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The ability of primates to detect transient changes in a visual scene can be influenced by the allocation of attention, as well as by the presence of distractors. We investigated the neural substrates of these effects by recording the responses of neurons in the middle temporal area (MT) of two monkeys while they detected a transient motion direction change in a moving target. We found that positioning a distractor near the target impaired the change-detection performance of the animals. This impairment monotonically decreased as the distractor's contrast decreased. A neural correlate of this effect was a decrease in the ability of MT neurons to signal the direction change (detection sensitivity or DS) when a distractor was near the target, both located inside the neuron's receptive field. Moreover, decreasing distractor contrast increased neuronal DS. On the other hand, directing attention away from the target decreased neuronal DS. At the level of individual neurons, we found a negative correlation between the degree of response normalization and the DS. Finally, the intensity of a neuron's response to the change was predictive of the animal's reaction time, suggesting that the activity of our recorded neurons was linked to the animal's detection performance. Our results suggest that the ability of an MT neuron to signal a transient direction change is regulated by the degree of inhibitory drive into the cell. The presence of distractors, their contrast and the allocation of attention influence such inhibitory drive, therefore modulating the ability of the neurons to signal transient changes in stimulus features and consequently behavioral performance.

Plaid stimuli are often used to investigate the mechanisms involved in the integration and segregation of motion information. Considering the perceptual importance of such mechanisms, only a very limited number of visual brain areas have been found to be specifically involved in motion integration. These are the human (h)MT+ complex, area V3 and the pulvinar. The hMT+ complex can be functionally subdivided into two separate areas, middle temporal area (MT) and medial superior temporal area (MST); however, it is currently unclear whether these distinct sub-regions have different responses to plaid stimuli. To address this issue we used functional magnetic resonance imaging to quantify the relative response of MT and MST to component and pattern motion. Participants viewed plaid stimuli that were constrained to result in the perception of either component motion (segregation of motion information) or pattern motion (integration of motion information). MT/MST segregation was achieved using a moving dot stimulus that allowed stimulation of each visual hemifield either in unison or separately. We found pattern motion selective responses in both MT and MST. Consistent with previous reports, activity indicative of pattern motion selectivity was also found in the pulvinar as well as in other extrastriate areas. These results demonstrate that MT, MST and the pulvinar are involved in the complex motion integration mechanisms that are triggered by plaid stimuli. This reinforces the concept that integrative computations take place in a distributed neuronal circuit both in cortical and sub-cortical networks.

Camouflaged animals remain inconspicuous only insofar as they remain static. This demonstrates that motion is a powerful cue for figure-ground segregation, allowing detection of moving objects even when their luminance and texture characteristics are matched to the background. We investigated the neural processes underlying this phenomenon by testing the responses of neurons in the middle temporal area (MT) to 'camouflaged' bars, which were rendered visible by motion. These responses were compared with those elicited by 'solid' bars, which also differed from background in terms of their mean luminance. Most MT neurons responded strongly to camouflaged bars, and signaled their direction of motion with precision, with direction-tuning curves being only slightly wider than those measured with solid bars. However, the tuning of most MT cells to stimulus length and speed depended on the type of stimulus - in comparison with solid bars, responses to camouflaged bars typically showed more extensive length summation, weak end-inhibition, and stronger attenuation at high speeds. Moreover, the emergence of direction selectivity was delayed in trials involving camouflaged bars, relative to solid bars. Comparison with results obtained in the first (V1) and second (V2) visual areas, using similar stimuli, indicates that neural computations performed in MT result in significantly stronger and more accurate signals about camouflaged objects, particularly in situations in which these are relatively large and slow moving. These computations are likely to represent an important step in enabling cue-invariant perception of moving objects, particularly in biologically relevant situations.

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