Visual area MT in the Cebus monkey: location, visuotopic organization, and variability

Sex differences in white matter tracts of capuchin monkey brains

Nonhuman primates exhibit sexual dimorphism in behavior, suggesting that there could be underlying differences in brain organization and function. Understanding this neuroanatomical variation is critical for enhancing our understanding of the evolution of sex differences in the human brain. Tufted capuchin monkeys (Sapajus [Cebus] apella) represent a phylogenetically diverse taxa of neotropical primates that converge on several behavioral characteristics with humans relevant to social organization, making them an important point of comparison for studying the evolution of sex differences in primates. While anatomical sex differences in gray matter have previously been found in capuchin monkeys, the current study investigates sex differences in white matter tracts. We carried out tract-based spatial statistical analysis on fractional anisotropy images of tufted capuchin monkeys (15 female, 5 male). We found that females showed significantly higher fractional anisotropy than males in regions of frontal-parietal white matter in the right cerebral hemisphere. Paralleling earlier findings in gray matter, male and female fractional anisotropy values in these regions were nonoverlapping. This complements prior work pointing toward capuchin sex differences in limbic circuitry and higher-order visual regions. We propose that these sex differences are related to the distinct socioecological niches occupied by male and female capuchins. Capuchin neuroanatomical sex differences appear to be more pronounced than in humans, which we suggest may relate to human adaptations for prolonged neurodevelopmental trajectories and increased plasticity.

Cortical maps as a fundamental neural substrate for visual representation

Visual perception is the product of serial hierarchical processing, parallel processing, and remapping on a dynamic network involving several topographically organized cortical visual areas. Here, we will focus on the topographical organization of cortical areas and the different kinds of visual maps found in the primate brain. We will interpret our findings in light of a broader representational framework for perception. Based on neurophysiological data, our results do not support the notion that vision can be explained by a strict representational model, where the objective visual world is faithfully represented in our brain. On the contrary, we find strong evidence that vision is an active and constructive process from the very initial stages taking place in the eye and from the very initial stages of our development. A constructive interplay between perceptual and motor systems (e.g., during saccadic eye movements) is actively learnt from early infancy and ultimately provides our fluid stable visual perception of the world.

Robust natural depth for anticorrelated random dot stereogram for edge stimuli, but minimal reversed depth for embedded circular stimuli, irrespective of eccentricity

The small differences between the images formed in our left and right eyes are an important cue to the three-dimensional structure of scenes. These disparities are encoded by binocular neurons in the visual cortex. At the earliest stage of processing, these respond to binocular correlation between images. We assessed the perception of depth in anticorrelated stimuli, in which the contrast polarity in one eye is reversed, as a function of their location in the retinal image, and their depth configuration (a horizontal edge or a circle surrounded by an annulus) We found that, regardless of stimulus eccentricity, participants perceived depth in the natural direction for edge stimuli, and weakened, reversed depth for circular stimuli.

Myelin densities in retinotopically defined dorsal visual areas of the macaque

The visuotopic organization of dorsal visual cortex rostral to area V2 in primates has been a longstanding source of controversy. Using sub-millimeter phase-encoded retinotopic fMRI mapping, we recently provided evidence for a surprisingly similar visuotopic organization in dorsal visual cortex of macaques compared to previously published maps in New world monkeys (Zhu and Vanduffel, Proc Natl Acad Sci USA 116:2306–2311, 2019). Although individual quadrant representations could be robustly delineated in that study, their grouping into hemifield representations remains a major challenge. Here, we combined in-vivo high-resolution myelin density mapping based on MR imaging (400 µm isotropic resolution) with fine-grained retinotopic fMRI to quantitatively compare myelin densities across retinotopically defined visual areas in macaques. Complementing previously documented differences in populational receptive-field (pRF) size and visual field signs, myelin densities of both quadrants of the dorsolateral posterior area (DLP) and area V3A are significantly different compared to dorsal and ventral area V3. Moreover, no differences in myelin density were observed between the two matching quadrants belonging to areas DLP, V3A, V1, V2 and V4, respectively. This was not the case, however, for the dorsal and ventral quadrants of area V3, which showed significant differences in MR-defined myelin densities, corroborating evidence of previous myelin staining studies. Interestingly, the pRF sizes and visual field signs of both quadrant representations in V3 are not different. Although myelin density correlates with curvature and anticorrelates with cortical thickness when measured across the entire cortex, exactly as in humans, the myelin density results in the visual areas cannot be explained by variability in cortical thickness and curvature between these areas. The present myelin density results largely support our previous model to group the two quadrants of DLP and V3A, rather than grouping DLP- with V3v into a single area VLP, or V3d with V3A+ into DM.

The role of feedback projections in feature tuning and neuronal excitability in the early primate visual system

A general assumption in visual neuroscience is that basic receptive field properties such as orientation and direction selectivity are constructed within intrinsic neuronal circuits and feedforward projections. In addition, it is assumed that general neuronal excitability and responsiveness in early visual areas is to a great extent independent of feedback input originating in areas higher in the stream. Here, we review the contribution of feedback projections from MT, V4 and pulvinar to the receptive field properties of V2 neurons in the anesthetized and paralyzed monkey. Importantly, our results contradict both of these assumptions. We separately inactivated each of these three brain regions using GABA pressure injections, while simultaneously recording V2 single unit activity before and hours after inactivation. Recordings and GABA injections were carried out in topographically corresponding regions of the visual field. We outline the changes in V2 activity, responsiveness and receptive field properties for early, mid and late post-injection phases. Immediately after injection, V2 activity is globally suppressed. Subsequently, there is an increase in stimulus-driven relative to spontaneous neuronal activity, which improves the signal-to-noise coding for the oriented moving bars. Notably, V2 tuning properties change substantially relative to its pre-injection selectivity profile. The resulting increase or decrease in selectivity could not be readily predicted based on the selectivity profile of the inactivated site. Finally, V2 activity rebounds before returning to it pre-injection profile Our results show that feedback projections profoundly impact neuronal circuits in early visual areas, and may have been heretofore largely underestimated in their physiological role.

Early recurrence enables figure border ownership

Rubin's face-vase illusion demonstrates how one can switch back and forth between two different interpretations depending on how the figure outlines are assigned. In the primate visual system, assigning ownership along figure borders is encoded by neurons called the border ownership (BO) cells. Studies show that the responses of these neurons not only depend on the local features within their receptive fields, but also on contextual information. Despite two decades of studies on BO neurons, the ownership assignment mechanism in the brain is still unknown. Here, we propose a hierarchical recurrent model grounded on the hypothesis that neurons in the dorsal stream provide the context required for ownership assignment. Our proposed model incorporates early recurrence from the dorsal pathway as well as lateral modulations within the ventral stream. While dorsal modulations initiate the response difference to figure on either side of the border, lateral modulations enhance the difference. We found responses of our dorsally-modulated BO cells, similar to their biological counterparts, are invariant to size, position and solid/outlined figures. Moreover, our model BO cells exhibit comparable levels of reliability in the ownership signal to biological BO neurons. We found dorsal modulations result in high levels of accuracy and robustness for BO assignments in complex scenes compared to previous models based on ventral feedback. Finally, our experiments with illusory contours suggest that BO encoding could explain the perception of such contours in higher processing stages in the brain.

Effects of MT lesions on visuomotor performance in macaques

Monkeys with selective bilateral lesions of area MT were trained on tasks designed to examine visuomotor function. They were required to: 1- retrieve a small food pellet from a narrow slot; 2- locate and retrieve a loose peanut mounted on a background of fixed peanuts; and 3- retrieve an erratically moving food pellet from a spinning bowl. After the lesions, these monkeys were behaviorally impaired relative to their own preoperative performances and also relative to the postoperative performances of the control monkeys with lesions in optic radiation fibers (OR) under MT or lesions in the posterior parietal cortex (PP). Although their performance improved with practice and time, the MT-lesioned monkeys showed long-term impairments twenty weeks after surgery. Control monkeys performed no worse on the tasks after their lesions. Another task which required the monkeys to retrieve a food pellet without visual guidance revealed that all the animals performed equally poorly when visual cues were unavailable, but that only the control monkeys benefited when visual cues were available. None of the monkeys were impaired on a pattern discrimination learning task. Besides that, direct observations revealed that the MT-lesioned animals grasped peanuts in a manner different from the control animals.

Tangential distribution of cell type and direction selectivity in monkey area MT

We studied the multiunit responses to moving and static stimuli from 585 cell clusters in area MT using multi-electrode arrays. Our aim was to explore if MT columns exhibit any larger-scale tangential organization or clustering based on their response properties. Neurons showing both motion and orientation selectivity were classified into four categories: 1- Type I (orientation selectivity orthogonal to the axis of motion); 2- Type II (orientation selectivity coaxial to the axis of motion); 3- Type DS (significant response to moving stimuli, but non-significant response to static stimuli); and 4- Type OS (significant orientation selectivity, but non-significant direction selectivity). Type I (34%), Type II (24%) and Type DS (32%) clusters were the most predominant and may be associated with different stages of motion processing in MT. On the other hand, the rarer Type OS (9%) may be integrating motion and form processing. Type I and unidirectional sites were the only classes to exhibit significant clustering. Type OS sites showed a trend for clustering, which did not reach statistical significance. We also found a trend for unidirectional sites to have bidirectional sites as neighbors. In conclusion, neuronal clustering associated with these four categories may be related to distinct MT functional circuits.

Sex differences in the brains of capuchin monkeys (Sapajus [Cebus] apella)

This study reports an analysis of 20 T1-weighted magnetic resonance imaging scans from tufted capuchin monkeys (5 male, 15 female). We carried out a data-driven, whole-brain volumetric analysis on regional gray matter anatomy using voxel-based morphometry. This revealed that males showed statistically significant expansion of a region of the hypothalamus, while females showed significant expansion in a distributed set of regions, including the cerebellum, early visual cortex, and higher-order visual regions spanning occipital and temporal cortex. In order to elucidate the network connectivity of these regions, we employed probabilistic tractography on diffusion tensor imaging data. This showed that the female-enlarged regions connect with distributed association networks across the brain. Notably, this contrasts with rodent studies, where sex differences are focused in deep, ancestral limbic regions involved in the control of reproductive behavior. Additionally, in our data set, for several regions, male and female volumetric measures were completely nonoverlapping. This contrasts with human studies, where sex differences in cortical regions have been reported but are characterized by overlapping rather than divergent male and female values. We suggest that these results can be understood in the context of the different lifetime experiences of males and females, which may produce increased experience-dependent cortical plasticity in capuchins compared to rodents, and in humans compared to capuchins.

Cortical Afferents of Area 10 in Cebus Monkeys: Implications for the Evolution of the Frontal Pole

Area 10, located in the frontal pole, is a unique specialization of the primate cortex. We studied the cortical connections of area 10 in the New World Cebus monkey, using injections of retrograde tracers in different parts of this area. We found that injections throughout area 10 labeled neurons in a consistent set of areas in the dorsolateral, ventrolateral, orbital, and medial parts of the frontal cortex, superior temporal association cortex, and posterior cingulate/retrosplenial region. However, sites on the midline surface of area 10 received more substantial projections from the temporal lobe, including clear auditory connections, whereas those in more lateral parts received >90% of their afferents from other frontal areas. This difference in anatomical connectivity reflects functional connectivity findings in the human brain. The pattern of connections in Cebus is very similar to that observed in the Old World macaque monkey, despite >40 million years of evolutionary separation, but lacks some of the connections reported in the more closely related but smaller marmoset monkey. These findings suggest that the clearer segregation observed in the human frontal pole reflects regional differences already present in early simian primates, and that overall brain mass influences the pattern of cortico-cortical connectivity.

Feedback from higher to lower visual areas for visual recognition may be weaker in the periphery: Glimpses from the perception of brief dichoptic stimuli

Eye movements bring attended visual inputs to the center of vision for further processing. Thus, central and peripheral vision should have different functional roles. Here, we use observations of visual perception under dichoptic stimuli to infer that there is a difference in the top-down feedback from higher brain centers to primary visual cortex. Visual stimuli to the two eyes were designed such that the sum and difference of the binocular input from the two eyes have the form of two different gratings. These gratings differed in their motion direction, tilt direction, or color, and duly evoked ambiguous percepts for the corresponding feature. Observers were more likely to perceive the feature in the binocular summation rather than the difference channel. However, this perceptual bias towards the binocular summation signal was weaker or absent in peripheral vision, even when central and peripheral vision showed no difference in contrast sensitivity to the binocular summation signal relative to that to the binocular difference signal. We propose that this bias can arise from top-down feedback as part of an analysis-by-synthesis computation. The feedback is of the input predicted using prior information by the upper level perceptual hypothesis about the visual scene; the hypothesis is verified by comparing the feedback with the actual visual input. We illustrate this process using a conceptual circuit model. In this framework, a bias towards binocular summation can arise from the prior knowledge that inputs are usually correlated between the two eyes. Accordingly, a weaker bias in the periphery implies that the top-down feedback is weaker there. Testable experimental predictions are presented and discussed.

Controversies about the visual areas located at the anterior border of area V2 in primates

Anatomical and electrophysiological studies have provided us with detailed information regarding the extent and topography of the primary (V1) and secondary (V2) visual areas in primates. The consensus about the V1 and V2 maps, however, is in sharp contrast with controversies regarding the organization of the cortical areas lying immediately rostral to V2. In this review, we address the contentious issue of the extent of the third visual area (V3). Specifically, we will argue for the existence of both ventral (V3v) and dorsal (V3d) segments of V3, which are located, respectively, adjacent to the anterior border of ventral and dorsal V2. V3v and V3d would together constitute a single functional area with a complete representation of both upper and lower visual hemifields. Another contentious issue is the organization of the parietal-occipital (PO) area, which also borders the rostral edge of the medial portion of dorsal V2. Different from V1, V2, and V3, which exhibit a topography based on the defined lines of isoeccentricity and isopolar representation, area PO only has a systematic representation of polar angles, with an emphasis on the peripheral visual field (isoeccentricity lines are not well defined). Based on the connectivity patterns of area PO with distinct cytochrome oxidase modules in V2, we propose a subdivision of the dorsal stream of visual information processing into lateral and medial domains. In this model, area PO constitutes the first processing instance of the dorsal-medial stream, coding for the full-field flow of visual cues during navigation. Finally, we compare our findings with those in other species of Old and New World monkeys and argue that larger animals, such as macaque and capuchin monkeys, have similar organizations of the areas rostral to V2, which is different from that in smaller New World monkeys.

Visual motion serves but is not under the purview of the dorsal pathway

Visual motion processing is often attributed to the dorsal visual pathway despite visual motion's involvement in almost all visual functions. Furthermore, some visual motion tasks critically depend on the structural integrity of regions outside the dorsal pathway. Here, based on numerous studies, I propose that visual motion signals are swiftly transmitted via multiple non-hierarchical routes to primary motion-dedicated processing regions (MT/V5 and MST) that are not part of the dorsal pathway, and then propagated to a multiplicity of brain areas according to task demands, reaching these regions earlier than the dorsal/ventral hierarchical flow. This not only places MT/V5 at the same or even earlier visual processing stage as that of V1, but can also elucidate many findings with implications to visual awareness. While the integrity of the non-hierarchical motion pathway is necessary for all visual motion perception, it is insufficient on its own, and the transfer of visual motion signals to additional brain areas is crucial to allow the different motion perception tasks (e.g. optic flow, visuo-vestibular balance, movement observation, dynamic form detection and perception, and even reading). I argue that this lateral visual motion pathway can be distinguished from the dorsal pathway not only based on faster response latencies and distinct anatomical connections, but also based on its full field representation. I also distinguish between this primary lateral visual motion pathway sensitive to all motion in the visual field, and a much less investigated optic flow sensitive medial processing pathway (from V1 to V6 and V6A) that appears to be part of the dorsal pathway. Multiple additional predictions are provided that allow testing this proposal and distinguishing between the visual pathways.

A problem of overlap

Here we propose that earlier-demonstrated details in the primate visual cortical map may account for an otherwise puzzling (and problematic) finding in the current human fMRI literature. Specifically, the well-known regions LO and MT(+) reportedly overlap in the human cortical visual map, when those two regions are localized using standard stimulus comparisons in conventional fMRI experiments. Here we describe evidence supporting the idea that the apparent functional overlap between LO and MT arises from a third area (the MT crescent: “MTc”), which is well known to surround posterior MT based on earlier histological, neuroanatomical, and electrophysiological studies in nonhuman primates. If we assume that MTc also exists in human visual cortex, and that it has a location and functional properties intermediate to those in LO and MT, simplistic modeling confirmed that this arrangement could produce apparent overlap between localizers for LO and MT in conventional fMRI maps in human visual cortex.

Retinotopic organization of extrastriate cortex in the owl monkey--dorsal and lateral areas

Dense retinotopy data sets were obtained by microelectrode visual receptive field mapping in dorsal and lateral visual cortex of anesthetized owl monkeys. The cortex was then physically flatmounted and stained for myelin or cytochrome oxidase. Retinotopic mapping data were digitized, interpolated to a uniform grid, analyzed using the visual field sign technique-which locally distinguishes mirror image from nonmirror image visual field representations-and correlated with the myelin or cytochrome oxidase patterns. The region between V2 (nonmirror) and MT (nonmirror) contains three areas-DLp (mirror), DLi (nonmirror), and DLa/MTc (mirror). DM (mirror) was thin anteroposteriorly, and its reduced upper field bent somewhat anteriorly away from V2. DI (nonmirror) directly adjoined V2 (nonmirror) and contained only an upper field representation that also adjoined upper field DM (mirror). Retinotopy was used to define area VPP (nonmirror), which adjoins DM anteriorly, area FSTd (mirror), which adjoins MT ventrolaterally, and TP (mirror), which adjoins MT and DLa/MTc dorsoanteriorly. There was additional retinotopic and architectonic evidence for five more subdivisions of dorsal and lateral extrastriate cortex-TA (nonmirror), MSTd (mirror), MSTv (nonmirror), FSTv (nonmirror), and PP (mirror). Our data appear quite similar to data from marmosets, though our field sign-based areal subdivisions are slightly different. The region immediately anterior to the superiorly located central lower visual field V2 varied substantially between individuals, but always contained upper fields immediately touching lower visual field V2. This region appears to vary even more between species. Though we provide a summary diagram, given within- and between-species variation, it should be regarded as a guide to parsing complex retinotopy rather than a literal representation of any individual, or as the only way to agglomerate the complex mosaic of partial upper and lower field, mirror- and nonmirror-image patches into areas.

Effect of microstimulation of the superior colliculus on visual space attention

We investigated the effect of microstimulation of the superficial layers of the superior colliculus (SC) on the performance of animals in a peripheral detection paradigm while maintaining fixation. In a matching-to-sample paradigm, a sample stimulus was presented at one location followed by a brief test stimulus at that (relevant) location and a distractor at another (irrelevant) location. While maintaining fixation, the monkey indicated whether the sample and the test stimulus matched, ignoring the distractor. The relevant and irrelevant locations were switched from trial to trial. Cells in the superficial layers of SC gave enhanced responses when the attended test stimulus was inside the receptive field compared with when the (physically identical) distractor was inside the field. These effects were found only in an "automatic" attentional cueing paradigm, in which a peripheral stimulus explicitly cued the animal as to the relevant location in the receptive field. No attentional effects were found with block of trials. The transient enhancement to the attended stimulus was observed at the onset and not at the offset of the stimulus. Electrical stimulation at the site corresponding to the irrelevant distractor location in the SC causes it to gain control over attention, causing impaired performance of the task at the relevant location. Stimulation at unattended sites without the presence of a distractor stimulus causes little or no impairment in performance. The effect of stimulation decays with successive stimulations. The animals learn to ignore the stimulation unless the parameters of the task are varied.

Visually evoked responses in extrastriate area MT after lesions of striate cortex in early life

Lesions of striate cortex [primary visual cortex (V1)] in adult primates result in blindness. In contrast, V1 lesions in neonates typically allow much greater preservation of vision, including, in many human patients, conscious perception. It is presently unknown how this marked functional difference is related to physiological changes in cortical areas that are spared by the lesions. Here we report a study of the middle temporal area (MT) of adult marmoset monkeys that received unilateral V1 lesions within 6 weeks of birth. In contrast with observations after similar lesions in adult monkeys, we found that virtually all neurons in the region of MT that was deprived of V1 inputs showed robust responses to visual stimulation. These responses were very similar to those recorded in neurons with receptive fields outside the lesion projection zones in terms of firing rate, signal-to-noise ratio, and latency. In addition, the normal retinotopic organization of MT was maintained. Nonetheless, we found evidence of a very specific functional deficit: direction selectivity, a key physiological characteristic of MT that is known to be preserved in many cells after adult V1 lesions, was absent. These results demonstrate that lesion-induced reorganization of afferent pathways is sufficient to develop robust visual function in primate extrastriate cortex, highlighting a likely mechanism for the sparing of vision after neonatal V1 lesions. However, they also suggest that interactions with V1 in early postnatal life are critical for establishing stimulus selectivity in MT.

Automatic mapping of visual cortex receptive fields: a fast and precise algorithm

An important issue for neurophysiological studies of the visual system is the definition of the region of the visual field that can modify a neuron's activity (i.e., the neuron's receptive field - RF). Usually a trade-off exists between precision and the time required to map a RF. Manual methods (qualitative) are fast but impose a variable degree of imprecision, while quantitative methods are more precise but usually require more time. We describe a rapid quantitative method for mapping visual RFs that is derived from computerized tomography and named back-projection. This method finds the intersection of responsive regions of the visual field based on spike density functions that are generated over time in response to long bars moving in different directions. An algorithm corrects the response profiles for latencies and allows for the conversion of the time domain into a 2D-space domain. The final product is an RF map that shows the distribution of the neuronal activity in visual-spatial coordinates. In addition to mapping the RF, this method also provides functional properties, such as latency, orientation and direction preference indexes. This method exhibits the following beneficial properties: (a) speed; (b) ease of implementation; (c) precise RF localization; (d) sensitivity (this method can map RFs based on few responses); (e) reliability (this method provides consistent information about RF shapes and sizes, which will allow for comparative studies); (f) comprehensiveness (this method can scan for RFs over an extensive area of the visual field); (g) informativeness (it provides functional quantitative data about the RF); and (h) usefulness (this method can map RFs in regions without direct retinal inputs, such as the cortical representations of the optic disc and of retinal lesions, which should allow for studies of functional connectivity, reorganization and neural plasticity). Furthermore, our method allows for precise mapping of RFs in a 30° by 30° area of the visual field for an array of microelectrodes of any size in less than 6 min.

GABA-induced inactivation of Cebus apella V2 neurons: effects on orientation tuning and direction selectivity

We investigated the GABA-induced inactivation of V2 neurons and terminals on the receptive field properties of this area in an anesthetized and paralyzed Cebus apella monkey. Extracellular single-unit activity was recorded using tungsten microelectrodes in a monkey before and after pressure-injection of a 0.25 or 0.5 M GABA solution. The visual stimulus consisted of a bar moving in 8 possible directions. In total, 24 V2 neurons were studied before and after blocker injections in 4 experimental sessions following GABA injection into area V2. A group of 10 neurons were studied over a short period. An additional 6 neurons were investigated over a long period after the GABA injection. A third group of 8 neurons were studied over a very long period. Overall, these 24 neurons displayed an early (1-20 min) significant general decrease in excitability with concomitant changes in orientation or direction selectivity. GABA inactivation in area V2 produced robust inhibition in 80% and a significant change in directional selectivity in 60% of the neurons examined. These GABA projections are capable of modulating not only levels of spontaneous and driven activity of V2 neurons but also receptive field properties such as direction selectivity.

Color discrimination in the tufted capuchin monkey, Sapajus spp

The present study evaluated the efficacy of an adapted version of the Mollon-Reffin test for the behavioral investigation of color vision in capuchin monkeys. Ten tufted capuchin monkeys (Sapajus spp., formerly referred to as Cebus apella) had their DNA analyzed and were characterized as the following: one trichromat female, seven deuteranope dichromats (six males and one female), and two protanope males, one of which was identified as an “ML protanope.” For their behavioral characterization, all of the subjects were tested at three regions of the Commission International de l'Eclairage (CIE) 1976 u′v′ diagram, with each test consisting of 20 chromatic variation vectors that were radially distributed around the chromaticity point set as the test background. The phenotypes inferred from the behavioral data were in complete agreement with those predicted from the genetic analysis, with the threshold distribution clearly differentiating between trichromats and dichromats and the estimated confusion lines characteristically converging for deuteranopes and the “classic” protanope. The discrimination pattern of the ML protanope was intermediate between protan and deutan, with confusion lines horizontally oriented and parallel to each other. The observed phenotypic differentiation confirmed the efficacy of the Mollon-Reffin test paradigm as a useful tool for evaluating color discrimination in nonhuman primates. Especially noteworthy was the demonstration of behavioral segregation between the “classic” and “ML” protanopes, suggesting identifiable behavioral consequences of even slight variations in the spectral sensitivity of M/L photopigments in dichromats.

GABA inactivation of visual area MT modifies the responsiveness and direction selectivity of V2 neurons in Cebus monkeys

We investigated the contribution of the projections from area MT to the receptive field properties of cells in visual area V2 in anesthetized and paralyzed Cebus apella monkeys. We recorded extracellular single-unit activity using tungsten microelectrodes in three monkeys before and after pressure injection of a 0.25-mol/l GABA solution. The visual stimulus consisted of a single bar moving in one of eight directions. In total, 72 V2 neurons were studied in 18 sessions of GABA injection into area MT. A group of 22 neurons was investigated over a shorter period of time ranging from 15 to 60 min, during which the activity did not return to baseline levels. The remaining 50 neurons were studied over a period of at least 2 h, and no statistical difference was observed in the neuronal response before and long after GABA inactivation. The effects on these 50 neurons consisted of an early (1-20 min) significant general decrease in excitability with changes in either orientation or direction selectivity. The differential decrease in excitability resulted in an intermediate improvement (20-40 min) of the signal-to-noise ratio for the stimulus-driven activity. The inactivation depended on the quantity of GABA injected into area MT and persisted for a period of 2 h. The GABA inactivation in area MT produced inhibition of most cells (72%) and a significant change of direction tuning in the majority (56%) of V2 neurons. Both increases and also decreases in the direction tuning of V2 neurons were observed. These feedback projections are capable of modulating not only the levels of spontaneous and driven activity of V2 neurons but also the V2 receptive field properties, such as direction selectivity.

GABA inactivation of area V4 changes receptive-field properties of V2 neurons in Cebus monkeys

To investigate the contribution of feedback circuits from area V4 to the receptive-field properties of V2 neurons, we used tungsten microelectrodes to record extracellular single units in these visual areas, before and after pressure injections of a solution of 0.25 mol/L of GABA in two anesthetized and paralyzed Cebus apella monkeys. The visual stimulus consisted of a single bar moving in one of eight directions. Using a device made of four stainless steel pipettes and one central tungsten electrode, we inactivated, with different amounts of GABA, topographically corresponding areas of V4, while studying V2 neurons. We studied a total of 36 V2 neurons during six sessions of GABA injections into area V4. GABA inactivation of visual area V4 produced a general decrease in the excitability of the neurons, which included a decrease in spontaneous and driven activities, followed by changes in direction selectivity. The changes in selectivity were toward an increase in directional selectivity and decrease in orientation selectivity. Thus, feedback connections arising from V4, an area of the ventral steams of visual information processing, are capable of not only modulating the spontaneous and driven activity of V2 neurons, but also of modifying V2 receptive field properties, such as its direction and/or orientation selectivity.

The relationship between cortical magnification factor and population receptive field size in human visual cortex: constancies in cortical architecture

Receptive field (RF) sizes and cortical magnification factor (CMF) are fundamental organization properties of the visual cortex. At increasing visual eccentricity, RF sizes increase and CMF decreases. A relationship between RF size and CMF suggests constancies in cortical architecture, as their product, the cortical representation of an RF (point image), may be constant. Previous animal neurophysiology studies of this question yield conflicting results. Here, we use fMRI to determine the relationship between the population RF (pRF) and CMF in humans. In average and individual data, the product of CMF and pRF size, the population point image, is near constant, decreasing slightly with eccentricity in V1. Interhemisphere and subject variations in CMF, pRF size, and V1 surface area are correlated, and the population point image varies less than these properties. These results suggest a V1 cortical processing architecture of approximately constant size between humans. Up the visual hierarchy, to V2, V3, hV4, and LO1, the population point image decreases with eccentricity, and both the absolute values and rate of change increase. PRF sizes increase between visual areas and with eccentricity, but when expressed in V1 cortical surface area (i.e., corticocortical pRFs), they are constant across eccentricity in V2/V3. Thus, V2/V3, and to some degree hV4, sample from a constant extent of V1. This may explain population point image changes in later areas. Consequently, the constant factor determining pRF size may not be the relationship to the local CMF, but rather pRF sizes and CMFs in visual areas from which the pRF samples.

Cone photopigment variations in Cebus apella monkeys evidenced by electroretinogram measurements and genetic analysis

We investigated the color vision pattern in Cebus apella monkeys by means of electroretinogram measurements (ERG) and genetic analysis. Based on ERG we could discriminate among three types of dichromatic males. Among females, this classification is more complex and requires additional genetic analysis. We found five among 10 possible different phenotypes, two trichromats and three dichromats. We also found that Cebus present a new allele with spectral peak near 552nm, with the amino acid combination SFT at positions 180, 277 and 285 of the opsin gene, in addition to the previously described SYT, AFT and AFA alleles.

Distribution of neurofilament proteins in the lateral geniculate nucleus, primary visual cortex, and area MT of adult Cebus monkeys

We investigated the distribution pattern of SMI-32-immunopositive cells in the lateral geniculate nucleus (LGN) and in the primary (V1) and middle temporal (MT) cortical visual areas of the adult New World monkey Cebus apella. In the LGN, the reaction for SMI-32 labeled cells in both the magnocellular (M) and parvocellular (P) layers. However, the cellular label was heavier in M layers, which also showed a more intense labeling in the neuropil. In V1, the reaction showed a lamination pattern, with the heaviest labeling occurring in layer 4B and upper layer 6 (layers that project to area MT). Area MT shows a dense band of labeled neuropil and large pyramidal neurons in layer 3, large darkly labeled but less densely packed neurons in layer 5, and a population of small, lightly labeled cells in layer 6. These results resemble those found in other New and Old World monkeys, which suggest that the preferential labeling of projection neurons associated with fast-conducting pathways to the extrastriate dorsal stream is a common characteristic of simian primates. In the superficial layers of V1 in Cebus monkeys, however, SMI-32-labeled neurons are found in both cytochrome oxidase blobs and interblob regions. In this aspect, our results in Cebus are similar to those found in the Old World monkey Macaca and different from those described for squirrel monkey, a smaller New World Monkey. In Cebus, as well as in Macaca, there is no correlation between SMI-32 distribution and the blob pattern.

Cortical connections of the middle temporal and the middle temporal crescent visual areas in prosimian galagos (Otolemur garnetti)

While considerable progress has been made in understanding the organization of visual cortex in monkeys, less is known about the visual systems of prosimians. The middle temporal visual area (MT), an area involved in motion perception, is common to all primates. We placed injections of tracers in MT and just caudal to MT in cortex expected to be the MT crescent (MTc), an area previously identified in monkeys but not in prosimians. We analyzed the patterns of projections in sections of the flattened cortex and used sections stained for cytochrome oxidase (CO) and myelin to identify the borders of MT, MTc, middle superior temporal (MST), superior temporal sulcus (FST), and V1 and V2 and to identify possible subdivisions of these areas. As in owl monkeys, MTc is a belt around most of MT that consists of a single row of CO-dense patches in a CO-light surround. Injections placed in MT revealed connections with V1, V2, V3, FST, MST, MTc, dorsomedial, dorsolateral (DL), posterior parietal cortex, and inferotemporal (IT) cortex. Injections localized to MTc displayed a slightly different pattern of connections with more involvement of DL and IT cortex, though other aspects, including patchy connections with V1 and V2, were similar to MT connections. The results indicate that prosimian galagos have an MT area with connection patterns that are similar to those in New and Old World monkeys. The MTc, initially described in owl monkeys, is present in galagos and is likely to be a common component of primate visual cortex.

Cortical connections of area V4 in the macaque

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

Two-Dimensional Mapping of the Central and Parafoveal Visual Field to Human Visual Cortex

Primate visual cortex contains a set of maps of visual space. These maps are fundamental to early visual processing, yet their form is not fully understood in humans. This is especially true for the central and most important part of the visual field—the fovea. We used functional magnetic resonance imaging (fMRI) to measure the mapping geometry of human V1 and V2 down to 0.5° of eccentricity. By applying automated atlas fitting procedures to parametrize and average retinotopic measurements of eight brains, we provide a reference standard for the two-dimensional geometry of human early visual cortex of unprecedented precision and analyze this high-quality mean dataset with respect to the 2-dimensional cortical magnification morphometry. The analysis indicates that 1) area V1 has meridional isotropy in areal projection: equal areas of visual space are mapped to equal areas of cortex at any given eccentricity. 2) V1 has a systematic pattern of local anisotropies: cortical magnification varies between isopolar and isoeccentricity lines, and 3) the shape of V1 deviates systematically from the complex-log model, the fit of which is particularly poor close to the fovea. We therefore propose that human V1 be fitted by models based on an equal-area principle of its two-dimensional magnification. 4) V2 is elongated by a factor of 2 in eccentricity direction relative to V1 and has significantly more local anisotropy. We propose that V2 has systematic intrinsic curvature, but V1 is intrinsically flat.

Variations in the structure of the prelunate gyrus in Old World monkeys

Anatomical and electrophysiological studies have revealed a complex organization in the macaque prelunate gyrus. We investigated the morphology and architecture of the prelunate gyrus in Old World monkeys. In Macaca nemestrina, we observed a sulcus crossing the prelunate gyrus within 2 mm of the vertical meridian representation. In other macaque species and other cercopithecines, we observed substantial variations in sulcal morphology across the prelunate gyrus. We did not find a sulcus in all species, and the location and depth of that indentation on the gyrus varied among species. A deep sulcus was observed in all species that emerged earlier in evolution than macaques, such as guenons, baboons, and colobines. We analyzed the regional and parcellation features of the prelunate gyrus in three macaque species, M. maura, M. mulatta, and M. radiata, and in Erythrocebus patas, with emphasis on the relation of structure to the distribution of prelunate visual areas. Nonphosphorylated neurofilament protein immunoreactivity permitted the delineation of a novel area in the prelunate gyrus of Old World monkeys, located around the prelunate sulcus. Species-specific patterns were also observed in the prelunate gyrus of the patas monkey compared to macaques. These observations, as well as a cladistic analysis of the data, suggest an expanded and diversified organization of the prelunate gyrus in some cercopithecoids that may reflect adaptation to specific ecological environments. It was, however, progressively lost in most macaques, being retained only in species that diverged early in the evolution of the genus Macaca, such as M. nemestrina and M. maura.

A distinct anatomical network of cortical areas for analysis of motion in far peripheral vision

We defined cortical areas involved in the analysis of motion in the far peripheral visual field, a poorly understood aspect of visual processing in primates. This was accomplished by small tracer injections within and around the representations of the monocular field of vision ('temporal crescents') in the middle temporal area (MT) of marmoset monkeys. Quantitative analyses demonstrate that the representation of the far periphery receives specific connections from the retrosplenial cortex (areas 23v and prostriata), as well as comparatively stronger inputs from the primary visual area (V1) and from areas surrounding MT (in particular, the medial superior temporal area, MST). In contrast, the far peripheral representation receives little or no input from most other extrastriate areas, including the second visual area (V2), the densely myelinated areas of the dorsomedial cortex, and ventral stream areas; these areas are shown to have robust projections to other parts of MT. Our results demonstrate that the responses of cells in different parts of a same visual area can be determined by different combinations of synaptic inputs, in terms of areas of origin. They also suggest that the interconnections responsible for motion processing in the far periphery of the visual field convey information that is crucial for rapid-response aspects of visual function such as orienting, postural and defensive reactions.

Distribution of calbindin-28kD and parvalbumin in V1 in normal adult Cebus apella monkeys and in monkeys with retinal lesions

Several proteins have their normal patterns of distributions altered by monocular visual deprivation. We studied the distribution of the calcium-binding proteins calbindin-28kD (Cb) and parvalbumin (Pv) in V1 in normal adult Cebus apella monkeys and in monkeys with monocular retinal lesions. In normal monkeys, the interblobs regions in layers 2/3 and the layer 4B are intensely labeled for Cb, while Pv reaction showed a complementary labeling pattern with a stronger staining in layers 4A, 4C and in the blob regions in layers 2/3. In monkeys with monocular retinal lesion, the laminar distribution of these proteins was differentially affected, although both reactions resulted in stronger labeling in non-deprived ocular dominance columns. While Cb reaction resulted in stronger labeling in layers 1 through 5, Pv labeling was heavier in layers 2/3, 4A and 4C. There was a clear reduction in the intensity of neuropil staining for both Pv and Cb in deprived ocular dominance columns with little or no reduction in number of labeled cells. This reduction could thus be attributed to activity-dependent changes at synapses level.

Retinotopic organization of ferret suprasylvian cortex

The retinotopic organization of striate and several extrastriate areas of ferret cortex has been established. Here we describe the representation of the visual field on the Suprasylvian visual area (Ssy). This cortical region runs mediolaterally along the posterior bank of the suprasylvian sulcus, and is distinct from adjoining areas in anatomical architecture. The Ssy lies immediately rostral to visual area 21, medial to lateral temporal areas, and lateral to posterior parietal areas. In electrophysiological experiments we made extracellular recordings in adult ferrets. We find that single and multiunit receptive fields range in size from 2 deg x 4 deg to 21 deg x 52 deg. The total visual field representation in Ssy spans over 70 deg in azimuth in the contralateral hemifield (with a small incursion into the ipsilateral hemifield), and from +36 deg to -30 deg in elevation. There are often two representations of the horizontal meridian. Furthermore, the location of the transition from upper to lower fields varies among animals. General features of topography are confirmed in anatomical experiments in which we made tracer injections into different locations in Ssy, and determined the location of retrograde label in area 17. Both isoelevation and isoazimuth lines can span substantial rostrocaudal and mediolateral distances in cortex, sometimes forming closed contours. This topography results in cortical magnifications averaging 0.07 mm/deg in elevation and 0.06 mm/deg in azimuth; however, some contours can run in such a way that it is possible to move a large distance on cortex without moving in the visual field. Because of these irregularities, Ssy contains a coarse representation of the contralateral visual field.

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

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

Cortical visual areas in monkeys: location, topography, connections, columns, plasticity and cortical dynamics

The visual system is constantly challenged to organize the retinal pattern of stimulation into coherent percepts. This task is achieved by the cortical visual system, which is composed by topographically organized analytic areas and by synthetic areas of the temporal lobe that have more holistic processing. Additional visual areas of the parietal lobe are related to motion perception and visuomotor control. V1 and V2 represent the entire visual field. MT represents only the binocular field, and V4 only the central 30 degrees-40 degrees. The parietal areas represent more of the periphery. For any eccentricity, the receptive field grows at each step of processing, more at anterior areas in the temporal lobe. Minimal point image size increases towards the temporal lobe, but remains fairly constant toward the parietal lobe. Patterns of projection show asymmetries. Central V2 and V4 project mainly to the temporal lobe, while peripherals V2 (more than 30 degrees) and V4 (more than 10 degrees) also project to the parietal lobe. Visual information that arrives at V1 projects to V2, MT and PO, which then project to other areas. Local lateral propagation and recursive loops corroborate to perceptual completion and filling in. Priority connections to temporal, parietal and parieto-temporal cortices help construct crude early representations of objects, trajectories and movements.

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

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

Optical imaging of visually evoked responses in prosimian primates reveals conserved features of the middle temporal visual area

Optical imaging of intrinsic cortical responses to visual stimuli was used to characterize the organization of the middle temporal visual area (MT) of a prosimian primate, the bush baby (Otolemur garnetti). Stimulation with moving gratings revealed a patchwork of oval-like domains in MT. These orientation domains could, in turn, be subdivided into zones selective to directional movements that were mainly orthogonal to the preferred orientation. Similar, but not identical, zones were activated by movements of random dots in the preferred direction. Orientation domains shifted in preference systematically either around a center to form pinwheels or as gradual linear shifts. Stimuli presented in different portions of the visual field demonstrated a global representation of visual space in MT. As optical imaging has revealed similar features in MT of New World monkeys, MT appears to have retained these basic features of organization for at least the 60 million years since the divergence of prosimian and simian primates.

Three streams of visual information processing in V2 of Cebus monkey

Gattass and collaborators (Gattass R, Rosa MGP, Souza APB, Piñon MCG, Neuenschwander S [1990a] Braz J Med Biol Res 23:375-393) proposed that the dorsal stream of visual processing, as defined by Ungerleider and Mishkin (Ungerleider LG, Mishkin M [1982] In: Ingle DJ, Goodale MA, Mansfield RJW, editors. Analysis of visual behavior. Cambridge: Massachusetts Institute of Technology. p 549-586), can be subdivided into dorsolateral and dorsomedial streams, and suggested that they may be involved in different aspects of the processing of motion and spatial perception, respectively. The goal of the present study was to provide additional evidence for this hypothesis by using cytochrome oxidase immunohistochemistry combined with retrograde tracing techniques. In Old World monkeys, the locations of visual area 4 (V4; ventral stream) and middle temporal area (MT; dorsal stream) projecting neurons in V2 supports the hypothesis that the cytochrome oxidase (CytOx)-rich thin stripes and the CytOx-poor interstripes are associated with the ventral stream, and that the CytOx-rich thick stripes belong to the dorsal stream. In this study we describe, in the New World monkey Cebus, the distribution of retrogradely labeled cells in V2 relative to the CytOx compartments after fluorescent tracers were placed in areas V4, MT, and the parietooccipital area (PO). We found PO-projecting neurons in CytOx-rich thick stripes and CytOx-poor interstripes in V2, whereas MT-projecting neurons appeared almost exclusively in thick stripes. In contrast, V4-projecting neurons were located mostly in CytOx-poor interstripes and CytOx-rich thin stripes. In addition, V4- and MT-projecting neurons were located mainly in supragranular layers, whereas PO-projecting neurons were located in supragranular and infragranular layers. These results support the hypothesis for the existence of three distinct streams of visual processing: ventral (including V4), dorsolateral (including MT), and dorsomedial (including PO).

Electrophysiological imaging of functional architecture in the cortical middle temporal visual area of Cebus apella monkey

We studied the spatial organization of directionally selective neurons in the cortical middle temporal visual area (area MT) of the Cebus monkey. We recorded neuronal activity from multielectrode arrays as they were stepped through area MT. The set of recording sites in each array penetration described a plane parallel to the cortical layers. At each recording site, we determined the preferred direction of motion. Responses recorded at successive locations from the same electrode in the array revealed gradual changes in preferred direction, along with occasional directional reversals. Comparisons of responses from adjacent electrodes at successive locations enabled electrophysiological imaging of the two-dimensional pattern of preferred directions across the cortex. Our results demonstrate a systematic organization for directionality in area MT of the New World Cebus monkey, which is similar to that known to exist in the Old World macaque. In addition, our results provide electrophysiological confirmation of map features that have been documented in other cortical areas and primate species by optical imaging. Specifically, the tangential organization of directional selectivity is characterized by slow continuous changes in directional preference, as well as lines (fractures) and points (singularities) that fragment continuous regions into patches. These electrophysiological methods also allowed a direct investigation of neuronal selectivities that give rise to map features. In particular, our results suggest that inhibitory mechanisms may be involved in the generation of fractures and singularities.

Two-dimensional map of direction selectivity in cortical visual area MT of Cebus monkey

We studied the spatial organization of direction of motion in visual area MT of the Cebus apella monkey. We used arrays of 6 (700 micro m apart) parallel electrodes in penetrations tangential to the cortical layers to record multi-unit responses to moving bars, at 200 micro m steps. We determined the direction selectivity at each recording site. The data from single penetrations showed cyclic and gradual changes in the direction selectivity of clusters of cells, intermixed with abrupt 180 degrees discontinuities along the electrode track. In order to obtain maps of direction of motion selectivity, we examined the spatial distribution of direction of motion in MT and we applied a method to determine the location of the centers of radial arrangements of direction selectivity. This tangential organization is characterized by slow continuous changes in direction of motion, interrupted by discontinuities. The changes in direction selectivity are organized radially in a pinwheel fashion and in slabs of linear variation. The pinwheel arrangements have 800-1400 micro m in diameter. The size of the radial arrangement is comparable to the point image size in area MT at each eccentricity.

Distribution of calbindin, parvalbumin and calretinin in the lateral geniculate nucleus and superior colliculus in Cebus apella monkeys

We studied the distribution of the calcium-binding proteins calbindin, parvalbumin and calretinin, in the superior colliculus and in the lateral geniculate nucleus of Cebus apella, a diurnal New World monkey. In the superior colliculus, these calcium-binding proteins show different distribution patterns throughout the layers. After reaction for calretinin one observes a heavy staining of the neuropil with few labeled cells in superficial layers, a greater number of large and medium-sized cells in the stratum griseum intermediale, and small neurons in deep layers. The reaction for calbindin revealed a strong staining of neuropil with a large number of small and well stained cells, mainly in the upper half of the stratum griseum superficiale. Intermediate layers were more weakly stained and depicted few neurons. There were few immunopositive cells and little neuropil staining in deep layers. The reaction for parvalbumin showed small and medium-sized neurons in the superficial layers, a predominance of large stellate cells in the stratum griseum intermediale, and medium-sized cells in the deep layers. In the lateral geniculate nucleus of Cebus, parvalbumin is found in the cells of both the P and M pathways, whereas calbindin is mainly found in the interlaminar and S layers, which are part of the third visual pathway. Calretinin was only found in cells located in layer S. This pattern is similar to that observed in Macaca, showing that these calcium-binding proteins reveal different components of the parallel visual pathways both in New and Old World monkeys.

Topographic organisation of extrastriate areas in the flying fox: implications for the evolution of mammalian visual cortex

The organisation of extrastriate cortex was studied in anaesthetised flying foxes (Pteropus poliocephalus) by using multiunit recording techniques. Based on the visuotopic organisation and response characteristics, the cortex immediately rostral to the second visual area (V2) was subdivided into two fields: visual area 3 (V3) laterally and the occipitoparietal area (OP) medially. Area V3 is a 1.0-1.5 mm wide strip of cortex that represents the entire contralateral hemifield as a mirror image of the representation found in V2. The representation of the vertical meridian and the area centralis form the rostral border of V3. In area OP, receptive fields are much larger than those of V3 and form a separate visuotopic map, with the upper quadrant represented rostral to the lower quadrant. Multiunit clusters in the cortex rostral to area OP (posterior parietal area) respond to both visual and somatosensory stimuli. Farther laterally, in the cortex rostral to V3, the occipitotemporal area (OT) was found to form yet another map of the visual field. Similar to the middle temporal area in primates, area OT in the flying fox forms a first-order representation of the visual field, with the lower quadrant represented medially, the upper quadrant represented laterally, the area centralis represented caudally, and the visual field periphery represented rostrally. The cortex surrounding area OT rostrally and ventrally is also visually responsive but could not be subdivided due to the large receptive fields. Finally, visual responses were elicited from an area adjacent to the peripheral representation in the first visual area (V1) in the splenial sulcus. These results demonstrate that nearly half of the flying fox cortex is related to vision, which contrasts with that of microchiropteran bats, in which auditory areas predominate. A comparison of the flying fox with other mammals suggests that several areas, including homologues of V1, V2, V3, OT, and the splenial area, may have originated early in mammalian evolution and have been inherited by most present-day eutherians. However, studies in other species will be needed to distinguish patterns of common ancestry from parallel evolution.

Spatial representation in the normal visual field: a study of hemifield line bisection

We examined bisection of lines viewed in only one hemifield by normal subjects. Subjects first performed a traditional version of line bisection, by indicating the perceived midpoint of a line on paper with a penmark. Bisection was accurate when they were allowed to shift their gaze over the stimulus, but it was biased towards the central visual field (centripetally) when gaze was fixed so that the line was seen in only one hemifield. In a second experiment, lines with transectors at various locations were presented briefly on a screen and subjects had to indicate on which side of the perceived midpoint the transector was located. A centripetal bias was still found, indicating that it has a perceptual origin. The interaction between bias and effects of tangent line presentation suggested that subjects were performing an angle bisection rather than a line bisection. Also, there was bias in not only right and left hemifields but also upper and lower hemifields. In a third experiment, increasing the width of the stimulus bars peripherally did not eliminate this bias. Bias was size-invariant along the horizontal meridian. This spatial version of Weber's law was modeled by a magnification function using an exponential equation. The slope of this function is much shallower than those currently known for V1, V4 and V5. We conclude that a centripetal bias exists for hemifield line bisection and that this bias likely contributes to the contralateral bias of line bisection by hemianopic patients found in other studies.

The second visual area in the marmoset monkey: visuotopic organisation, magnification factors, architectonical boundaries, and modularity

The organisation of the second visual area (V2) in marmoset monkeys was studied by means of extracellular recordings of responses to visual stimulation and examination of myelin- and cytochrome oxidase-stained sections. Area V2 forms a continuous cortical belt of variable width (1-2 mm adjacent to the foveal representation of V1, and 3-3.5 mm near the midline and on the tentorial surface) bordering V1 on the lateral, dorsal, medial, and tentorial surfaces of the occipital lobe. The total surface area of V2 is approximately 100 mm2, or about 50% of the surface area of V1 in the same individuals. In each hemisphere, the receptive fields of V2 neurones cover the entire contralateral visual hemifield, forming an ordered visuotopic representation. As in other simians, the dorsal and ventral halves of V2 represent the lower and upper contralateral quadrants, respectively, with little invasion of the ipsilateral hemifield. The representation of the vertical meridian forms the caudal border of V2, with V1, whereas a field discontinuity approximately coincident with the horizontal meridian forms the rostral border of V2, with other visually responsive areas. The bridge of cortex connecting dorsal and ventral V2 contains neurones with receptive fields centred within 1 degree of the centre of the fovea. The visuotopy, size, shape and location of V2 show little variation among individuals. Analysis of cortical magnification factor (CMF) revealed that the V2 map of the visual field is highly anisotropic: for any given eccentricity, the CMF is approximately twice as large in the dimension parallel to the V1/V2 border as it is perpendicular to this border. Moreover, comparison of V2 and V1 in the same individuals demonstrated that the representation of the central visual field is emphasised in V2, relative to V1. Approximately half of the surface area of V2 is dedicated to the representation of the central 5 degrees of the visual field. Calculations based on the CMF, receptive field scatter, and receptive field size revealed that the point-image size measured parallel to the V1/V2 border (2-3 mm) equals the width of a full cycle of cytochrome oxidase stripes in V2, suggesting a close correspondence between physiological and anatomical estimates of the dimensions of modular components in this area.

Visual field representation in striate and prestriate cortices of a prosimian primate (Galago garnetti)

Microelectrode mapping techniques were used to study the visuotopic organization of the first and second visual areas (V1 and V2, respectively) in anesthetized Galago garnetti, alorisiform prosimian primate. 1) V1 occupies approximately 200 mm2 of cortex, and is pear shaped, rather than elliptical as in simian primates. Neurons in V1 form a continuous (1st-order) representation of the visual field, with the vertical meridian forming most of its perimeter. The representation of the horizontal meridian divides V1 into nearly equal sectors representing the upper quadrant ventrally, and the lower quadrant dorsally. 2) The emphasis on representation of central vision is less marked in Galago than in simian primates, both diurnal and nocturnal. The decay of cortical magnification factor with increasing eccentricity is almost exactly counterbalanced by an increase in average receptive field size, such that a point anywhere in the visual field is represented by a compartment of similar diameter in V1. 3) Although most of the cortex surrounding V1 corresponds to V2, one-quarter of the perimeter of V1 is formed by agranular cortex within the rostral calcarine sulcus, including area prostriata. Although under our recording conditions virtually every recording site in V2 yielded visually responsive cells, only a minority of those in area prostriata revealed such responses. 4) V2 forms a cortical belt of variable width, being narrowest (approximately 1 mm) in the representation of the area centralis and widest (2.5-3 mm) in the representation of the midperiphery (>20 degrees eccentricity) of the visual field. V2 forms a second-order representation of the visual field, with the area centralis being represented laterally and the visual field periphery medially, near the calcarine sulcus. Unlike in simians, the line of field discontinuity in Galago V2 does not exactly coincide with the horizontal meridian: a portion of the lower quadrant immediately adjacent to the horizontal meridian is represented at the rostral border of ventral V2, instead of in dorsal V2. Despite the absence of cytochrome oxidase stripes, the visual field map in Galago V2 resembles the ones described in simians in that the magnification factor is anisotropic. 5) Receptive field progressions in cortex rostral to dorsal V2 suggest the presence of a homologue of the dorsomedial area, including representations of both quadrants of the visual field. These results indicate that many aspects of organization of V1 and V2 in simian primates are shared with lorisiform prosimians, and are therefore likely to have been present in the last common ancestor of living primates. However, some aspects of organization of the caudal visual areas in Galago are intermediate between nonprimates and simian primates, reflecting either an intermediate stage of differentiation or adaptations to a nocturnal niche. These include the shape and the small size of V1 and V2, the modest degree of emphasis on central visual field representation, and the relatively large area prostriata.