Functionally matched domains in parietal-frontal cortex of monkeys project to overlapping regions of the striatum.

Projections of small regions (domains) of primary motor cortex (M1), premotor cortex (PMC) and posterior parietal cortex (PPC) to the striatum of squirrel monkeys were revealed by restricted injections of anterograde tracers. As many as 8 classes of action-specific domains can be identified in PPC, as well as in PMC and M1, and some have been identified for injections by the action evoked by 0.5 s trains of electrical microstimulation. Injections of domains in all three cortical regions labeled dense patches of terminations in the matrix of the ipsilateral putamen, while providing sparse or no projections to corresponding regions of the contralateral putamen. When two or three of these domains were injected with different tracers, projection fields in the putamen were highly overlapped for injections in functionally matched domains across cortical areas, but were highly segregated for injections placed in functionally mismatched domains. While not all classes of domains were studied, the results suggest that the striatum potentially has separate representations of eight or more classes of actions that receive inputs from domains in three or more cortical regions in sensorimotor cortex. The overlap/segregation of cortico-striatal projections correlates with the strength of cortico-cortical connections between injected motor areas.

Cortical networks subserving upper limb movements in primates.

In all primates, the cortical control of hand and arm movements is initiated and controlled by a network of cortical regions including primary motor cortex (M1), premotor cortex (PMC), and posterior parietal cortex (PPC). These interconnected regions are influenced by inputs from especially visual and somatosensory cortical areas, and prefrontal cortex. Here we discuss recent evidence showing M1, PMC, and PPC can be subdivided into a number of functional zones or domains, including several that participate in guiding and controlling hand and arm movements. Functional zones can be defined by the movement sequences evoked by microstimulation within them, and functional zones related to the same type of movement in all three cortical regions are interconnected. The inactivation of a functional zone in each of the regions has a different impact on motor behavior. Finally, there is considerable plasticity within the networks so that behavioral recoveries can occur after damage to functional zones within a network.

Validation of diffusion tensor MRI in the central nervous system using light microscopy: quantitative comparison of fiber properties.

Diffusion tensor imaging (DTI) provides an indirect measure of tissue structure on a microscopic scale. To date, DTI is the only imaging method that provides such information in vivo, and has proven to be a valuable tool in both research and clinical settings. In this study, we investigated the relationship between white matter structure and diffusion parameters measured by DTI. We used micrographs from light microscopy of fixed, myelin-stained brain sections as a gold standard for direct comparison with data from DTI. Relationships between microscopic tissue properties observed with light microscopy (fiber orientation, density and coherence) and fiber properties observed by DTI (tensor orientation, diffusivities and fractional anisotropy) were investigated. Agreement between the major eigenvector of the tensor and myelinated fibers was excellent in voxels with high fiber coherence. In addition, increased fiber spread was strongly associated with increased radial diffusivity (p = 6 × 10(-6)) and decreased fractional anisotropy (p = 5 × 10(-8)), and was weakly associated with decreased axial diffusivity (p = 0.07). Increased fiber density was associated with increased fractional anisotropy (p = 0.03), and weakly associated with decreased radial diffusivity (p < 0.06), but not with axial diffusivity (p = 0.97). The mean diffusivity was largely independent of fiber spread (p = 0.24) and fiber density (p = 0.34).

Thalamic connections of the dorsal and ventral premotor areas in New World owl monkeys.

Thalamic connections of two premotor cortex areas, dorsal (PMD) and ventral (PMV), were revealed in New World owl monkeys by injections of fluorescent dyes or wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The injections were placed in the forelimb and eye-movement representations of PMD and in the forelimb representation of PMV as determined by microstimulation mapping. For comparison, injections were also placed in the forelimb representation of primary motor cortex (M1) of two owl monkeys. The results indicate that both PMD and PMV receive dense projections from the ventral lateral (VL) and ventral anterior (VA) thalamus, and sparser projections from the ventromedial (VM), mediodorsal (MD) and intralaminar (IL) nuclei. Labeled neurons in VL were concentrated in the anterior (VLa) and the medial (VLx) nuclei, with only a few labeled cells in the dorsal (VLd) and posterior (VLp) nuclei. In VA, labeled neurons were concentrated in the parvocellular division (VApc) dorsomedial to VLa. Labeled neurons in MD were concentrated in the most lateral and posterior parts of the nucleus. VApc projected more densely to PMD than PMV, especially to rostral PMD, whereas caudal PMD received stronger projections from neurons in VLx and VLa. VLd projected exclusively to PMD, and not to PMV. In addition, neurons labeled by PMD injections tended to be more dorsal in VL, IL, and MD than those labeled by PMV injections. The results indicate that both premotor areas receive indirect inputs from the cerebellum (via VLx, VLd and IL) and globus pallidus (via VLa, VApc, and MD). Comparisons of thalamic projections to premotor and M1 indicate that both regions receive strong projections from VLx and VLa, with the populations of cells projecting to M1 located more laterally in these nuclei. VApc, VLd, and MD project mainly to premotor areas, while VLp projects mainly to M1. Overall, the thalamic connectivity patterns of premotor cortex in New World owl monkeys are similar to those reported for Old World monkeys.

The thalamic connections of motor, premotor, and prefrontal areas of cortex in a prosimian primate (Otolemur garnetti).

Connections of motor areas in the frontal cortex of prosimian galagos (Otolemur garnetti) were determined by injecting tracers into sites identified by microstimulation in the primary motor area (M1), dorsal premotor area (PMD), ventral premotor area (PMV), supplementary motor area (SMA), frontal eye field (FEF), and granular frontal cortex. Retrogradely labeled neurons for each injection were related to architectonically defined thalamic nuclei. Nissl, acetylcholinesterase, cytochrome oxidase, myelin, parvalbumin, calbindin, and Cat 301 preparations allowed the ventral anterior and ventral lateral thalamic regions, parvocellular and magnocellular subdivisions of ventral anterior nucleus, and anterior and posterior subdivisions of ventral lateral nucleus of monkeys to be identified. The results indicate that each cortical area receives inputs from several thalamic nuclei, but the proportions differ. M1 receives major inputs from the posterior subdivision of ventral lateral nucleus while premotor areas receive major inputs from anterior parts of ventral lateral nucleus (the anterior subdivision of ventral lateral nucleus and the anterior portion of posterior subdivision of ventral lateral nucleus). PMD and SMA have connections with more dorsal parts of the ventral lateral nucleus than PMV. The results suggest that galagos share many subdivisions of the motor thalamus and thalamocortical connection patterns with simian primates, while having less clearly differentiated subdivisions of the motor thalamus.

Topographic patterns of v2 cortical connections in a prosimian primate (Galago garnetti).

Topographic patterns of cortical connections of the second visual area (V2) were examined in a lorisiform prosimian primate (Galago garnetti). Up to five different tracers were injected into dorsal and ventral V2. Tracers included wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and up to four fluorochromes. Tracer injections consistently labeled neurons and terminals in primary visual cortex (V1), V2, the middle temporal area (MT), and the dorsolateral visual area (DL). Labeled neurons were also found in other proposed extrastriate areas such as the dorsomedial visual area (DM), dorsointermediate area (DI), middle temporal crescent (MTc), medial superior temporal area (MST), ventral posterior parietal area (VPP), and caudal inferotemporal cortex (ITc), but these connections were more variable and less dependent on the retinotopic position of injection sites in V2. Areal boundaries were identified by differences in cytochrome oxidase (CO) and myelin staining. We conclude that V2 cortical connections in prosimian galagos are similar to those in simian primates, suggesting that prosimians and other lines of primate evolution have retained several visual areas from a common ancestor that relate to V2 in similar ways. Architectural features of striate and extrastriate areas in prosimian galagos are similar to simian primates, with notable exceptions such as stripes in V2, which appear to be less differentiated in galagos.

Reorganization of primary motor cortex in adult macaque monkeys with long-standing amputations.

The organization of primary motor cortex (M1) of adult macaque monkeys was examined years after therapeutic amputation of part of a limb or digits. For each case, a large number of sites in M1 were electrically stimulated with a penetrating microelectrode, and the evoked movements and levels of current needed to evoke the movements were recorded. Results from four monkeys with the loss of a forelimb near or above the elbow show that extensive regions of cortex formerly devoted to the missing hand evoked movements of the stump and the adjoining shoulder. Threshold current levels for stump movements were comparable to those for normal arm movements. Few or no sites in the estimated former territory of the hand evoked face movements. Similar patterns of reorganization were observed in all four cases, which included two monkeys injured as adults, one as a juvenile, and one as an infant. In a single monkey with a hindlimb amputation at the knee as an infant, stimulation of cortex in the region normally devoted to the foot moved the leg stump, again at thresholds in the range for normal movements. Finally, in a monkey that had lost digit 5 and the distal phalanges of digits 2-4 at 2 yr of age, much of the hand portion of M1 was devoted to movements of the digit stumps.

Projections of the superior colliculus to subdivisions of the inferior pulvinar in New World and Old World monkeys.

Patterns of terminals labeled after WGA-HRP injections in the superior colliculus (SC) in squirrel monkeys and macaque monkeys, and after DiI application in marmosets, were related to the architecture of the pulvinar and dorsal lateral geniculate nucleus (LGN). In all studied species, the SC projects densely to two architectonic subdivisions of the inferior pulvinar, the posterior inferior pulvinar nucleus (PIp) and central medial inferior pulvinar nucleus (PIcM). These projection zones expressed substance P. Thus, sections processed for substance P reveal SC termination zones in the inferior pulvinar. The medial subdivision of the inferior pulvinar, PIm, which is known to project to visual area MT, does not receive a significant collicular input. Injections in MT of a squirrel monkey revealed no overlap between SC terminals and neurons projecting to area MT. Thus, PIm is not the significant relay station of visual input from the SC to MT. The SC also sends an input to the LGN, however, this projection is sparser than the input directed to pulvinar.

Pallidal and cerebellar afferents to pre-supplementary motor area thalamocortical neurons in the owl monkey: a multiple labeling study.

In the present study, we determined where thalamic neurons projecting to the pre-supplementary motor area (pre-SMA) are located relative to pallidothalamic and cerebellothalamic inputs and nuclear boundaries. We employed a triple-labeling technique in the same owl monkey (Aotus trivirgatus). The cerebellothalamic projections were labeled with injections of wheat germ agglutinin conjugated to horseradish peroxidase, and the pallidothalamic projections were labeled with biotinylated dextran amine. The pre-SMA was identified by location and movement patterns evoked by intracortical microstimulation and injected with the retrograde tracer cholera toxin subunit B. Brain sections were processed sequentially using different chromogens to visualize all three tracers in the same section. Alternate sections were processed for Nissl cytoarchitecture or acetylcholinesterase chemoarchitecture for nuclear boundaries. The cerebellar nuclei primarily projected to posterior (VLp), medial (VLx), and dorsal (VLd) divisions of the ventral lateral nucleus; the pallidum largely projected to the anterior division (VLa) of the ventral lateral nucleus and the parvocellular part of the ventral anterior nucleus (VApc). However, we also found zones of overlapping projections, as well as interdigitating foci of pallidal and cerebellar label, particularly in border regions of the VLa and VApc. Thalamic neurons labeled by pre-SMA injections occupied a wide band and were especially concentrated in the VLx and VApc, cerebellar and pallidal territories, respectively. Labeled thalamocortical neurons overlapped cerebellar inputs in the VLd and VApc and overlapped pallidal inputs in the VLa and the ventral medial nucleus. The results demonstrate that inputs from both the cerebellum and globus pallidus are relayed to the pre-SMA.

Callosal connections of the parabelt auditory cortex in macaque monkeys.

Auditory cortex of macaque monkeys is located on the lower bank of the lateral sulcus and the adjoining superior temporal gyrus. This region of cortex contains a core of primary-like areas surrounded by a narrow belt of associated fields. Adjacent to the lateral belt on the superior temporal gyrus is a parabelt region which contains at least two subdivisions (rostral and caudal). In previous studies we defined the parabelt region as cortex with topographic cortical connections with the belt areas surrounding the core, and connections with the dorsal and magnocellular divisions of the medial geniculate complex, but minimal connections with the core region and ventral division of the medial geniculate complex. The callosal connections of the parabelt auditory cortex were determined by placing injections, of up to six distinguishable tracers, into different locations of the parabelt region in each of four macaque monkeys. The results indicated that the strongest callosal projections arise from homotopic areas in parabelt cortex, and they roughly matched the rostrocaudal levels of the medial and lateral belt cortex. Weaker callosal inputs to the parabelt originate from the corresponding levels of the superior temporal gyrus and superior temporal sulcus. The core region does not contribute significant callosal projections to the parabelt region. The results provide further support for the conclusion that the parabelt region represents a third level of auditory cortical processing beyond direct activation by primary subcortical and cortical auditory structures.

Do superior colliculus projection zones in the inferior pulvinar project to MT in primates?

In order to determine the relationship of superior colliculus inputs to thalamic neurons projecting to the middle temporal visual area (MT), injections of wheat germ agglutinin conjugated with horseradish peroxidase were placed in the superior colliculus of three owl monkeys, with injections of Fast Blue in the MT. The locations of labelled terminals and neurons in the posterior thalamus were related to four architectonically distinct nuclei of the inferior pulvinar (Stepniewska & Kaas, Vis. Neurosci. 14, pp.1043-1060, 1997). Fast Blue injections in the MT labelled neurons largely in the medial nucleus of the inferior pulvinar. A few labelled neurons were found in the adjoining central medial nucleus of the inferior pulvinar, as well as in the lateral pulvinar and the dorsal lateral geniculate nucleus. Superior colliculus inputs were most dense in the posterior and medial nuclei of the inferior pulvinar. There were sparser inputs to the central lateral nucleus of the inferior pulvinar, locations in the lateral and medial pulvinar, and the dorsal lateral geniculate nucleus. The results indicate that the medial nucleus of the inferior pulvinar, the major projection zone to the MT, does not receive a significant input from the superior colliculus.

Prefrontal connections of the parabelt auditory cortex in macaque monkeys.

In the present study, we determined connections of three newly defined regions of auditory cortex with regions of the frontal lobe, and how two of these regions in the frontal lobe interconnect and connect to other portions of frontal cortex and the temporal lobe in macaque monkeys. We conceptualize auditory cortex as including a core of primary areas, a surrounding belt of auditory areas, a lateral parabelt of two divisions, and adjoining regions of temporal cortex with parabelt connections. Injections of several different fluorescent tracers and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were placed in caudal (CPB) and rostral (RPB) divisions of the parabelt, and in cortex of the superior temporal gyrus rostral to the parabelt with parabelt connections (STGr). Injections were also placed in two regions of the frontal lobe that were labeled by a parabelt injection in the same case. The results lead to several major conclusions. First, CPB injections label many neurons in dorsal prearcuate cortex in the region of the frontal eye field and neurons in dorsal prefrontal cortex of the principal sulcus, but few or no neurons in orbitofrontal cortex. Fine-grain label in these same regions as a result of a WGA-HRP injection suggests that the connections are reciprocal. Second, RPB injections label overlapping prearcuate and principal sulcus locations, as well as more rostral cortex of the principal sulcus, and several locations in orbitofrontal cortex. Third, STGr injections label locations in orbitofrontal cortex, some of which overlap those of RPB injections, but not prearcuate or principal sulcus locations. Fourth, injections in prearcuate and principal sulcus locations labeled by a CPB injection labeled neurons in CPB and RPB, with little involvement of the auditory belt and no involvement of the core. In addition, the results indicated that the two frontal lobe regions are densely interconnected. They also connect with largely separate regions of the frontal pole and more medial premotor and dorsal prefrontal cortex, but not with the extensive orbitofrontal region which has RPB and STGr connections. The results suggest that both RPB and CPB provide the major auditory connections with the region related to directing eye movements towards stimuli of interest, and the dorsal prefrontal cortex for working memory. Other auditory connections to these regions of the frontal lobe appear to be minor. RPB has connections with orbitofrontal cortex, important in psychosocial and emotional functions, while STGr primarily connects with orbital and polar prefrontal cortex.

Thalamocortical connections of the parabelt auditory cortex in macaque monkeys.

The auditory cortex of macaque monkeys contains a core of primary-like areas surrounded by a narrow belt of associated fields that encompass much of the superior temporal plane in these animals. Adjacent to the lateral belt on the superior temporal gyrus is a parabelt region that contains at least two subdivisions (rostral and caudal). In a previous study (Hackett et al. [1998] J. Comp. Neurol. 394:475-495), we determined that the parabelt has topographic connections with the belt areas surrounding the core, but minimal connections with the core itself. In this study, we describe the thalamocortical connections of the parabelt auditory cortex based on multiple injections of neuronal tracers into this region in each of five macaque monkeys. Injections confined to the parabelt labeled large numbers of neurons in the dorsal (MGd) and magnocellular (MGm) divisions of the medial geniculate complex (MGC), suprageniculate (Sg), limitans (Lim), and medial pulvinar (PM) nuclei. Only when injections encroached on the lateral belt cortex were substantial numbers of labeled neurons found in the ventral (MGv) division of the MGC, consistent with the absence of significant connections between the parabelt and core fields. The rostrocaudal topography of the parabelt region was maintained in the thalamocortical connections, supporting the parcellation of this region of cortex. The results suggest that the parabelt region represents a third level of auditory cortical processing, which is not influenced by direct inputs from primary cortical or subcortical auditory structures.

Subdivisions of auditory cortex and ipsilateral cortical connections of the parabelt auditory cortex in macaque monkeys.

Auditory cortex of macaque monkeys can be divided into a core of primary or primary-like areas located on the lower bank of the lateral sulcus, a surrounding narrow belt of associated fields, and a parabelt region just lateral to the belt on the superior temporal gyrus. We determined patterns of ipsilateral cortical connections of the parabelt region by placing injections of four to seven distinguishable tracers in each of five monkeys. Results were related to architectonic subdivisions of auditory cortex in brain sections cut parallel to the surface of artificially flattened cortex (four cases) or cut in the coronal plane (one case). An auditory core was clearly apparent in these sections as a 16- to 20-mm rostrocaudally elongated oval, several millimeters from the lip of the sulcus, that stained darkly for parvalbumin, myelin, and acetylcholinesterase. These features were most pronounced caudally in the cortex assigned to auditory area I, only slightly reduced in the rostral area, and most reduced in the narrower rostral extension we define as the rostrotemporal area. A narrow band of cortex surrounding the core stained more moderately for parvalbumin, acetylcholinesterase, and myelin. Two regions of the caudal belt, the caudomedial area, and the mediolateral area, stained more darkly, especially for parvalbumin. Rostromedial and medial rostrotemporal, regions of the medial belt stained more lightly for parvalbumin than the caudomedial area or the lateral belt. The parabelt region stained less darkly than the core and belt fields. Injections confined to the parabelt region labeled few neurons in the core, but large numbers in parts of the belt, the parabelt, and adjacent portions of the temporal lobe. Injections that encroached on the belt labeled large numbers of neurons in the core and helped define the width of the belt. Caudal injections in the parabelt labeled caudal portions of the belt, rostral injections labeled rostral portions, and both caudal and rostral injections labeled neurons in the rostromedial area of the medial belt. These observations support the concept of dividing the auditory cortex into core, belt, and parabelt; provide evidence for including the rostral area in the core; suggest the existence of as many as seven or eight belt fields; provide evidence for at least two subdivisions of the parabelt; and identify regions of the temporal lobe involved in auditory processing.

Multiple divisions of macaque precentral motor cortex identified with neurofilament antibody SMI-32.

In brain sections stained with monoclonal antibody SMI-32, which recognizes non-phosphorylated neurofilament protein, we distinguished separate caudal, intermediate, and rostral subdivisions of gigantocellular precentral cortex (areas 4c, 4i, and 4r) in macaque monkeys. The divisions form bands extending mediolaterally across the major body-region representations of the primary motor cortex (M1). These observations provide additional evidence that primary motor cortex is not a single, structurally homogeneous cortical area.

Architectonic subdivisions of the inferior pulvinar in New World and Old World monkeys.

Architectonic subdivisions of the inferior pulvinar (PI) complex were delineated in New World owl and squirrel monkeys and Old World macaque monkeys. Brain sections were processed for Nissl substance, myelin, cytochrome oxidase (CO), acetylcholinesterase (AChE), calbindin-D28K (Cb), or with the monoclonal antibody Cat-301. In all three primates, we identified the posterior nucleus (PIp) and the medial nucleus (PIm) of previous reports, and divided the previously recognized central nucleus (PIc) into two subdivisions, medial (PIcM) and lateral (PIcL). Each nucleus had several features that allowed it to be readily distinguished. (1) PIp was dark in Cb, and moderately dark in AChE and CO preparations. (2) PIm was Cb light, and AChE and CO dark. (3) PIcM was Cb dark, and AChE and CO light. (4) PIcL was Cb moderate with a scattering of dark neurons, and moderately dark for AChE and CO. (5) In sections processed for Cat-301, PIm in macaque monkeys and PIcM and PIp in squirrel monkeys stained darkly, while little staining was apparent in owl monkeys. The results allowed subdivisions of the inferior pulvinar to be more clearly defined, homologized, and compared across taxa. All monkeys appear to have the same four subdivisions of the PI, although properties vary.

Lateral division of the lateral posterior region: connections with area 18 in cats.

To establish the topography of the lateral division of the lateral posterior region (LP1) projections to area 18, up to five different anatomical tracers were injected in separate rostrocaudal locations in area 18 of four adult cats, and patterns of retrogradely labeled LP1 cells were identified. LP1 inputs to area 18 arose from both caudal and rostral nuclei and were topological, organized in patterns that indicate that lower visual space is represented anteroventrally, and more central and upper visual space is represented caudodorsally. In the caudal LP1 nucleus, patches of labeled cells formed bands that ran parallel to the medial and lateral LP1 borders and encompassed medial portions of the nucleus. In rostral LP1, the patches of labeled cells formed clusters giving the connections with area 18 a more modular appearance, and were nearer the lateral LP1 border. Injections made nearest area centralis representations in area 18 labeled more neurons than injections in cortex representing more peripheral visual space. Also, neighboring injections in area 18 labeled overlapping patches of cells, but no double-labeled cells were observed. These findings are consistent with previous conclusions based on electrophysiological mapping studies, that two retinotopically organized nuclei constitute LP1.

Movement representation in the dorsal and ventral premotor areas of owl monkeys: a microstimulation study.

We used intracortical microstimulation to investigate the lateral premotor cortex and neighboring areas in 14 hemispheres of owl monkeys, focusing on the somatotopic distribution of evoked movements, thresholds for forelimb movements, and the relative representation of proximal and distal forelimb movements. We elicited movements from the dorsal and ventral premotor areas (PMD, PMV), the caudal and rostral divisions of primary motor cortex (M1c, M1r), the frontal eye field (FEF), the dorsal oculomotor area (OMD; area 8b), the supplementary motor area (SMA), and somatosensory cortex (areas 3a and 3b). Area PMD was composed of architectonically distinguishable caudal and rostral subdivisions (PMDc, PMDr). Stimulation of PMD elicited movements of the hindlimb, forelimb, neck and upper trunk, face, and eyes. Hindlimb and forelimb movements were represented in the caudalmost part of PMDc. Face, neck, and eye movements were represented in the lateral and rostral parts of PMDc and in PMDr. Stimulation of PMV elicited forelimb and orofacial movements, but not hindlimb movements. Both proximal and distal forelimb movements were elicited from PMDc and PMV, although PMD stimulation elicited mainly shoulder and elbow movements, while PMV stimulation evoked primarily wrist and digit movements. Distal movements were evoked more frequently from PMV than from M1r or M1c. Across cases, the median forelimb thresholds for PMDc and PMV were 60 and 36 microA, respectively, values that differ significantly from each other and from the value of 11 microA obtained for M1r. Our observations indicate that premotor cortex is much more responsive to electrical stimulation than commonly thought, and contains a large territory from which eye movements can be elicited. These results suggest that in humans, much of the electrically excitable cortex located on the precentral gyrus, including cortex sometimes considered part of the frontal eye field, is probably homologous to the premotor cortex of nonhuman primates.

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.

Surface-view connectivity patterns of area 18 in cats.

To determine surface-view connectivity patterns of area 18, separate injections of up to six anatomical tracers were delivered to various rostrocaudal locations of area 18 in six normal cats. Subsequently, cortex was separated from subcortical structures, manually flattened, and cut parallel to the surface. Results reveal that ipsilateral cortical connections of area 18 with three regions of cortex are topological. In areas 17 and 19, separate patches of cells labeled with different tracers progressed in a rostrocaudal sequence corresponding to the order of the injections. A similar but less precise pattern of rostrocaudal labeling occurred in more lateral visual cortex, even though several presumptive visual areas were involved. Thus, anteromedial suprasylvian cortex projected to anterior area 18 while more posterolateral suprasylvian cortex projected to posterior area 18. There was no evidence of double-labeled cells projecting to separate regions in area 18. These results are more consistent with the concept of a single suprasylvian area projecting to area 18 cortex than several.