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Christopher C. Pack, Richard T. Born; Integration of motion signals over regions of uniform luminance by MT neurons in the alert macaque. Journal of Vision 2002;2(7):412. doi: 10.1167/2.7.412.
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© ARVO (1962-2015); The Authors (2016-present)
Object velocity can be measured by tracking changes in local contrast, but many objects contain relatively large regions of uniform luminance. We investigated the representation of motion in such regions by recording MT cell responses to three types of moving stimuli: discs of uniform luminance, annuli, and dot fields. All stimuli varied in diameter from 5 to 35 deg, and were centered on the receptive fields of individual MT neurons, while the monkeys maintained fixation. By slowly drifting the stimuli in the preferred and null directions for each cell, we measured the strength of direction selectivity for each stimulus. As expected all cells responded in a direction-selective manner to both the discs and the annuli when the stimuli were confined to their classical receptive fields. Neuronal responses typically decreased with increasing stimulus diameter. Surprisingly, however, when the stimulus edges were far outside the classical receptive fields many cells still exhibited direction selectivity for the discs, but not for the annuli, even though the local motion signals were similar for both stimuli. The preferential responses to uniform-luminance discs were most prevalent in the cells with inhibitory surrounds (as measured with the dot fields), and were seen primarily in the later phases of the temporal response profiles. These differences may be related to the different local luminance profiles that define the disc and annulus edge. However, varying the width of the luminance step defining the annulus edge did not influence the responses in any obvious way. In a smaller sample of V1 cells, we observed similar responses to stimuli that extended as much as 6 deg beyond the classical receptive fields. These results suggest that a population of neurons in macaque visual cortex can represent the motion of objects, even in regions lacking luminance contrast. Such a representation could help to define figure and ground for large moving objects.
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