areas 35 and 36) and parahippocampal (areas TH and TF) cortices contribute importantly to normal memory function. Unfortunately, neuroanatomical information concerning the cytoarchitectonic organization and extrinsic connectivity of these cortical regions is meager. We investigated the organization of cortical inputs to the macaque monkey perirhinal and parahippocampal cortices by placing discrete injections of the retrograde tracers fast blue, diamidino yellow, and wheat germ agglutinin conjugated to horseradish peroxidase throughout these areas. We found that the macaque monkey perirhinal and parahippocampal cortices receive different complements of cortical inputs. The major cortical inputs to the perirhinal cortex arise from the unimodal visual areas TE and rostral TEO and from area TF of the parahippocampal cortex. The perirhinal cortex also receives projections from the dysgranular and granular subdivisions of the insular cortex and from area 13 of the orbitofrontal cortex. In contrast, area TF of the parahippocampal cortex receives its strongest input from more caudal visual areas V4, TEO, and caudal TE, as well as prominent inputs from polymodal association cortices, including the retrosplenial cortex and the dorsal bank of the superior temporal sulcus. Area TF also receives projections from areas 7a and LIP of the posterior parietal lobe, insular cortex, and areas 46, 13, 45, and 9 of the frontal lobe. As with area TF, area TH receives substantial projections from the retrosplenial cortex as well as moderate projections from the dorsal bank of the superior temporal sulcus; unlike area TF, area TH receives almost no innervation from areas TE and TEO. It does, however, receive relatively strong inputs from auditory association areas on the convexity of the superior temporal gyrus.

Parietal, temporal, and occipital projections to cortex of the superior temporal sulcus in the rhesus monkey: a retrograde tracer study

The afferent cortical connections of individual cytoarchitectonic areas within the superior temporal sulcus (STS) of the rhesus monkey were studied by retrograde tracer techniques, including double tracer experiments. Rostral superior temporal polysensory (STP) cortex (area TPO-1) receives input from the rostral superior temporal gyrus (STG), cortex of the circular sulcus, and parahippocampal gyrus (PHG) (areas 35, TF, and TL). Mid-STP cortex (areas TPO-2 and -3) has input from the mid-STG, cortex of the mid-circular sulcus, caudal inferior parietal lobule (IPL), cingulate gyrus (areas, 23, 24, retrosplenial cortex), and mid-PHG (areas 28, TF, TH, and TL). Caudal STP cortex (area TPO-4) has afferent connections with the caudal STG, cortex of the caudal insula and caudal circular sulcus, caudal IPL, lower bank of the intraparietal sulcus (IPS), medial parietal lobe, cingulate gyrus, and mid- and caudal PHG (areas TF, TH, TL; prostriate area). The most rostral cortex of the lower bank of the STS (areas TEa and TEm), a presumed visual association area, receives input from the rostral inferotemporal (IT) region; more caudal portions of areas TEa and TEm have afferent connections with the caudal IT region, PHG, preoccipital gyrus, and cortex of the lower bank of the IPS.

Divergent feedback connections from areas V4 and TEO in the macaque

Extrastriate areas TEO and V4 have been associated with form and color vision. Area V4 has also been suggested to participate in processes concerned with attention, stimulus salience, and perceptual learning. In a continuing effort to elucidate the connectional interactions and microcircuitry of these areas, we describe in this report the pattern of feedback connections from TEO and V4. Connections were demonstrated by injections of the high-resolution anterograde tracers PHA-L or biocytin and further analyzed by reconstruction of 25 individual axons through serial sections. This analysis yielded several new results: (1) Both areas TEO and V4 have widespread feedback connections (defined by their preferential termination in layer 1 and avoidance of layer 4). From TEO, there are dense projections to area V4 and moderate ones to V2 and V1. From V4, there are dense projections to V2 and moderate ones to V3 and V1. (2) Terminal fields span large territories in area V1, up to 6.0 mm in the case of axons originating from TEO; up to 5.0 mm in the case of axons originating from V4. In V2, fields tend to be smaller, between 3.0-5.0 mm. (3) Many axons from TEO and some from V4 have terminations in both areas V1 and V2. (4) Because individual terminal clusters and segments are often larger than cytochrome oxidase compartments, especially in V1, we suggest they may not be correlated with this compartmental organization. These results are consistent with the hypothesis that feedback connections may contribute to processes other than perceptual discrimination.

Cortical projections to anterior inferior temporal cortex in infant macaque monkeys

Inferior temporal (IT) cortex is a "high-order" region of extrastriate visual cortex important for visual form perception and recognition in adult primates. The pattern of cortical afferents from both ipsilateral and contralateral hemispheres to anterior IT cortex was determined in infant macaque monkeys 7-18 weeks of age following injections of wheat-germ agglutinin-HRP. Within the ipsilateral hemisphere, the locations and laminar distribution of labeled cells were similar to those observed after comparable injections in adult monkeys. Specifically, ipsilateral afferents derived from visual areas V4, TEO, anterior and posterior IT, and STP, from parahippocampal, perirhinal, and parietal zones, and from several anterior zones including lateral and ventral frontal cortex, the insula, and cingulate cortex. Within the contralateral hemisphere, we observed labeled cells in homotopic regions of IT and in parahippocampal and perirhinal areas, as has been reported for adult monkeys. However, we also identified additional contralateral regions not previously known to provide input to anterior IT, including lateral and ventral frontal cortex, cingulate cortex, and STP. Overall, the strongest and most widespread projections from outside the temporal lobe were found in the youngest monkey, suggesting that some of these projections may represent transient circuitry necessary for the development of complex visual response properties in anterior IT.

Subcortical connections of inferior temporal areas TE and TEO in macaque monkeys

To investigate the subcortical connections of inferior temporal cortex, we injected its anterior and posterior portions (Bonin and Bailey's cytoarchitectonic areas TE and TEO, respectively) in 6 rhesus monkeys with retrograde and anterograde tracers. The results indicate that both areas TE and TEO receive nonreciprocal inputs from several thalamic nuclei, including paracentralis, ventralis anterior, centralis, and limitans, and that TE also receives input from reuniens. Additional nonreciprocal inputs to both areas arise from the hypothalamus, basal nucleus of Meynert, dorsal and median raphe, locus coeruleus, and reticular formation. TE and TEO are reciprocally connected with the lateral, medial, and inferior nuclei of the pulvinar and with the ventral portion of the claustrum. The main subcortical nonreciprocal output from TE and TEO is to the striatum and from TEO to the superior colliculus. TE also sends a very limited projection to nucleus medialis dorsalis magnocellularis of the thalamus. Although the connections of areas TE and TEO are overlapping in most subcortical structures, they are partially segregated in the pulvinar, the reticular nucleus of the thalamus, and the striatum. Specifically, relative to those of TE, the projections of TEO are located more laterally in the medial, lateral, and inferior nuclei of the pulvinar, more ventrally in the reticular nucleus, and more caudally in both the ventral putamen and tail and head of the caudate nucleus.

Cortical connections of inferior temporal area TEO in macaque monkeys

In macaque monkeys, lesions involving the posterior portion of the inferior temporal cortex, cytoarchitectonic area TEO, produce a severe impairment in visual pattern discrimination. Recently, this area has been shown to contain a complete, though coarse, representation of the contralateral visual field (Boussaoud, Desimone, and Ungerleider: J. Comp. Neurol. 306:554-575, '91). Because the inputs and outputs of area TEO have not yet been fully described, we injected a variety of retrograde and anterograde tracers into 11 physiologically identified sites within TEO of seven rhesus monkeys and analyzed the areal and laminar distribution of its cortical connections. Our results show that TEO receives feedforward, topographically organized inputs from prestriate areas V2, V3, and V4. Additional sparser feedforward inputs arise from areas V3A, V4t, and MT. Each of these inputs is reciprocated by a feedback projection from TEO. TEO was also found to have reciprocal intermediate-type connections with the fundus of the superior temporal area (area FST), cortex in the most posteromedial portion of the superior temporal sulcus (the posterior parietal sulcal zone [area PP]), cortex in the intraparietal sulcus (including the lateral intraparietal area [area LIP]), the frontal eye field, and area TF on the parahippocampal gyrus. The connections with V3A, V4t, and PP were found only after injections in the peripheral field representations of TEO. Finally, TEO was found to project in a feedforward pattern to area TE and to areas anterior to FST on the lateral bank and floor of the superior temporal sulcus (areas TEm, TEa, and IPa, Seltzer and Pandya: Brain Res. 149:1-24, '78), all of which send feedback projections to TEO. Feedback projections also arise from parahippocampal area TH, and areas TG, 36, and possibly 35. These are complemented by only sparse feedforward projections to TG from central field representations in TEO and to TH from peripheral field representations. The results thus indicate that TEO forms an important link in the occipitotemporal pathway for object recognition, sending visual information forward from V1 and prestriate relays in V2-V4 to anterior inferior temporal area TE.

Subcortical connections of subdivisions of inferior temporal cortex in squirrel monkeys

On the basis of cortical connections and architectonics, inferior temporal (IT) cortex of squirrel monkeys consists of a caudal, prestriate-recipient region, ITC; a rostral region, ITR; and possibly an intermediate region along the border of ITC and ITR, "ITI" (Weller & Steele, 1992). ITC contains dorsal (ITCd) and ventral (ITCV) areas. The subcortical connections of these subdivisions of IT cortex were determined in the present study from the results of cortical injections of wheat-germ agglutinin conjugated to horseradish peroxidase, [3H]-amino acids and fast blue. ITC and ITR receive afferents from the locus coeruleus, dorsal raphe, nucleus annularis, central superior nucleus, pontine reticular formation, lateral hypothalamus, paracentral nucleus, and central medial nucleus; send efferents to the superior colliculus, reticular nucleus, and striatum; and have both afferent and efferent connections with the pretectum, pulvinar, claustrum, amygdala, and basal nucleus of Meynert. ITC and ITR have different patterns of connections with a number of subcortical structures, including the pulvinar and amygdala. Injections in ITC strongly label multiple nuclei of the inferior pulvinar and the medial division of the lateral pulvinar (PLM), and moderately label the medial pulvinar (PM), whereas injections in ITR strongly label PM and moderately label PLM. Injections in ITC label sparse projections to the lateral nucleus of the amygdala, in contrast to injections in ITR that label strong projections to the lateral and basal nuclei of the amygdala. Injections in "ITI" produce a pattern of subcortical label that has some features of that observed from injections in ITC and that observed from injections in ITR. Although most of the connections of ITCd and ITCv appear similar, only injections involving ITCd label the middle nucleus of the inferior pulvinar (PIM). Comparison of the subcortical connections of subdivisions of IT cortex in squirrel monkeys and what is presently known of the subcortical connections of subdivisions of IT cortex in macaque monkeys supports the previous suggestion that ITC of squirrel monkeys may be comparable to area TEO of macaques, ITI may be comparable to posterior area TE, and ITR may be comparable to anterior area TE (Weller & Steele, 1992).