Effects of Inferior Temporal Lesions on Two Types of Orientation Discrimination in the Macaque Monkey

Rhesus monkeys with transection of the forebrain commissures were trained in two different tasks in which grating orientation was the discriminandum. In the temporal same—different task, the monkeys had to judge whether or not two successively presented gratings differed in orientation. In the identification task, we measured how well the monkey could judge the orientation of the grating. The performance in any task was affected neither by a unilateral anterior temporal cortical area lesion nor by a subsequent posterior temporal cortical area lesion in the same hemisphere resulting in a two-stage inferior temporal (IT) lesion. However, a single stage IT (combined anterior and posterior temporal cortical areas) lesion of the other hemisphere severely disrupted the performance in the temporal same-different task, but only barely increased just noticeable differences in orientation in the identification task. This indicates that the impairment in a temporal comparison task after an IT lesion is not due to a perceptual coding deficit, but is related to the temporal comparison per se. Thus, IT is involved in the temporal comparison of successively presented stimuli. On the other hand, the two IT lesions, each having a different history (single versus two stage) had dramatically different behavioural effects, suggesting an important role for adult brain plasticity in determining the behavioural outcome of a brain lesion.     

The Effect of Removing Superior Temporal Cortical Motion Areas in the Macaque Monkey: I. Motion Discrimination Using Simple Dots

We compared the pre- and postoperative performance of macaque monkeys on visual discrimination tasks entailing the perception of differences in the motion of two luminous spots. Animals in which area MT and adjacent regions had been surgically removed were not significantly impaired as long as there were differences in the temporal frequency of the two stimuli. When the latter were eliminated the postoperative performance of the MT animals was significantly impaired compared to their preoperative performance and compared to the animals in the control groups. The same animals were also impaired at perceiving which of two moving dots, presented in the dark, changed its direction.     

The Effect of Removing Superior Temporal Cortical Motion Areas in the Macaque Monkey: II. Motion Discrimination Using Random Dot Displays

Nine macaque monkeys were trained to discriminate between a display in which all the dots moved randomly (visual dynamic noise) and an adjacent display in which a proportion of the dots oscillated coherently from side to side. Two monkeys in which area MT (middle temporal visual area) and adjacent regions were surgically removed were unable to perform even the simplest version of this task where the coherent motion was not masked by any visual dynamic noise. The other seven animals tolerated between 60% and 65% of random movement in the otherwise coherently oscillating display before they failed to discriminate between the two displays. In contrast, shape discrimination tasks containing luminance boundaries were unaffected by the removal of area MT. However, the removal of area MT did impair the animals' ability to use a kinetic boundary as the basis for shape discrimination. Their performance with a kinetic boundary between a moving and a stationary random dot field and a kinetic boundary between two random dot fields moving in opposite directions was the most severely impaired. Their performance with a kinetic boundary defined by two random dot fields moving orthogonally to each other was unimpaired. None of the animals from either group was able to use the kinetic boundary defined by coherent motion and visual dynamic noise as the basis for shape discrimination. The ease with which the control animals were able to employ the different types of kinetic boundaries to perform the shape discrimination reflected previously published proportions of cells in area MT that are specialized for detecting the different types of kinetic boundaries.     

Shape and Spatial Distribution of Receptive Fields and Antagonistic Motion Surrounds in the Middle Temporal Area (V5) of the Macaque

The spatial organization of receptive fields in the middle temporal (MT) area of anaesthetized and paralysed macaque monkeys was studied. In all, 288 neurons were successfully recorded. The size and shape of the receptive field (RF) was mapped with small patches of translating random dots and the resulting data were fitted with a generalized Gaussian. Results show that the RF area increases with eccentricity, and is larger in lamina 5 than in other layers. Most of these RFs are elongated, and the axis of elongation tends to be orthogonal to the preferred direction of motion. The direction selectivity is maintained in all positions in the RF, but layer 5 cells are less direction-selective than cells in other layers. In a second series of experiments, radial dimensions of the classical RF and the antagonistic surround were estimated from area summation tests. These data were fitted with the difference of the integrals of two Gaussians. Surrounds were weakest in layer 4 and strongest in layer 2. Optimal stimulus diameters, also estimated from the area summation curve, were larger in the infragranular layers than in the other layers. The maximum sensitivity of the surround was clearly displaced from the classical RF (CRF) centre, indicating that the surround is not concentric with the CRF. This radial offset and the extent of the surround were largest in layers 2 and 5 and smallest in 3a. The extent of the surround half-height equalled, on average, 3–4 times that of the CRF. These results suggest that antagonistic surrounds are constructed in MT, probably through horizontal connections, and that a strong vertical organization exists in area MT, as has been shown for V1.     

Effects of Visual Cortex Lesions on Orientation Discrimination of Illusory Contours in the Cat

We have trained five cats in orientation discrimination using different contours, and compared the deficits caused by lesions of cortical areas 17 and 18 (tier I) to the deficits induced by removal of those areas receiving afferents originating in areas 17 and 18 (tier II). As contour stimuli we used two types of illusory contours and a luminance bar. The two illusory contours were defined by opposed line-ends. One of them coincided with a luminance gradient whereas the other did not. Tier I lesions destroyed the capacity to discriminate the orientation of both illusory contours, and also caused an important, though less severe, deficit in bar orientation discrimination. The deficits induced by tier I lesions were permanent. Tier II lesions also caused significant deficits in orientation discrimination of illusory contours, but only a negligible deficit in bar orientation discrimination, and this result was not a mere consequence of a difference in difficulty between the tasks involving bars and illusory contours. In addition, tier II lesions differentiated between illusory contour types, the deficit being more pronounced for the illusory contour without luminance gradient than for the one with luminance gradient. In contrast to tier I lesions, tier II lesions allowed significant recovery, leading to small final deficits for all contour types tested.     

Does Practice in Orientation Discrimination Lead to Changes in the Response Properties of Macaque Inferior Temporal Neurons?

We trained two rhesus monkeys in a task in which they had to judge whether or not two successively presented gratings differed in orientation. In a first experiment, we trained a monkey for only a restricted set of orientations and then recorded from the temporal cortical visual area (TE) while he made discriminations at trained and untrained orientations. Although this orientation-selective practice induced a marked anisotropy in his behavioural performance, this was not matched by a similar anisotropy in single-cell response properties. In a second experiment, we compared the response properties of TE cells in two monkeys before and after practice in the discrimination of small orientation differences. The training had no effect on either the responsiveness or the orientation tuning. We did, however, observe alterations in the pattern of response modulations induced by the behavioural context. However, these changes with practice, although present in both monkeys, were not consistent from animal to animal. The relevance of these findings for the functional significance of behavioural context dependencies of TE cells, as well as for the plasticity of TE responses, is discussed.     

Synaptogenesis in the Occipital Cortex of Macaque Monkey Devoid of Retinal Input From Early Embryonic Stages

The role of input from the retina on the development of synaptic organization in the primate striate cortex was examined in macaque monkeys enucleated at embryonic (E) day 67 and E59. Both the prenatally operated animals and their age-matched controls were delivered at term (E165) and killed either at 3 months (at the end of the rapid phase of synaptogenesis) or 3 years (at the end of the plateau phase of synaptogenesis). As expected, in the operated animals the striate cortex had a smaller surface area but a normal thickness and complement of layers. The present study revealed that the mean densities of synaptic contacts per unit area and volume of neuropil in the striate cortex of the two operated animals were similar to those of age-matched controls (～30/100 μm² or 100/100 μm³ of neuropil). Thus, the absence of retinal input via the lateral geniculate nucleus did not affect the schedule and magnitude of synaptogenesis. Likewise, the ratio of symmetrical versus asymmetrical synapses and mean lengths of synaptic junctions were within the normal range of variation in both group of animals. The proportions of synaptic contacts situated on dendritic spines and shafts were also similar in supra- and infragranular cortical layers of normal and enucleated animals. However, the ratio of synapses situated on dendritic spines and shafts in the sublayers IVAB and IVC, which normally become reversed during late adolescence, were not reversed in the enucleates. Therefore, our study indicates that certain parameters of synaptic development, such as the density of contacts per unit volume of neuropil and the proportion of basic types and their size, in the supra- and infragranular layers of the striate cortex develop to an optimal normal level in the absence of both retinae from early embryonic stages. However, in the thalamorecipient sublayers the details of the synaptic circuits, such as their localization on dendritic spines or shafts, fail to mature properly in the absence of normal functional input from the periphery.     

Cortical projections to the superior colliculus in prosimian galagos (Otolemur garnetti)

The superior colliculus (SC) is a key structure within the extrageniculate pathway of visual information to cortex and is highly involved in visuomotor functions. Previous studies in anthropoid primates have shown that superficial layers of the SC receive direct inputs from various visual cortical areas such as V1, V2, and middle temporal (MT), while deeper layers receive direct inputs from visuomotor cortical areas within the posterior parietal cortex and the frontal eye fields. Very little is known, however, about the corticotectal projections in prosimian primates. In the current study we investigated the sources of cortical inputs to the SC in prosimian galagos (Otolemur garnetti) using retrograde anatomical tracers placed into the SC. The superficial layers of the SC in galagos received the majority of their inputs from early visual areas and visual areas within the MT complex. Yet, surprisingly, MT itself had relatively few corticotectal projections. Deeper layers of the SC received direct projections from visuomotor areas including the posterior parietal cortex and premotor cortex. However, relatively few corticotectal projections originated within the frontal eye fields. While prosimian galagos resemble other primates in having early visual areas project to the superficial layers of the SC, with higher visuomotor regions projecting to deeper layers, the results suggest that MT and frontal eye field projections to the SC were sparse in early primates, remained sparse in present-day prosimian primates, and became more pronounced in anthropoid primates.     

A Metabolic Mapping Study of Orientation Discrimination and Detection Tasks in the Cat

Increasing evidence suggests that a large number of distinct cortical areas and associated subcortical structures participate in the processing of visual information and that different aspects of visual scenes are evaluated in different areas. This necessitates identification of cortical and subcortical regions cooperating in particular visual tasks. Using the 2-deoxyglucose technique, we monitored the differential activation of areas in the cat visual cortex participating in an orientation discrimination and a detection task. Concordant with previous lesion studies, we found increased activity levels in area 17 in the discrimination condition relative to the detection condition. In addition, the 2-deoxyglucose technique revealed discrimination-related increased activations in the claustrum, the putamen and in parts of the anteromedial, anterolateral and posterolateral lateral suprasylvian visual areas. Regions activated differentially with the detection task comprised subdivisions of areas 17, 18, 19 and 21, posterior area 7 (7p), several areas of the posterior part of the middle and posterior suprasylvian sulcus, the pulvinar complex and the superior colliculus. These results show that the 2-deoxyglucose technique is useful to investigate cognitive brain functions, and that different sets of cortical and subcortical regions are activated during two visual tasks with similar visual stimulation.     

Selectivity of Macaque MT/V5 Neurons for Surface Orientation in Depth Specified by Motion

Area MT/V5 in the macaque brain is one of the major cortical regions involved in the analysis of retinal image motion. The majority of the neurons in this cortical area have non-uniform antagonistic surrounds as components of their receptive field complexes. Theoretical studies indicate that such asymmetrical surrounds should enable neurons to extract orientation in depth from motion. Here we show that nearly half of the MT/V5 neurons encode the tilt component of the orientation in depth of a plane specified by motion. Furthermore, we show that such selectivity for depth from motion depends on the presence of an asymmetrical surround and on the speed tuning of those asymmetrical surround influences.     

Oscillatory Neuronal Responses in the Visual Cortex of the Awake Macaque Monkey

An important step in early visual processing is the segmentation of scenes. Features constituting individual objects have to be grouped together and segregated from those of other figures or the background. It has been proposed that this grouping could be achieved by synchronizing the fine temporal structure of responses from neurons excited by an individual figure. In the cat visual cortex evidence has been obtained that responses of feature-selective neurons have a distinctive oscillatory structure and can synchronize both within and across cortical areas, the synchronization depending on stimulus configuration. Here we investigate the generality of oscillatory responses and their synchronization and specifically whether these phenomena occur in extrastriate areas of the visual cortex of the awake behaving primate. We find in the caudal superior temporal sulcus of the macaque monkey ( Macaca fascicularis ) that adjacent neurons can synchronize their responses, in which case their discharges exhibit an oscillatory temporal structure. During such periods of local synchrony spatially separated cell groups can also synchronize their responses if activated with a single stimulus. These findings resemble those described previously for the cat visual cortex, except that in the awake monkey the oscillatory episodes tend to be of shorter duration and exhibit more variability of oscillation frequency.     

State-Dependent Regulation of Cortical Processing Speed via Gain Modulation

To thrive in dynamic environments, animals must be capable of rapidly and flexibly adapting behavioral responses to a changing context and internal state. Examples of behavioral flexibility include faster stimulus responses when attentive and slower responses when distracted. Contextual or state-dependent modulations may occur early in the cortical hierarchy and may be implemented via top-down projections from corticocortical or neuromodulatory pathways. However, the computational mechanisms mediating the effects of such projections are not known. Here, we introduce a theoretical framework to classify the effects of cell type-specific top-down perturbations on the information processing speed of cortical circuits. Our theory demonstrates that perturbation effects on stimulus processing can be predicted by intrinsic gain modulation, which controls the timescale of the circuit dynamics. Our theory leads to counterintuitive effects, such as improved performance with increased input variance. We tested the model predictions using large-scale electrophysiological recordings from the visual hierarchy in freely running mice, where we found that a decrease in single-cell intrinsic gain during locomotion led to an acceleration of visual processing. Our results establish a novel theory of cell type-specific perturbations, applicable to top-down modulation as well as optogenetic and pharmacological manipulations. Our theory links connectivity, dynamics, and information processing via gain modulation.SIGNIFICANCE STATEMENT To thrive in dynamic environments, animals adapt their behavior to changing circumstances and different internal states. Examples of behavioral flexibility include faster responses to sensory stimuli when attentive and slower responses when distracted. Previous work suggested that contextual modulations may be implemented via top-down inputs to sensory cortex coming from higher brain areas or neuromodulatory pathways. Here, we introduce a theory explaining how the speed at which sensory cortex processes incoming information is adjusted by changes in these top-down projections, which control the timescale of neural activity. We tested our model predictions in freely running mice, revealing that locomotion accelerates visual processing. Our theory is applicable to internal modulation as well as optogenetic and pharmacological manipulations and links circuit connectivity, dynamics, and information processing.     

A Model for the Origin of Motion Direction Selectivity in Visual Cortex

Motion perception is a vital part of our sensory repertoire in that it contributes to navigation, awareness of moving objects, and communication. Motion sense in carnivores and primates originates with primary visual cortical neurons selective for motion direction. More than 60 years after the discovery of these neurons, there is still no consensus on the mechanism underlying direction selectivity. This paper describes a model of the cat''s visual system in which direction selectivity results from the well-documented orientation selectivity of inhibitory neurons: inhomogeneities in the orientation preference map for inhibitory neurons leads to spatially asymmetric inhibition, and thus to direction selectivity. Stimulation of the model with a drifting grating shows that direction selectivity results from the relative timing of excitatory and inhibitory inputs to a neuron. Using a stationary contrast-reversing grating reveals that the inhibitory input is spatially displaced in the preferred direction relative to the excitatory input, and that this asymmetry leads to the timing difference. More generally, the model yields physiologically realistic estimates of the direction selectivity index, and it reproduces the critical finding with contrast-reversing gratings that response phase advances with grating spatial phase. It is concluded that a model based on intracortical inhibition can account well for the known properties of direction selectivity in carnivores and primates.SIGNIFICANCE STATEMENT Motion perception is vital for navigation, communication, and the awareness of moving objects. Motion sense depends on cortical neurons that are selective for motion direction, and this paper describes a model for the physiological mechanism underlying cortical direction selectivity. The essence of the model is that intracortical inhibition of a direction-selective cell is spatially inhomogeneous and therefore depends on whether a stimulus generates inhibition before or after reaching the cell''s receptive field: the response is weaker in the former than in the latter case. If the model is correct, it will contribute to the understanding of motion processing in carnivores and primates.     

Functional Differentiation of Mouse Visual Cortical Areas Depends upon Early Binocular Experience

The mammalian visual cortex contains multiple retinotopically defined areas that process distinct features of the visual scene. Little is known about what guides the functional differentiation of visual cortical areas during development. Recent studies in mice have revealed that visual input from the two eyes provides spatiotemporally distinct signals to primary visual cortex (V1), such that contralateral eye-dominated V1 neurons respond to higher spatial frequencies than ipsilateral eye-dominated neurons. To test whether binocular visual input drives the differentiation of visual cortical areas, we used two-photon calcium imaging to characterize the effects of juvenile monocular deprivation (MD) on the responses of neurons in V1 and two higher visual areas, LM (lateromedial) and PM (posteromedial). In adult mice of either sex, we find that MD prevents the emergence of distinct spatiotemporal tuning in V1, LM, and PM. We also find that, within each of these areas, MD reorganizes the distinct spatiotemporal tuning properties driven by the two eyes. Moreover, we find a relationship between speed tuning and ocular dominance in all three areas that MD preferentially disrupts in V1, but not in LM or PM. Together, these results reveal that balanced binocular vision during development is essential for driving the functional differentiation of visual cortical areas. The higher visual areas of mouse visual cortex may provide a useful platform for investigating the experience-dependent mechanisms that set up the specialized processing within neocortical areas during postnatal development.SIGNIFICANCE STATEMENT Little is known about the factors guiding the emergence of functionally distinct areas in the brain. Using in vivo Ca²⁺ imaging, we recorded visually evoked activity from cells in V1 and higher visual areas LM (lateromedial) and PM (posteromedial) of mice. Neurons in these areas normally display distinct spatiotemporal tuning properties. We found that depriving one eye of normal input during development prevents the functional differentiation of visual areas. Deprivation did not disrupt the degree of speed tuning, a property thought to emerge in higher visual areas. Thus, some properties of visual cortical neurons are shaped by binocular experience, while others are resistant. Our study uncovers the fundamental role of binocular experience in the formation of distinct areas in visual cortex.     

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.     

Cortical connections of visual area MT in the macaque

We have identified the cortical connections of area MT and determined their topographic organization and relationship to myeloarchitectural fields. Efferents of MT were examined in seven macaques that had received injections of tritiated amino acids, and afferents were examined in one macaque that had received injections of two fluorescent dyes. The injection sites formed an orderly sequence from the representation of central to that of peripheral vision in the upper and lower visual fields. In addition to connections with the striate cortex (V1), connections were found between MT and a variety of extrastriate areas, including V2, V3, V3A, V4, V4t, VIP, MST, FST, possibly PO, and, finally, the frontal eye field. The connections of MT with V1, V2, and the dorsal and ventral portions of V3 were topographically organized and consistent with the visuotopic arrangement reported previously in these areas. V2 could be distinguished from V3 by the distinctive myeloarchitectural appearance of the former. Connections with areas V4 and V4t also displayed at least a coarse visuotopic organization, in that the central representation of MT projected laterally in these areas and the peripheral representation projected medially. The lower visual field representation of V4 was located dorsally, on the prelunate convexity, while the upper field representation was located primarily on the ventral aspect of the hemisphere. V4t had a distinctively light myeloarchitecture and received projections from only the lower field representation of MT. The remaining connections of MT were with areas located entirely in the dorsal half of the hemisphere. There were widespread connections with areas MST and FST in the superior temporal sulcus, with some evidence for a crude visuotopic organization in MST. Connections were also found with area VIP in the intraparietal sulcus, with area V3A on the annectent gyrus, possibly with area PO in the dorsomedial prestriate cortex, and, finally, with the frontal eye field on the anterior bank of the lower limb of the arcuate sulcus. Area FST and parts of both MST and VIP had a distinctive myeloarchitecture. The pattern of laminar connections with V1, V2, and V3 indicated that MT projects "back" to these areas and they project "forward" to MT. That is, the projections to these areas from MT terminated in both the supragranular and infragranular layers and the projections to MT from these areas originated predominantly from cells located above granular layer IV (above layer IVC in V1).(ABSTRACT TRUNCATED AT 400 WORDS)     

Hunger Modulates the Responses to Gustatory Stimuli of Single Neurons in the Caudolateral Orbitofrontal Cortex of the Macaque Monkey

1. In order to determine whether the responsiveness of neurons in the caudolateral orbitofrontal cortex (a secondary cortical gustatory area) is influenced by hunger, the activity evoked by prototypical taste stimuli (glucose, NaCl, HCl, and quinine hydrochloride) and fruit juice was recorded in single neurons in this cortical area before, while, and after cynomolgous macaque monkeys were fed to satiety with glucose or fruit juice. 2. It was found that the responses of the neurons to the taste of the glucose decreased to zero while the monkey ate it to satiety during the course of which his behaviour turned from avid acceptance to active rejection. 3. This modulation of responsiveness of the gustatory responses of the neurons to satiety was not due to peripheral adaptation in the gustatory system or to altered efficacy of gustatory stimulation after satiety was reached, because modulation of neuronal responsiveness by satiety was not seen at earlier stages of the gustatory system, including the nucleus of the solitary tract, the frontal opercular taste cortex, and the insular taste cortex. 4. The decreases in the responsiveness of the neurons were relatively specific to the food with which the monkey had been fed to satiety. For example, in seven experiments in which the monkey was fed glucose solution, neuronal responsiveness decreased to the taste of the glucose but not to the taste of blackcurrant juice. Conversely, in two experiments in which the monkey was fed to satiety with fruit juice, the responses of the neurons decreased to fruit juice but not to glucose. 5. These and earlier findings lead to a proposed neurophysiological mechanism for sensory-specific satiety in which the information coded by single neurons in the gustatory system becomes more specific through the processing stages consisting of the nucleus of the solitary tract, the taste thalamus, and the frontal opercular and insular taste primary taste cortices, until neuronal responses become relatively specific for the food tasted in the caudolateral orbitofrontal cortex (secondary) taste area. Then sensory-specific satiety occurs because in this caudolateral orbitofrontal cortex taste area (but not earlier in the taste system) it is a property of the synapses that repeated stimulation results in a decreased neuronal response. 6. Evidence was obtained that gustatory processing involved in thirst also becomes interfaced to motivation in the caudolateral orbitofrontal cortex taste projection area, in that neuronal responses here to water were decreased to zero while water was drunk until satiety was produced.     

Functional Properties of Neurons in the Anterior Bank of the Parieto-occipital Sulcus of the Macaque Monkey

Extracellular recordings were made in the anterior bank of the parieto-occipital sulcus of two waking monkeys trained to perform fixation tasks in normal illumination or in complete darkness. Of the recorded neurons, 73% (251/343) were responsive to visual stimulation, but their overall organization did not conform to a simple, continuous retinotopic map. Most of the visual neurons showed a high degree of orientation and direction sensitivity, higher than that found in areas V1, V2 and V3A under the same experimental conditions. Whether they had a resolvable receptive field or not, the discharge rate of many neurons in the anterior bank of the parieto-occipital sulcus was influenced by oculomotor activity. The animals were required to execute pursuit or saccadic eye movements in darkness. Saccadic eye movements were found to influence 19% of the neurons tested (29/156); by contrast, pursuit eye movements were without effect (0/64). Saccade responses were direction-tuned and, in several cases, the neuronal discharge started before the onset of eye movement. The animals were also required to gaze, in darkness, at nine different positions on the screen they faced. Of the neurons tested, 59% (102/174) were affected by the direction of gaze. Higher discharge rates were generally observed when the animals looked towards the lower part of the field of view. Given the functional properties of its neurons, its connections with area V3A-where neural signals appropriate for building an objective map of the visual space are present (Galletti and Battaglini, 1989, J. Neurosci., 9, 1112-1125)-and its output to the visuomotor centres involved in the generation of saccades (frontal eye fields and superior colliculus), we infer that the cortex of the anterior bank of the parieto-occipital sulcus might be part of the network involved in the control of gaze in order to locate objects in visual space.     

A callosal projection of area 17 upon the border region of area MT in the marmoset monkey, Callithrix jacchus

Injections of the retrograde tracer HRP into the border region of the temporal visual area MT and adjoining cortex in Callithrix labeled pyramidal neurons in area 17 of the contralateral hemisphere. Evidence is presented that this newly discovered heterotopic callosal projection of the monkey striate cortex connects regions of representation of the zero vertical meridian of the visual field in a retinotopic order.