Processing of color, form and disparity information in visual areas VP and V2 of ventral extrastriate cortex in the macaque monkey

The responses of single cells to light bars of different orientation, direction of motion, speed, binocular disparity, and wavelength were systematically analyzed in areas V2 and VP of ventral extrastriate visual cortex in the macaque monkey. Selectivity for each of these parameters was assessed quantitatively using computer-controlled procedures. In both VP and V2 (both representing the superior contralateral quadrant), more than half of the cells studied were selective for stimulus color and more than half for stimulus orientation. In contrast, only a small minority of the VP and V2 cells were selective for the direction of stimulus motion. Comparison with reports of single-unit properties in dorsal extrastriate cortex suggests there are no major differences in the incidence of orientation, direction, and color selectivity between ventral and dorsal subdivisions of V2. Between V3 and VP, though, there are marked differences: Color-selective cells are much less common in V3 than VP, whereas direction-selective cells are more common in V3. This dorsoventral difference in the distribution of neuronal response properties suggests a significant asymmetry in the way visual information is processed in upper and lower parts of the visual field. The properties of cells in VP suggest that it plays an important role in both form and color vision, similar to that attributed to area V4.     

Pure homonymous hemiachromatopsia

Summary The study describes neuro-ophthalmologic findings in two patients with brain infarction who developed homonymous hemiachromatopsio with resolution to pure homonymous achromatopsia in an upper quadrant. All other visual parameter were normal; only color perimetry was capable of demonstrating the visual disorder. The results arc presented with special emphasis on the maculae region. Computed tomography studies and magnetic resonance imaging revealed lesions in the caudal and medial occipitotemporal gyri as well as in adjacent cortical regions. The lesions were secondary to disordered circulation in a proximal occipitotempora branch of the posterior cerebral artery. The anatomical findings and functional relations of color vision it man are discussed in the light of animal findings.     

The projections from striate cortex (V1) to areas V2 and V3 in the macaque monkey: asymmetries, areal boundaries, and patchy connections

The organization of projections from V1 to areas V2 and V3 in the macaque monkey was studied with a combination of anatomical techniques, including lesions and tracer injections made in different portions of V1 and V2 in 20 experimental hemispheres. Our results indicate that dorsal V1 (representing the inferior contralateral visual quadrant) consistently projects in topographically organized fashion to V3 in the lunate and parietooccipital sulci as well as to the middle temporal area (MT) and dorsal V2. In contrast, ventral V1 (representing the superior contralateral quadrant) projects only to MT and ventral V2. A corresponding dorsoventral asymmetry in myeloarchitecture supports the idea that V3 is an area that is restricted to dorsal extrastriate cortex and lacks a complete representation of the visual field. The average surface area of myeloarchitectonically identified V3 was 89 mm2. Additional information was obtained concerning the laminar distribution of connections from V1 to V2 and V3, the patchiness of these projections, and the consistency of projections to other extrastriate areas, including V4 and V3A.     

Ventral visual cortex in humans: cytoarchitectonic mapping of two extrastriate areas

The extrastriate visual cortex forms a complex system enabling the analysis of visually presented objects. To gain deeper insight into the anatomical basis of this system, we cytoarchitectonically mapped the ventral occipital cortex lateral to BA 18/V2 in 10 human postmortem brains. The anatomical characterization of this part of the ventral stream was performed by examination of cell-body-stained histological sections using quantitative cytoarchitectonic analysis. First, the gray level index (GLI) was measured in the ventral occipital lobe. Cytoarchitectonic borders, i.e., significant changes in the cortical lamination pattern, were then identified using an observer-independent algorithm based on multivariate analysis of GLI profiles. Two distinct cytoarchitectonic areas (hOC3v, hOC4v) were characterized in the ventral extrastriate cortex lateral to BA 18/V2. Area hOC3v was found in the collateral sulcus. hOC4v was located in this sulcus and also covered the fusiform gyrus in more occipital sections. Topographically, these areas thus seem to represent the anatomical substrates of functionally defined areas, VP/V3v and V4/V4v. Following histological analysis, the delineated cytoarchitectonic areas were transferred to 3D reconstructions of the respective postmortem brains, which in turn were spatially normalized to the Montreal Neurological Institute reference space. A probabilistic map was generated for each area which describes how many brains had a representation of this area in a particular voxel. These maps can now be used to identify the anatomical correlates of functional activations observed in neuroimaging experiments to enable a more informed investigation into the many open questions regarding the organization of the human visual cortex.     

Ventral posterior visual area of the macaque: visual topography and areal boundaries

Using both physiological and anatomical techniques, we have studied the topographic organization of extrastriate visual cortex on the ventral surface of the occipital lobe in macaque monkeys. Our results show that a topographically organized representation of the superior contralateral quadrant of the visual field lies immediately anterior to the ventral half of V2. This area is organized in a mirror symmetric fashion to ventral V2: it shares a horizontal meridian representation with V2 and a representation of the superior vertical meridian forms its anterior border. A well-defined strip of callosal inputs runs along the vertical meridian representation, thereby providing a reliable anatomical marker for areal boundaries in ventral extrastriate cortex. We refer to this area as the ventral posterior area (VP) because it is, in all these respects, notably similar to VP in the owl monkey. Ventral V2 has strong reciprocal connections with VP, and the topography of the V2 projection agrees closely with the topography revealed in our physiological mapping experiments. The visual field representation in VP is strikingly anisotropic, with linear magnification factor being much larger along contours of constant polar angle than along contours of constant eccentricity.     

Fibromuscular dysplasia of the basilar artery presenting as cerebral infarction in a young female]

We reported a 20-year-old woman with fibromuscular dysplasia (FMD) of the basilar artery presenting multiple cerebral infarctions. A sudden onset of consciousness disturbance and right hemiparesis was experienced. A neurological examination on day 2 revealed an absence of light and corneal reflexes on the left side, homonymous left upper quadrant anopsia and right hemiparesis with Babinski sign: she was also somnolent. On head MRI, multiple high signal intensity lesions were seen in the right occipital lobe, bilateral thalami and left pons on T2- and diffusion weighted images. Brain angiogram revealed the string of beads sign of the basilar artery, suggesting FMD. Neurological deficits gradually improved in the 2 months that followed, leaving slight hemiparesis and homonymous left upper quadrant anopsia In the following 3 years, no recurrence was seen with aspirin (81 mg/day). FMD in the head and neck usually affects extracranial segments of the carotid and vertebral arteries, while FMD of the basilar artery is extremely rare. To the best of our knowledge, 12 cases with FMD of the basilar artery have been reported; of these, 11 were symptomatic and 5 died. Since FMD of the basilar artery has poor prognosis, attention needs to be paid for FMD in young adults as a differential diagnosis of cerebral infarction in the territory of the basilar artery.     

Modular Organization of Human Extrastriate Visual Cortex: Evidence from Cytochrome Oxidase Pattern in Normal and Macular Degeneration Cases

The human extrastriate occipital cortex contains several visual areas that are probably analogues of macaque areas V2, V3, VP, V4 and V5. Tracing of callosal connections has led to the anatomical identification of these areas and to the characterization of some of them by cyto- and myeloarchitecture (Clarke and Miklossy, 1990). The pattern of cytochrome oxidase activity in these visual areas is now described in a normal case and in a case of age-related bilateral macular degeneration. In normal cortex, the laminar distribution of cytochrome oxidase activity was similar in V2, V3, VP, V4 and V5; a prominent dark band covered most of layers III and IV, and its upper and lower limits were gradual. In V2, V3, V4 and V5 but not VP, layer II tended to be darker than the infragranular layers. The overall intensity of the staining varied between areas: VP was very light, V2, V3 and V4 were darker, and V5 was very dark. A different, two-band pattern of cytochrome oxidase activity was found in a restricted region of the posterosuperior precuneus. The bilateral age-related macular degeneration had led to a great loss of ganglion cells in the central, but not in the peripheral retinae. The central representation in the lateral geniculate nuclei showed abnormally weak staining for cytochrome oxidase, particularly in the parvocellular layers. In the cortex, the contrast between lightly and darkly stained regions was greater than in the normal case. In particular, V5 was very heavily stained, and in V1 and V2 there were two different types of dark stripes that may represent compartments driven predominantly by the magnocellular system.     

Evidence from V1 connections for both dorsal and ventral subdivisions of V3 in three species of New World monkeys

We used patterns of connections of primary visual cortex (V1) to reevaluate differing proposals on the organization of extrastriate cortex in three species of New World monkeys. Several fluorescent tracers and the bidirectional tracer cholera toxin B subunit (CTB) were injected into dorsal V1 (representing the lower visual quadrant) and ventral V1 (representing the upper visual quadrant) of titi, squirrel, and owl monkeys. Labeled cells and terminals were plotted on brain sections cut parallel to the surface of flattened cortex and were related to architectonic boundaries. The results provided compelling evidence for both dorsal V3 with dorsal V1 connections and ventral V3 with ventral V1 connections. The connection pattern indicated that V3 represents the visual hemifield as a mirror image of V2. In addition, V3 could be recognized by a weak banding pattern in brain sections processed for cytochrome oxidase. V1 has connections with at least 12 subdivisions of visual cortex, with half of the connections involving V2 and 20% V3. Comparable results were obtained from all three species, suggesting that visual cortex is similarly organized.     

Cerebral polyopia with extrastriate quadrantanopia: report of a case with magnetic resonance documentation of V2/V3 cortical infarction.

This is a case report of the occurrence of cerebral diplopia with right-side superior homonymous quadrantanopia in a young woman after chiropractic neck manipulation. Magnetic resonance imaging confirmed an infarct in the left inferior V2/V3 (extrastriate) cortex. The characteristics of the diplopia are illustrated with the patient's drawings, and persisting abnormalities in perception are described in the area of the initial field defect after static (computed) visual field testing yielded normal results.     

The visual pulvinar in tree shrews II. Projections of four nuclei to areas of visual cortex

Patterns of thalamocortical connections were related to architectonically defined subdivisions of the pulvinar complex and the dorsolateral geniculate nucleus (LGN) in tree shrews (Tupaia belangeri). Tree shrews are of special interest because they are considered close relatives of primates, and they have a highly developed visual system. Several distinguishable tracers were injected within and across cortical visual areas in individual tree shrews in order to reveal retinotopic patterns and cortical targets of subdivisions of the pulvinar. The results indicate that each of the three architectonic regions of the pulvinar has a distinctive pattern of cortical connections and that one of these divisions is further divided into two regions with different patterns of connections. Two of the pulvinar nuclei have similar retinotopic patterns of projections to caudal visual cortex. The large central nucleus of the pulvinar (Pc) projects to the first and second visual areas, V1 and V2, and an adjoining temporal dorsal area (TD) in retinotopic patterns indicating that the upper visual quadrant is represented dorsal to the lower quadrant in Pc. The smaller ventral nucleus (Pv) which stains darkly for the Cat-301 antigen, projects to these same cortical areas, with a retinotopic pattern. Pv also projects to a temporal anterior area, TA. The dorsal nucleus (Pd), which densely expresses AChE, projects to posterior and ventral areas of temporal extrastriate cortex, areas TP and TPI. A posterior nucleus, Pp, projects to anterior areas TAL and TI, of the temporal lobe, as well as TPI. Injections in different cortical areas as much as 6 mm apart labeled overlapping zones in Pp and double-labeled some cells. These results indicate that the visual pulvinar of tree shrews contains at least four functionally distinct subdivisions, or nuclei. In addition, the cortical injections revealed that the LGN projects topographically and densely to V1 and that a significant number of LGN neurons project to V2 and TD.     

Resolving the organization of the New World monkey third visual complex: the dorsal extrastriate cortex of the marmoset (Callithrix jacchus)

We tested current hypotheses on the functional organization of the third visual complex, a particularly controversial region of the primate extrastriate cortex. In anatomical experiments, injections of retrograde tracers were placed in the dorsal cortex immediately rostral to the second visual area (V2) of New World monkeys (Callithrix jacchus), revealing the topography of interconnections between the "third tier" cortex and the primary visual area (V1). The data indicate the presence of a dorsomedial area (DM), which represents the entire upper and lower quadrants of the visual field, and which receives strong, topographically organized projections from the superficial layers of V1. The visuotopic organization and boundaries of DM were confirmed by electrophysiological recordings in the same animals and by architectural characteristics which were distinct from those found in ventral extrastriate cortex rostral to V2. There was no electrophysiological or histological evidence for a transitional area between V2 and DM. In particular, the central representation of the upper quadrant in DM was directly adjacent to the representation of the horizontal meridian that marks the rostral border of V2. The present results argue in favor of the hypothesis that the third visual complex in New World monkeys contains different areas in its dorsal and ventral components: area DM, near the dorsal midline, and a homolog of area 19 of other mammals, located more lateral and ventrally. The characteristics of DM suggest that it may correspond to visual area 6 (V6) of Old World monkeys.     

Organization and connections of V1 in Monodelphis domestica

We examined the internal organization and connections of the primary visual area, V1, in the South American marsupial Monodelphis domestica. Multiunit electrophysiological recording techniques were used to record from neurons at multiple sites. Receptive field location, size, progressions, and reversals were systematically examined to determine the visuotopic organization of V1 and its boundaries with adjacent visual areas. As in other mammals, a virtually complete representation of the visual hemifield was observed in V1, which was coextensive with a region of dense myelination. The vertical meridian was represented at the rostrolateral boundary of the field, the upper visual quadrant was represented caudolaterally, whereas the lower visual quadrant was represented rostromedially. Injections of fluorescent tracers into V1 revealed dense connections with cortex immediately adjacent to the rostrolateral boundary, in peristriate cortex (PS or V2). Connections were also consistently observed with a caudotemporal area (CT), entorhinal cortex (EC), and multimodal cortex (auditory/visual, A/V). These results demonstrate that M. domestica possess a highly differentiated neocortex with clear functional and architectonic cortical field boundaries, as well as discrete patterns of cortical connections. Some connections of V1 are similar to those observed in eutherian mammals, such as connections with V2 and extrastriate areas (e.g., CT), which suggests that there are general features of visual system organization that all mammals possess due to retention from a common ancestor. On the other hand, connections of V1 with EC and multimodal cortex may be a primitive feature of visual cortex that was lost in some lineages, may be a derived feature of marsupial neocortex, or may be a feature particular to mammals with small brains.     

Ischemic lesions of the occipital cortex and optic radiations: positron emission tomography

We used 18-F-fluoro-2-deoxyglucose positron emission tomography (PET) and computed tomography (CT) to study eight patients with homonymous hemianopias or quadrantanopias due to ischemic lesions of the visual pathways. Four patients with ischemic damage to all or part of the occipital lobe had decreased glucose metabolism in the affected region. Three patients with ischemic damage limited to the optic radiations had decreased glucose metabolism in the portion of striate cortex appropriate for the visual field defect. Changes in glucose metabolism frequently occurred in the undamaged ipsilateral thalamus and visual association areas.     

fMRI reveals greater within- than between-hemifield integration in the human lateral occipital cortex

Early visual areas within each hemisphere (V1, V2, V3/VP, V4v) contain distinct representations of the upper and lower quadrants of the contralateral hemifield. As receptive field size increases, the retinotopy in higher-tier visual areas becomes progressively less distinct. Using functional magnetic resonance imaging (fMRI) to map the visual fields, we found that an intermediate level visual area, the lateral occipital region (LO), contains retinotopic maps with a contralateral bias, but with a combined representation of the upper and lower visual field. Moreover, we used the technique of fMRI adaptation to determine whether neurons in LO code for both the upper and lower contralateral quadrants. We found that even when visual stimulus locations are equivalent across comparisons, the LO was more sensitive to location changes that crossed hemifields than location changes within a hemifield. These results suggested that within high-tier visual areas the increasing integration of visual field information is a two-stage process. The upper and lower visual representations are combined first, in LO, then the left and right representations. Furthermore, these results provided evidence for a neural mechanism to explain behavioral findings of greater integration within than between hemifields.     

Cerebral color blindness: an acquired defect in hue discrimination.

In contrast to the traditional view that striate visual cortex (area 17) is surrounded by two homogeneous cortical areas (areas 18 and 19), recent studies have shown that mammalian extrastriate visual cortex contains several anatomically and functionally distinct subregions. One such region, the V-4 complex of the rhesus monkey, is highly specialized for the analysis of color information, suggesting that a lesion in a homologous region might produce a defect in color vision while sparing other visual functions. We have studied a patient whose clinical syndrome supports this suggestion: a 44-year-old man with normal color vision suffered two cerebral infarctions that produced first a right and then a left superior homonymous quadrantanopia and also caused prosopagnosia, topographical disorientation, and severely impaired color vision. Computed tomography demonstrated extensive lesions in both inferior occipital lobes in the territories of the lateral branches of the posterior cerebral arteries, involving the lingual and medial occipitotemporal gyri bilaterally; these gyri contain the inferior portion of striate cortex and segments of extrastriate visual cortex. The patient had no difficulty in giving the correct color names associated with common objects presented either verbally or in outline drawings. Standardized testing with the Farnsworth-Munsell 100-hue test, the Nagel anomaloscope, and a method that tests for just-noticeable differences between monochromatic stimuli all showed that the patient's ability to distinguish one color from another was markedly imparied but not totally absent. In contrast, visual acuity, reading, visually guided eye movements, and stereopsis were normal. Cells in the V-4 complex of monkey extrastriate cortex are highly specialized for distinguishing one color from another; the hue discrimination deficit that was demonstrated in this patient with cerebral color blindness indicates that a region or regions with similar function has been damaged.     

Eye-movement training-induced changes of visual field representation in patients with post-stroke hemianopia

Changes in neuronal activity have been described in patients with hemianopia following ischemic lesions of the visual cortex. This reorganization may facilitate compensation of lost visual function that is rarely fully restituted. Improving exploratory eye movements with appropriate training has been shown to partially compensate for the visuoperceptive impairment during daily life activities. The changes in cortical processing of visual stimuli that may be induced by these training strategies, however, are less well described. We used fMRI to study the training effects of eye-movement training on cortical representation of visual hemifields. Brain activation during hemifield stimulation was measured in eight patients with an occipital cortical lesion of the striate cortex causing homonymous hemianopia. Starting 8 weeks after the stroke, patients received 4 weeks of eye movement training. fMRI measurements were performed at baseline and after training. In five patients, follow-up fMRI was performed 4 weeks after the end of training. Differences in activation between rest and hemifield stimulation as well as before and after training were assessed with statistical parametric mapping. Twelve healthy subjects were scanned twice at a 4-week interval. During stimulation of the affected hemifield, significant activation at baseline was found bilaterally in extrastriate cortical areas, with the strongest increases in the contralesional hemisphere. This activation pattern was maintained after training. Four weeks after the end of training, there was an additional activation of the extrastriate cortex in the contralesional hemisphere compared to baseline. No changes in the size of visual field defects were found. In this group of patients, eye-movement training induced altered brain activation in the unaffected extrastriate cortex.     

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. © 1996 Wiley-Liss, Inc.     

fMRI Measures of perceptual filling-in in the human visual cortex

Filling-in refers to the tendency of stabilized retinal stimuli to fade and become replaced by their background. This phenomenon is a good example of central brain mechanisms that can selectively add or delete information to/from the retinal input. Importantly, such cortical mechanisms may overlap with those that are used more generally in visual perception. In order to identify cortical areas that contribute to perceptual filling-in, we used functional magnetic resonance imaging to image activity in the visual cortex while subjects experienced filling-in. Nine subjects viewed an achromatic disc with slightly higher luminance than the background and indicated the presence or absence of filling-in by a keypress. The disc was placed in either the upper or lower left quadrant. Similar high-contrast stimuli were used to map out the retinotopic representation of the disc. Unexpectedly, the lower-field high-contrast stimulus produced more parietal cortex activation than the upper-field condition, indicating preferential representation of the lower field by attentional control mechanisms. During perceptual filling-in, we observed significant contralateral reductions in activation in lower-tier retinotopic areas V1 and V2. In contrast, increased activation was consistently observed in visual areas V3A and V4v, higher-level cortex in the intraparietal sulcus, posterior superior temporal sulcus, and the ventral occipital-temporal region, as well as the pulvinar. The filling-in activation pattern was remarkably similar for both the upper- and lower-field conditions. Behaviorally, filling-in was reported to be easier for the lower visual field, and filling-in periods were longer for the lower than the upper quadrant. We suggest this behavioral asymmetry may be partially due to the preferential parietal representation of the lower field. The results lead us to propose that perceptual filling-in recruits high-level control mechanisms to reconcile competing percepts, and alters the normal image-related signals at the first stages of cortical processing. Moreover, the overall pattern of activation during filling-in resembles that seen in other studies of perceptually bistable stimuli, including binocular rivalry, indicating common control mechanisms.     

Single axon analysis of pulvinocortical connections to several visual areas in the macaque

The pulvinar nucleus is a major source of input to visual cortical areas, but many important facts are still unknown concerning the organization of pulvinocortical (PC) connections and their possible interactions with other connectional systems. In order to address some of these questions, we labeled PC connections by extracellular injections of biotinylated dextran amine into the lateral pulvinar of two monkeys, and analyzed 25 individual axons in several extrastriate areas by serial section reconstruction. This approach yielded four results: (1) in all extrastriate areas examined (V2, V3, V4, and middle temporal area [MT]/V5), PC axons consistently have 2-6 multiple, spatially distributed arbors; (2) in each area, there is a small number of larger caliber axons, possibly originating from a subpopulation of calbindin-positive giant projection neurons in the pulvinar; (3) as previously reported by others, most terminations in extrastriate areas are concentrated in layer 3, but they can occur in other layers (layers 4,5,6, and, occasionally, layer 1) as collaterals of a single axon; in addition, (4) the size of individual arbors and of the terminal field as a whole varies with cortical area. In areas V2 and V3, there is typically a single principal arbor (0.25-0.50 mm in diameter) and several smaller arbors. In area V4, the principal arbor is larger (2.0- to 2.5-mm-wide), but in area MT/V5, the arbors tend to be smaller (0.15 mm in diameter). Size differences might result from specializations of the target areas, or may be more related to the particular injection site and how this projects to individual cortical areas. Feedforward cortical axons, except in area V2, have multiple arbors, but these do not show any obvious size progression. Thus, in areas V2, V3, and especially V4, PC fields are larger than those of cortical axons, but in MT/V5 they are smaller. Terminal specializations of PC connections tend to be larger than those of corticocortical, but the projection foci are less dense. Further work is necessary to determine the differential interactions within and between systems, and how these might result in the complex patterns of suppression and enhancement, postulated as gating mechanisms in cortical attentional effects, or in different states of arousal.     

Topographic organization of cortical input to striate cortex in the Cebus monkey: a fluorescent tracer study

Cortical afferents to area V1 were studied in seven Cebus monkeys by means of retrograde fluorescent tracers. Injections were placed in V1, under electrophysiological guidance, in the regions of representation of both the upper and lower visual quadrants, at eccentricities that ranged from 0.5 to 64 degrees. In all cases retrogradely filled neurons were found in retinotopically corresponding portions of areas V2 and MT, as defined electrophysiologically (Rosa et al: J. Comp. Neurol. 275:326, 1988; Fiorani et al: J Comp Neurol 287:98, 1989). The results also revealed two other visual zones located anterior to V2 here named third and fourth visual areas. A topographical organization of the connections was observed in these areas, with upper quadrant located ventrally and lower quadrant located dorsally. A clear central-peripheral gradient, from the lateral to the medial cortical surface, was also observed in these areas. Lower field injections revealed crude topographic organization in area DZ and a diffuse projecting zone in the annectent gyrus. Peripheral injections in V1 revealed a clear upper and lower field segregation in areas PO and prostriata as well as a complex topography in MST. In addition, another region of labeling revealed the presence of an area, the temporal ventral posterior region, with an organized topographic representation of the upper field, with a central to peripheral gradient, from the lateral to the medial cortical surface. Three groups of cortical areas were distinguished according to the laminar distribution of neurons labeled from V1. In the first group, which is characterized by dense infra- and supragranular labeling, only V2 was included. The second group consists of areas V3, MT, and PO. These areas show dense labeling in the infragranular layers and occasionally sparse labeling in the supragranular layers. Finally, V4 and the other projecting areas, which are characterized by exclusive labeling of the infragranular layers were included in the third group.