Neurofilament protein is differentially distributed in subpopulations of corticocortical projection neurons in the macaque monkey visual pathways

Previous studies of the primate cerebral cortex have shown that neurofilament protein is present in pyramidal neuron subpopulations displaying specific regional and laminar distribution patterns. In order to characterize further the neurochemical phenotype of the neurons furnishing feedforward and feedback pathways in the visual cortex of the macaque monkey, we performed an analysis of the distribution of neurofilament protein in corticocortical projection neurons in areas V1, V2, V3, V3A, V4, and MT. Injections of the retrogradely transported dyes Fast Blue and Diamidino Yellow were placed within areas V4 and MT, or in areas V1 and V2, in 14 adult rhesus monkeys, and the brains of these animals were processed for immunohistochemistry with an antibody to nonphosphorylated epitopes of the medium and heavy molecular weight subunits of the neurofilament protein. Overall, there was a higher proportion of neurons projecting from areas V1, V2, V3, and V3A to area MT that were neurofilament protein-immunoreactive (57-100%), than to area V4 (25-36%). In contrast, feedback projections from areas MT, V4, and V3 exhibited a more consistent proportion of neurofilament protein-containing neurons (70-80%), regardless of their target areas (V1 or V2). In addition, the vast majority of feedback neurons projecting to areas V1 and V2 were located in layers V and VI in areas V4 and MT, while they were observed in both supragranular and infragranular layers in area V3. The laminar distribution of feedforward projecting neurons was heterogeneous. In area V1, Meynert and layer IVB cells were found to project to area MT, while neurons projecting to area V4 were particularly dense in layer III within the foveal representation. In area V2, almost all neurons projecting to areas MT or V4 were located in layer III, whereas they were found in both layers II-III and V-VI in areas V3 and V3A. These results suggest that neurofilament protein identifies particular subpopulations of corticocortically projecting neurons with distinct regional and laminar distribution in the monkey visual system. It is possible that the preferential distribution of neurofilament protein within feedforward connections to area MT and all feedback projections is related to other distinctive properties of these corticocortical projection neurons.     

Processing of first- and second-order motion signals by neurons in area MT of the macaque monkey

Extrastriate cortical area MT is thought to process behaviorally important visual motion signals. Psychophysical studies suggest that visual motion signals may be analyzed by multiple mechanisms, a "first-order" one based on luminance, and a "second-order" one based upon higher level cues (e.g. contrast, flicker). Second-order motion is visible to human observers, but should be invisible to first-order motion sensors. To learn if area MT is involved in the analysis of second-order motion, we measured responses to first- and second-order gratings of single neurons in area MT (and in one experiment, in area V1) in anesthetized, paralyzed macaque monkeys. For each neuron, we measured directional and spatio-temporal tuning with conventional first-order gratings and with second-order gratings created by spatial modulation of the flicker rate of a random texture. A minority of MT and V1 neurons exhibited significant selectivity for direction or orientation of second-order gratings. In nearly all cells, response to second-order motion was weaker than response to first-order motion. MT cells with significant selectivity for second-order motion tended to be more responsive and more sensitive to luminance contrast, but were in other respects similar to the remaining MT neurons; they did not appear to represent a distinct subpopulation. For those cells selective for second-order motion, we found a correlation between the preferred directions of first- and second-order motion, and weak correlations in preferred spatial frequency. These cells preferred lower temporal frequencies for second-order motion than for first-order motion. A small proportion of MT cells seemed to remain selective and responsive for second-order motion. None of our small sample of V1 cells did. Cells in this small population, but not others, may perform "form-cue invariant" motion processing (Albright, 1992).     

Responses of MT and MST neurons to one and two moving objects in the receptive field

To test the effects of complex visual motion stimuli on the responses of single neurons in the middle temporal visual area (MT) and the medial superior temporal area (MST) of the macaque monkey, we compared the response elicited by one object in motion through the receptive field with the response of two simultaneously presented objects moving in different directions through the receptive field. There was an increased response to a stimulus moving in a direction other than the best direction when it was paired with a stimulus moving in the best direction. This increase was significant for all directions of motion of the non-best stimulus and the magnitude of the difference increased as the difference in the directions of the two stimuli increased. Similarly, there was a decreased response to a stimulus moving in a non-null direction when it was paired with a stimulus moving in the null direction. This decreased response in MT did not reach significance unless the second stimulus added to the null direction moved in the best direction, whereas in MST the decrease was significant when the second stimulus direction differed from the null by 90 degrees or more. Further analysis showed that the two-object responses were better predicted by taking the averaged response of the neuron to the two single-object stimuli than by summation, multiplication, or vector addition of the responses to each of the two single-object stimuli. Neurons in MST showed larger modulations than did neurons in MT with stimuli moving in both the best direction and in the null direction and the average better predicted the two-object response in area MST than in area MT. This indicates that areas MT and MST probably use a similar integrative mechanisms to create their responses to complex moving visual stimuli, but that this mechanism is further refined in MST. These experiments show that neurons in both MT and MST integrate the motion of all directions in their responses to complex moving stimuli. These results with the motion of objects were in sound agreement with those previously reported with the use of random dot patterns for the study of transparent motion in MT and suggest that these neurons use similar computational mechanisms in the processing of object and global motion stimuli.     

Organization of disparity-selective neurons in macaque area MT

Neurons selective for binocular disparity are found in a number of visual cortical areas in primates, but there is little evidence that any of these areas are specialized for disparity processing. We have examined the organization of disparity-selective neurons in the middle temporal visual area (MT), an area shown previously to contain an abundance of disparity-sensitive neurons. We recorded extracellularly from MT neurons at regularly spaced intervals along electrode penetrations that passed through MT either normal to the cortical surface or at a shallow oblique angle. Comparison of multiunit and single-unit recordings shows that neurons are clustered in MT according to their disparity selectivity. Across the surface of MT, disparity-selective neurons are found in discrete patches that are separated by regions of MT that exhibit poor disparity tuning. Within disparity-selective patches of MT, we typically observe a smooth progression of preferred disparities (e.g. , near to far) as our electrode travels parallel to the cortical surface. In electrode penetrations normal to the cortical surface, on the other hand, MT neurons generally have similar disparity tuning, with little variation from one recording site to the next. Thus disparity-tuned neurons are organized into cortical columns by preferred disparity, and preferred disparity is mapped systematically within larger, disparity-tuned patches of MT. Combined with other recent findings, the data suggest that MT plays an important role in stereoscopic depth perception in addition to its well known role in motion perception.     

Modular organization of occipito-temporal pathways: cortical connections between visual area 4 and visual area 2 and posterior inferotemporal ventral area in macaque monkeys

The modular organization of cortical pathways linking visual area 4 (V4) with occipital visual area 2 (V2) and inferotemporal posterior inferotemporal ventral area (PITv) was investigated through an analysis of the patterns of retrogradely labeled cell bodies after injections of tracers into V4 and PITv. Although cytochrome oxidase or other stains have failed to yield reliable independent anatomical markers for cortical modules beyond V1 and V2, V4 and PITv seem to have modular compartments with specific patterns of cortico-cortical connectivity. Tracer injections of V4 labeled cells in V2 (1) thin stripes exclusively, (2) interstripes exclusively, or (3) specific combinations of interstripe and thin stripe subcompartments. These labeling patterns suggest (1) that there is a complicated organization of inputs to V4, (2) that projections from V2 to V4 display a submodular selectivity, and (3) that projections from V2 to V4 display some degree of cross-stream convergence. Consistent with this framework, extensive regions of PITv provide feedback projections to interstripe-recipient portions of V4, whereas more restricted portions of PITv provide feedback to thin stripe-recipient portions of V4. Similarly, the feedforward projection from V4 to PITv often arose from multiple cell clusters across a wide expanse of V4. When distinguishable fluorescent tracers were injected into two PITv sites separated by 3-5 mm, a variety of projection patterns was observed in V4. In most cases, labeled cells were found in multiple, interdigitating, nonoverlapping clusters of 1-3 mm width, whereas in other cases the two labeled fields were highly intermixed. These results suggest that V4 and PITv contain functional modules that can be characterized by the specific patterns of segregated and convergent projections they receive from lower cortical areas. These specific patterns of intercortical input, in conjunction with intrinsic cortical circuitry, may endow extrastriate cortical neurons with new and more complex receptive field properties.     

The constructive nature of vision: direct evidence from functional magnetic resonance imaging studies of apparent motion and motion imagery

Echoplanar functional magnetic resonance imaging was used to monitor activation changes of brain areas while subjects viewed apparent motion stimuli and while they were engaged in motion imagery. Human cortical areas MT (V5) and MST were the first areas of the 'dorsal' processing stream which responded with a clear increase in signal intensity to apparent motion stimuli as compared with flickering control conditions. Apparent motion of figures defined by illusory contours evoked greater activation in V2 and MT/MST than appropriate control conditions. Several areas of the dorsal pathway (V3A, MT/MST, areas in the inferior and superior parietal lobule) as well as prefrontal areas including FEF and BA 9/46 responded strongly when subjects merely imagined moving stimuli which they had seen several seconds before. The activation during motion imagery increased with the synaptic distance of an area from V1 along the dorsal processing stream. Area MT/MST was selectively activated during motion imagery but not during a static imagery control condition. The comparison between the results obtained with objective motion, apparent motion and imagined motion provides further insights into a complex cortical network of motion-sensitive areas driven by bottom-up and top-down neural processes.     

Optic flow processing in monkey STS: a theoretical and experimental approach

How does the brain process visual information about self-motion? In monkey cortex, the analysis of visual motion is performed by successive areas specialized in different aspects of motion processing. Whereas neurons in the middle temporal (MT) area are direction-selective for local motion, neurons in the medial superior temporal (MST) area respond to motion patterns. A neural network model attempts to link these properties to the psychophysics of human heading detection from optic flow. It proposes that populations of neurons represent specific directions of heading. We quantitatively compared single-unit recordings in area MST with single-neuron simulations in this model. Predictions were derived from simulations and subsequently tested in recorded neurons. Neuronal activities depended on the position of the singular point in the optic flow. Best responses to opposing motions occurred for opposite locations of the singular point in the visual field. Excitation by one type of motion is paired with inhibition by the opposite motion. Activity maxima often occur for peripheral singular points. The averaged recorded shape of the response modulations is sigmoidal, which is in agreement with model predictions. We also tested whether the activity of the neuronal population in MST can represent the directions of heading in our stimuli. A simple least-mean-square minimization could retrieve the direction of heading from the neuronal activities with a precision of 4.3 degrees. Our results show good agreement between the proposed model and the neuronal responses in area MST and further support the hypothesis that area MST is involved in visual navigation.     

Tuning bandwidths for near-threshold stimuli in area MT

It is not known whether psychophysical performance depends primarily on small numbers of neurons optimally tuned to specific visual stimuli, or on larger populations of neurons that vary widely in their properties. Tuning bandwidths of single cells can provide important insight into this issue, yet most bandwidth measurements have been made using suprathreshold visual stimuli, whereas psychophysical measurements are frequently obtained near threshold. We therefore examined the directional tuning of cells in the middle temporal area (MT, or V5) using perithreshold, stochastic motion stimuli that we have employed extensively in combined psychophysical and physiological studies. The strength of the motion signal (coherence) in these displays can be varied independently of its direction. For each MT neuron, we characterized the directional bandwidth by fitting Gaussian functions to directional tuning data obtained at each of several motion coherences. Directional bandwidth increased modestly as the coherence of the stimulus was reduced. We then assessed the ability of MT neurons to discriminate opposed directions of motion along six equally spaced axes of motion spanning 180 degrees. A signal detection analysis yielded neurometric functions for each axis of motion, from which neural thresholds could be extracted. Neural thresholds remained surprisingly low as the axis of motion diverged from the neuron's preferred-null axis, forming a plateau of high to medium sensitivity that extended approximately 45 degrees on either side of the preferred-null axis. We conclude that directional tuning remains broad in MT when motion signals are reduced to near-threshold values. Thus directional information is widely distributed in MT, even near the limits of psychophysical performance. These observations support models in which relatively large numbers of signals are pooled to inform psychophysical decisions.     

Callosally projecting neurons in the macaque monkey V1/V2 border are enriched in nonphosphorylated neurofilament protein

Previous immunohistochemical studies combined with retrograde tracing in macaque monkeys have demonstrated that corticocortical projections can be differentiated by their content of neurofilament protein. The present study analyzed the distribution of nonphosphorylated neurofilament protein in callosally projecting neurons located at the V1/V2 border. All of the retrogradely labeled neurons were located in layer III at the V1/V2 border and at an immediately adjacent zone of area V2. A quantitative analysis showed that the vast majority (almost 95%) of these interhemispheric projection neurons contain neurofilament protein immunoreactivity. This observation differs from data obtained in other sets of callosal connections, including homotypical interhemispheric projections in the prefrontal, temporal, and parietal association cortices, that were found to contain uniformly low proportions of neurofilament protein-immunoreactive neurons. Comparably, highly variable proportions of neurofilament protein-containing neurons have been reported in intrahemispheric corticocortical pathways, including feedforward and feedback visual connections. These results indicate that neurofilament protein is a prominent neurochemical feature that identifies a particular population of interhemispheric projection neurons at the V1/V2 border and suggest that this biochemical attribute may be critical for the function of this subset of callosal neurons.     

Visual interneurons, sensitive to the size of objects and the direction of their movement in dragonflies of the genus Sympetrum]

Experiments have been made on 4 dragonfly species -- Sympetrum vulgatum, S. flaveolum, S sanguineum, S. danae. A pair of neurons was found in the thoracic ganglia and connectives, which has symmetrical contralateral receptive fields. These neurons are selectively sensitive to swift upward motion of a target of 3--10 degrees in size. This type of response was originally described by Zenkin and Pigarev [1, 2]. The receptive field, 120X25 degrees in size, is oriented horizontally from the medial rim of the eye. The center of sensitivity has the following polar coordinates: 15 degrees laterally from the medial plane and 20 degrees above the equatorial one. The relation of detecting properties of the observed neurones to key stimuli which trigger hunting behaviour is discussed. It is suggested that filtration of single and small optic stimuli by specialized detector neurons results not from the processes in the own receptive field of the neuron, but from the interaction with other neurons which are sensitive to motion of large objects and complex patterns.     

Behavioral constraints in the development of neuronal properties: a cortical model embedded in a real-world device

The ability of organisms to categorize diverse and often novel stimuli depends on ongoing interactions with their environment. In a modality such as vision, categorization requires the generation of both selective and invariant responses of cortical neurons to complex visual stimuli. How does behavior contribute to shaping the responses of these neurons? Analysis of this question is made difficult by the complex multilevel interactions between many neural and behavioral variables. To mitigate this difficulty, we studied the development and ongoing plasticity of pattern-selective neuronal responses by means of synthetic neural modeling. For this purpose, we constructed Darwin V, which consists of a simulated neuronal model embedded in a real-world device that is capable of motion and autonomous behavior. The neuronal model consists of four major components: a visual system (containing cortical and subcortical networks); a taste system based on conductance; sets of motor neurons capable of triggering behavior; and a diffuse ascending (value) system. The modeled visual cortex consists of two areas: a topographic map responsive to elementary features connected to a higher-order map composed of initially non-selective neuronal units. During behavior over time in its environment, Darwin V encounters numerous objects consisting of black metal cubes displaying different patterns of white blobs and stripes. Initially, the lack of specific higher-order visual responses does not allow visual pattern discrimination, and appetitive and aversive behaviors are triggered by the 'taste' (surface conductivity of objects) alone. In the course of sensory experience, however, changes occur in visual and sensorimotor connection strengths, with two major consequences. First, units within the higher visual area acquire responses that are both pattern selective and translation invariant. Second, as a result of the operation of the value system, these responses become linked to appropriate behaviors. Analysis of Darwin V after such changes indicates that the continuity of self-generated movements is essential for the development of pattern-selective and translation-invariant responses. The concomitant development of a preference for foveal over parafoveal objects was found to be due to increased behavioral interactions with object cubes gripped by the centrally mounted effector (snout) of Darwin V. Finally, even after development of higher-order visual responses, visual responses to more frequently encountered objects continued to be enhanced, while other responses were diminished. Overall, the detailed study of Darwin V over multiple levels of organization provides a heuristically revealing example of the crucial role played by behavioral and environmental interactions in the development of complex responses by specialized neurons.     

Cortical area MT and the perception of stereoscopic depth

Stereopsis is the perception of depth based on small positional differences between images formed on the two retinae (known as binocular disparity). Neurons that respond selectively to binocular disparity were first described three decades ago, and have since been observed in many visual areas of the primate brain, including V1, V2, V3, MT and MST. Although disparity-selective neurons are thought to form the neural substrate for stereopsis, the mere existence of disparity-selective neurons does not guarantee that they contribute to stereoscopic depth perception. Some disparity-selective neurons may play other roles, such as guiding vergence eye movements. Thus, the roles of different visual areas in stereopsis remain poorly defined. Here we show that visual area MT is important in stereoscopic vision: electrical stimulation of clusters of disparity-selective MT neurons can bias perceptual judgements of depth, and the bias is predictable from the disparity preference of neurons at the stimulation site. These results show that behaviourally relevant signals concerning stereoscopic depth are present in MT.     

The effects of V4 and middle temporal (MT) area lesions on visual performance in the rhesus monkey

The effects of V4, MT, and combined V4 + MT lesions were assessed on a broad range of visual capacities that included measures of contrast sensitivity, wavelength and brightness discrimination, form vision, pattern vision, motion and flicker perception, stereopsis, and the selection of stimuli that were less prominent than those with which they appeared in stimulus arrays. The major deficit observed was a loss in the ability, after V4 lesions, to select such less prominent stimuli; this was the case irrespective of the manner in which the stimulus arrays were made visible, using either luminance, chrominance, motion, or stereoscopic depth as surface media. In addition, V4 lesions yielded mild deficits in color, brightness, and form vision whereas MT lesions yielded mild to moderate deficits in motion and flicker perception. Both lesions produced mild deficits in contrast sensitivity, shape-from-motion perception, and yielded increased reaction times on many of the tasks. The impairment resulting from combined V4 and MT lesions was not greater than the sum of the deficits of either lesion. None of the lesions produced significant deficits in stereopsis. The findings suggest that (1) area V4 is part of a neural system that is involved in extracting stimuli from the visual scene that elicit less neural activity early in the visual system than do other stimuli with which they appear and (2) several other extrastriate regions and more than just two major cortical processing streams contribute to the processing of basic visual functions in the extrastriate cortex.     

Recanalization of venule following experimental retinal branch vein occlusion

Wenderoth, Bray and Johnstone [(1988) Perception, 17, 81-91] measured motion aftereffects induced on stationary vertical sine-wave gratings by horizontally drifting two-dimensional patterns (plaids). The adapting plaid component gratings were simultaneously or alternately presented and were oriented left and right of vertical by 15, 45 or 75 degrees. It was found that aftereffects decreased linearly in the alternating conditions as the plaid component orientations changed but this was not the case in the simultaneous adaptation conditions, a finding taken to be consistent with the hypothesis that one-dimensional aftereffects have a low level site (possibly V1) whereas two-dimensional effects have a higher level site (possibly MT). In three experiments, we have examined in more detail the determinants of aftereffects induced by simultaneous and alternating plaid components. The data suggest that the mechanisms involved are more complex than those put forward by Wenderoth et al. and that plaid perception utilizes both higher and lower level processes which can be referred to, respectively, as an intersection of constraints algorithm and a moving "blob" detector.     

Ventral intraparietal area of the macaque: anatomic location and visual response properties

1. The middle temporal area (MT) projects to the intraparietal sulcus in the macaque monkey. We describe here a discrete area in the depths of the intraparietal sulcus containing neurons with response properties similar to those reported for area MT. We call this area the physiologically defined ventral intraparietal area, or VIP. In the present study we recorded from single neurons in VIP of alert monkeys and studied their visual and oculomotor response properties. 2. Area VIP has a high degree of selectivity for the direction of a moving stimulus. In our sample 72/88 (80%) neurons responded at least twice as well to a stimulus moving in the preferred direction compared with a stimulus moving in the null direction. The average response to stimuli moving in the preferred direction was 9.5 times as strong as the response to stimuli moving in the opposite direction, as compared with 10.9 times as strong for neurons in area MT. 3. Many neurons were also selective for speed of stimulus motion. Quantitative data from 25 neurons indicated that the distribution of preferred speeds ranged from 10 to 320 degrees/s. The degree of speed tuning was on average twice as broad as that reported for area MT. 4. Some neurons (22/41) were selective for the distance at which a stimulus was presented, preferring a stimulus of equivalent visual angle and luminance presented near (within 20 cm) or very near (within 5 cm) the face. These neurons maintained their preference for near stimuli when tested monocularly, suggesting that visual cues other than disparity can support this response. These neurons typically could not be driven by small spots presented on the tangent screen (at 57 cm). 5. Some VIP neurons responded best to a stimulus moving toward the animal. The absolute direction of visual motion was not as important for these cells as the trajectory of the stimulus: the best stimulus was one moving toward a particular point on the face from any direction. 6. VIP neurons were not active in relation to saccadic eye movements. Some neurons (10/17) were active during smooth pursuit of a small target. 7. The predominance of direction and speed selectivity in area VIP suggests that it, like other visual areas in the dorsal stream, may be involved in the analysis of visual motion.     

Gamma delta T cells of the murine vagina: T cell response in vivo in the absence of the expression of CD2 and CD28 molecules

While little is known about their activation requirements and function, the intraepithelial T cells of the murine vagina express TCR complexes in which the antigen recognition components and the signaling components have unusual features. These vaginal T cells express an invariant V gamma 4/V delta 1 TCR and appear to be the only intraepithelial gamma delta T cells that exclusively use FcR gamma chains in their TCR complex. To further characterize the vaginal gamma delta T cells we isolated them from normal mice and from mice injected systemically with an activation-inducing dose of anti-TCR mAb. The isolated gamma delta T cells were examined by flow cytometry for their surface expression of a panel of adhesion, proteins, activation antigens and cellular interaction molecules (CD44, CD62L, CD45RB, LFA-1, CD2 and CD28). The patterns of expression observed indicate that the vaginal gamma delta T cells of normal mice show the phenotype of effector T cells. The adhesion/co-stimulatory molecules CD28 and CD2 were not detected on vaginal gamma delta T cells, an interesting finding since the absence of CD2 from other T cells has been suggested to result in anergy. However, vaginal gamma delta T cells are responsive to TCR-mediated signals since injection of normal mice with pan-anti-TCR antibody or stimulating anti-gamma delta TCR antibody resulted in an increase in cell number and increased expression of transferrin and IL-2 receptors. These results indicate that vaginal gamma delta T cells might utilize other co-stimulatory molecules, if any, in connection with TCR-induced activation and differentiation. While the physiological function of vaginal gamma delta T cells remains unknown, the expression of an invariant V gamma 4/V delta 1 TCR, their exclusive use of gamma chain homodimers in their TCR, and the absence of CD2 and CD28 co-stimulatory molecules are a novel combination of properties that suggests specialized functional properties. Although vaginal gamma delta T cells share some features in common with gamma delta T cells that reside in other epithelial tissues, such as skin and intestine, the present studies provide additional evidence that vaginal gamma delta T cells are a highly specialized and distinct T cell population.     

Second order components of moving plaids activate extrastriate cortex: a positron emission tomography study

A moving plaid is a composite pattern produced by superimposing two sinusoidal gratings which differ in orientation and motion direction. The perceived drift direction of a plaid appears to be determined partly by a binocular mechanism, which follows intersection of constraint rules (Burke and Wenderoth, 1993b), and partly by a monocular mechanism, which tracks the dark and bright intersects of the plaid, the contrast envelopes. The first neurones that respond to plaids as patterns rather than component gratings are found in area V5, also known as MT, which is exclusively binocular. Therefore, the psychophysical evidence suggesting that the contrast envelope tracking mechanism is monocular is surprising but has been obtained consistently. We aimed to localize the contrast envelope tracking mechanism by undertaking a positron emission tomography (PET) activation experiment in which the subjects were presented with alternating plaid components during the control scan and with the moving plaid resulting from the superposition of these components as the activation scan. The results showed differential activation in area V3. Recent results from macaque single cell recordings have also demonstrated increased sensitivity to moving plaid stimuli compared to the plaid component gratings in V3 neurones.     

Recovery from object substitution masking induced by transient suppression of visual motion processing: A repetitive transcranial magnetic stimulation study.

Object substitution masking is a form of visual backward masking in which a briefly presented target is rendered invisible by a lingering mask that is too sparse to produce lower image-level interference. Recent studies suggested the importance of an updating process in a higher object-level representation, which should rely on the processing of visual motion, in this masking. Repetitive transcranial magnetic stimulation (rTMS) was used to investigate whether functional suppression of motion processing would selectively reduce substitution masking. rTMS-induced transient functional disruption of cortical area V5/MT+, which is important for motion analysis, or V1, which is reciprocally connected with V5/MT+, produced recovery from masking, whereas sham stimulation did not. Furthermore, masking remained undiminished following rTMS over the region 2 cm posterior to V5/MT+, ruling out nonspecific effects of real stimulation and confirming regional specificity of the rTMS effect. The results suggest that object continuity via the normal function of the visual motion processing system might in part contribute to this masking. The relation of these findings to the reentrant processing view of object substitution masking and other visual phenomena is discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of stimulus speed upon the antagonistic surrounds of area MT/V5 neurons

We investigated the effect of stimulus speed upon surround antagonism in macaque MT/V5 neurons, using probe stimuli placed at different positions in the surround. Their speed was varied, while the stimulation of the excitatory receptive field (RF) was held at optimal speed. Most Surrounds proved asymmetric, arising from a single region on one side of RF, although bilaterally and circularly symmetric surrounds were occasionally observed. Surround organization was generally retained at faster or slower surround speeds. Speed-dependent changes usually entailed diminished position dependence of surround influence, consequent to reduced surround effect at the position producing maximum inhibition. The effect of a stimulus covering the entire surround was much less dependent upon motion speed. Results show that surround non-uniformity is a robust finding in MT/V5 and endows neurons with multiple mechanisms for extracting surface orientation in depth.     

A model for encoding multiple object motions and self-motion in area MST of primate visual cortex

Many cells in the dorsal part of the medial superior temporal (MST) region of visual cortex respond selectively to specific combinations of expansion/contraction, translation, and rotation motions. Previous investigators have suggested that these cells may respond selectively to the flow fields generated by self-motion of an observer. These patterns can also be generated by the relative motion between an observer and a particular object. We explored a neurally constrained model based on the hypothesis that neurons in MST partially segment the motion fields generated by several independently moving objects. Inputs to the model were generated from sequences of ray-traced images that simulated realistic motion situations, combining observer motion, eye movements, and independent object motions. The input representation was based on the response properties of neurons in the middle temporal area (MT), which provides the primary input to area MST. After applying an unsupervised optimization technique, the units became tuned to patterns signaling coherent motion, matching many of the known properties of MST cells. The results of this model are consistent with recent studies indicating that MST cells primarily encode information concerning the relative three-dimensional motion between objects and the observer.