Primate Area MT Research Articles

Dynamic population codes of multiplexed stimulus features in primate area MT.

The middle-temporal area (MT) of primate visual cortex is critical in the analysis of visual motion. Single-unit studies suggest that the response dynamics of neurons within area MT depend on stimulus features, but how these dynamics emerge at the population level, and how feature representations interact, is not clear. Here, we used multivariate classification analysis to study how stimulus features are represented in the spiking activity of populations of neurons in area MT of marmoset monkey. Using representational similarity analysis we distinguished the emerging representations of moving grating and dot field stimuli. We show that representations of stimulus orientation, spatial frequency, and speed are evident near the onset of the population response, while the representation of stimulus direction is slower to emerge and sustained throughout the stimulus-evoked response. We further found a spatiotemporal asymmetry in the emergence of direction representations. Representations for high spatial frequencies and low temporal frequencies are initially orientation dependent, while those for high temporal frequencies and low spatial frequencies are more sensitive to motion direction. Our analyses reveal a complex interplay of feature representations in area MT population response that may explain the stimulus-dependent dynamics of motion vision.NEW & NOTEWORTHY Simultaneous multielectrode recordings can measure population-level codes that previously were only inferred from single-electrode recordings. However, many multielectrode recordings are analyzed using univariate single-electrode analysis approaches, which fail to fully utilize the population-level information. Here, we overcome these limitations by applying multivariate pattern classification analysis and representational similarity analysis to large-scale recordings from middle-temporal area (MT) in marmoset monkeys. Our analyses reveal a dynamic interplay of feature representations in area MT population response.

Feature attention for binocular disparity in primate area MT depends on tuning strength.

Attending to a stimulus modulates the responses of sensory neurons that represent features of that stimulus, a phenomenon named "feature attention." For example, attending to a stimulus containing upward motion enhances the responses of upward-preferring direction-selective neurons in the middle temporal area (MT) and suppresses the responses of downward-preferring neurons, even when the attended stimulus is outside of the spatial receptive fields of the recorded neurons (Treue S, Martinez-Trujillo JC. Nature 399: 575-579, 1999). This modulation renders the representation of sensory information across a neuronal population more selective for the features present in the attended stimulus (Martinez-Trujillo JC, Treue S. Curr Biol 14: 744-751, 2004). We hypothesized that if feature attention modulates neurons according to their tuning preferences, it should also be sensitive to their tuning strength, which is the magnitude of the difference in responses to preferred and null stimuli. We measured how the effects of feature attention on MT neurons in rhesus monkeys (Macaca mulatta) depended on the relationship between features-in our case, direction of motion and binocular disparity-of the attended stimulus and a neuron's tuning for those features. We found that, as for direction, attention to stimuli containing binocular disparity cues modulated the responses of MT neurons and that the magnitude of the modulation depended on both a neuron's tuning preferences and its tuning strength. Our results suggest that modulation by feature attention may depend not just on which features a neuron represents but also on how well the neuron represents those features.

Coding of changes in motion direction by local field potentials in primate area MT

Coding of changes in motion direction by local field potentials in primate area MT

Influence of visually guided tracking arm movements on single cell activity in area MT

The behavioral relevance of neuronal activity in primate area MT for motion perception and control of visually guided eye movements is well documented. The projections of area MT comprise connections to subcortical structures and to the parietal network, both of which play a role in visuospatial transformation for guiding eyes and hands. Here, we have investigated, whether area MT is involved in the network needed to control visually guided arm movements. Our results show that half of the neurons tested significantly modulated their activity during visually guided arm movements. We conclude that the main reason for the neuronal modulation is not the arm movement per se, but the use of information from MT for visual feedback in the tracking movement. Moreover, control experiments show that attentional effects cannot solely cause the neuronal modulation. Thus, our study provides strong evidence that area MT is involved in processing visual information for visually guided manual tracking movements.

Reconstructing the World: Switching from Segmentation to Integration Allows Neurons in Area MT to Make “Sense” of the Visual Scene

Huang et al. in this issue of Neuron show that primate area MT neurons exploit contextual cues to adequately interpret motion information. MT neurons switch from segmentation to integration when motion arises from single rather than multiple objects. This switching may help solve the aperture problem and bind distant object components into a perceptual whole.

Encoding of dynamic visual stimuli by primate area MT neurons

Neural stimulus selectivity is thought to be optimized for the representation of real-world stimuli. Neural coding properties, therefore, may adapt to different environments. Here, we address the question if tuning curves depend on the statistics of visual stimuli. This is done by studying the directional tuning of macaque area MT neurons exposed to dynamic motion stimuli of two different direction progression statistics. Despite an apparent difference of tuning curves across stimulus conditions, our results support the view that the underlying encoding system is robust and subject to only restricted malleability by stimulus statistics.

Lesions of extrastriate cortex and consequences for visual guidance during locomotion.

One of the functions of the putative motion pathway in visual cortex may be visual guidance during locomotion. Using cats, we have investigated the role of the first area in this pathway, the lateral suprasylvian visual area (LS, often considered analogous to the primate area MT). Visual function during locomotion was tested by looking at the accuracy of foot placement as cats walked down a cluttered alley. Cats were tested before and after LS was lesioned bilaterally using ibotenic acid. In all four cats tested, partial lesions of LS caused a significant deterioration in the accuracy of foot placement, though cats showed no deficit in acuity or in pattern discrimination. To determine whether the observed deficit was specific to the motion pathway, in two additional cats lesions were made in the central field representation in area 19, which lies adjacent to LS but belongs to the putative object recognition pathway. Surprisingly, both cats showed significant improvements in their accuracy of foot placement during locomotion. The reason for this improvement was unclear.

The role of neural mechanisms of attention in solving the binding problem.

We thank L. Chelazzi, V. Ferrera, P. Fries, S. Kastner, and A. Rossi for helpful comments on the manuscript.

Synchrony unbound: a critical evaluation of the temporal binding hypothesis.

Synchrony unbound: a critical evaluation of the temporal binding hypothesis.

Center–Surround Antagonism Based on Disparity in Primate Area MT

Most neurons in primate visual area MT have a large, modulatory region surrounding their classically defined receptive field, or center. The velocity tuning of this "surround" is generally antagonistic to the center, making it potentially useful for detecting image discontinuities on the basis of differential motion. Because classical MT receptive fields are also disparity-selective, one might expect to find disparity-based surround antagonism as well; this would provide additional information about image discontinuities. However, the effects of disparity in the MT surround have not been studied previously. We measured single-neuron responses to variable-disparity moving patterns in the MT surround while holding a central moving pattern at a fixed disparity. Of the 130 neurons tested, 84% exhibited a modulatory surround, and in 52% of these, responses were significantly affected by disparity in the surround. In most cases, disparity effects in the surround were antagonistic to the center; that is, neurons were generally suppressed when center and surround stimuli had the same disparity, with decreasing suppression as the center and surround stimuli became separated in depth. Also, the effects of disparity and direction were mainly additive; i.e., disparity effects were generally independent of direction, and vice versa. These results suggest that the MT center-surround apparatus provides information about image discontinuities, not only on the basis of velocity differences but on the basis of depth differences as well. This supports the hypothesis that MT surrounds have a role in image segmentation.

Encoding of three-dimensional structure-from-motion by primate area MT neurons.

We see the world as three-dimensional, but because the retinal image is flat, we must derive the third dimension, depth, from two-dimensional cues. Image movement provides one of the most potent cues for depth. For example, the shadow of a contorted wire appears flat when the wire is stationary, but rotating the wire causes motion in the shadow, which suddenly appears three-dimensional. The neural mechanism of this effect, known as 'structure-from-motion', has not been discovered. Here we study cortical area MT, a primate region that is involved in visual motion perception. Two rhesus monkeys were trained to fixate their gaze while viewing two-dimensional projections of transparent, revolving cylinders. These stimuli appear to be three-dimensional, but the surface order perceived (front as opposed to back) tends to reverse spontaneously. These reversals occur because the stimulus does not specify which surface is in front or at the back. Monkeys reported which surface order they perceived after viewing the stimulus. In many of the neurons tested, there was a reproducible change in activity that coincided with reversals of the perceived surface order, even though the stimulus remained identical. This suggests that area MT has a basic role in structure-from-motion perception.

Responses of primate area MT during the execution of optokinetic nystagmus and afternystagmus.

The directional selectivity of the visual response properties was determined in 148 neurons, all located in area MT of three hemispheres of two macaque monkeys. The preferred direction of every neuron was obtained by analyzing the response obtained by a circular movement of the background while the monkeys fixated a stationary target. The distribution of the preferred directions was isotropic and showed no ipsiversive bias. MT neurons were excited in a directionally selective manner during the execution of optokinetic nystagmus, in a similar way to that produced by visual stimulation during fixation. The majority of neurons showed a sensitivity to the velocity of retinal image slip. Activity during the execution of optokinetic nystagmus could be traced back to residual retinal image slip in the direction of optokinetic stimulation. No dynamic effects of the neuronal activity during the build-up of eye velocity in early optokinetic nystagmus were observed. Obviously, the activity in area MT did not reflect the charging of the velocity storage mechanism. Accordingly, following the cessation of stimulation, the activity dropped to the level of spontaneous activity and did not parallel the execution of optokinetic afternystagmus. These results suggest that area MT is not part of the velocity storage mechanism and, furthermore, that the storage mechanism must be downstream of area MT in the processing of visual motion for the generation of the optokinetic nystagmus and afternystagmus.

Theoretical studies of the formation of direction columns and receptive field properties in primate area MT

Theoretical studies of the formation of direction columns and receptive field properties in primate area MT

Experimental-anatomical studies on the "middle temporal visual area (MT)" in primates. I. Efferent cortico-cortical connections in the marmoset Callithrix jacchus.

AbstractThe efferent cortico‐cortical fiber connections of the middle temporal visual area (MT) have been studied. The primate area MT was shown by various authors to receive a fiber projection from area 17; physiologically it shows a retinotopical organization. Lesions placed in area MT of Callithrix revealed a surprising variety of cortico‐cortical connections. Termination sites of degenerating axons were found ipsilaterally in the frontal eye field and in eight regions of the temporal, parietal and occipital lobes all known to be photically responsive.Callosal fibers terminated in the contralateral area MT and in two adjoining areas. The termination site in area MT suggested a homotopic point to point connection meaning that in area MT the entire retinal representation is connected transcallosally, whereas in the classical visual areas only the regions of midline representation are connected.The projection of intrinsic interlaminar fibers from the supragranular layers upon layer V was weakly developed in area MT.