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Farhan Khawaja, Christopher Pack; Responses of MT neurons to type II plaid stimuli. Journal of Vision 2010;10(7):932. doi: 10.1167/10.7.932.
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© ARVO (1962-2015); The Authors (2016-present)
The dorsal pathway of the primate visual cortex contains a hierarchy of areas involved in the processing of motion signals that are useful for perception and behavior. Motion direction is first detected by V1, which sends specialized projections to cortical areas such as MT and MST. Motion processing in these extrastriate areas is often tested by stimulating individual neurons with plaid stimuli comprised of two drifting gratings. When the perceived direction of the plaid stimulus is rotated clockwise or counter-clockwise relative to both of the component gratings, the stimulus is categorized as a “Type II” plaid. Although much previous work has examined the perceptual aspects of “Type II” plaids, nothing is known about the underlying neurophysiological responses. Unikinetic plaids (constructed by summing a moving and a stationary grating) are Type II plaids stimuli that have been used in psychophysical experiments to measure short-latency ocular following (Masson, 2002). These stimuli present a unique challenge to the motion processing stream, as their velocity can only be recovered by taking into account information from stationary components of the stimuli. We have therefore compared the responses of MT neurons to unikinetic plaid stimuli and more standard “Type I” plaids, by recording spikes and LFPs from 38 MT neurons to both types of plaids from two awake, behaving macaque monkeys. Consistent with previous results, we found that 13/38 (34%) MT neurons were component-selective in response to Type I plaid stimuli, while 9/38 (24%) were pattern-selective. Surprisingly, many neurons that exhibited strong component tuning when stimulated with Type I plaids exhibited robust pattern-selective responses to Type II unikinetic plaids. As a result the majority of MT neurons were pattern-selective when tested with unikinetic plaids. This suggests that the processing of Type II plaid stimuli may involve additional cues that enable the detection of two-dimensional motion signals.
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