Oxyopia Abstract

April 12, 2002
Noon
489 Minor Hall

Kenneth Britten, PhD
Center for Neuroscience & Section of Neurobiology, Physiology, and Behavior, UC Davis
Host: Ralph Freeman

Title

"Summation in the receptive fields of MT and MST neurons"

Abstract

In the primate visual cortex, a series of anatomically connected cortical areas, collectively referred to as the "motion system", contains high proportions of directionally selective cells, and has been shown to be intimately involved in motion perception. Receptive fields (RFs) become larger and selectivities become more complex as one ascends this hierarchy of motion processing. This trend is exemplified forcefully by the difference in RF properties in areas MT and MST. MST cells' RFs are much larger than those in MT, and are often selective for complex motion patterns such as rotations, expansion or contraction. It is unclear how these complex RFs are assembled from the simpler RFs of the areas (such as MT) which supply afferents to MST.

To explore how the RFs of these extrastriate areas are constructed, we have been using combinations of simple "motion impulse" stimuli to probe mechanisms of spatial summation. The stimuli are small, moving spatial Gabor functions, briefly and locally presented. In our experiments, these are used either individually or in various combinations to investigate how responses depend on the locations, directions, and contrasts of the local component stimuli within the RF. We found that when presented with multiple stimuli within their RFs, both MT and MST cells responded well below what would have been predicted on the basis of linear summation. The reduction in response in both areas ("normalization") was contrast-dependent in MT, with a high contrast sensitivity. It was not strongly dependent on either stimulus location or direction, either in MT or MST. In both MT and MST, reasonable account of the response to combinations of stimuli could be made using a simple linear model incorporating this contrast normalization. Therefore, despite the profound differences in visual responses between the two areas, their basic mechanisms of spatial summation appear largely similar.

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