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Nicole C. Rust, Eero P. Simoncelli, J. Anthony Movshon; Neurons in MT compute pattern direction by pooling excitatory and suppressive inputs. Journal of Vision 2005;5(8):83. doi: 10.1167/5.8.83.
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Cells in MT are tuned for the direction of moving stimuli. In response to a superimposed pair of sinusoidal gratings (a plaid), component direction selective cells (CDS) respond in a manner predicted by summation of their responses to the constituent grating stimuli. In contrast, pattern direction selective cells (PDS), are tuned for the two-dimensional velocity corresponding to a rigid displacement of the plaid, consistent with the way we perceive these stimuli. To investigate the computation of pattern direction, we used a spike-triggered analysis to characterize the responses of individual MT neurons in terms of a linear weighting of signals elicited by sinusoidal gratings moving at different directions and speeds. On each trial, each of a large set of gratings was assigned a random phase and one of three contrasts: 0, C/2, or C. We recovered a linear weight for each stimulus dimension by computing the mean contrast of each grating before a spike (the spike-triggered average or STA). The arrangement of the positive and negative weights of the STA predicted whether the cell responded with pattern or component selectivity. Specifically, strong, broadly tuned inhibition in PDS cells suppressed responses to the individual plaid components, resulting in tuning for the direction of plaid motion. In CDS cells, such suppression was weak or absent. These results, which are consistent with the predictions of Simoncelli & Heeger (1998, Vis. Res.), suggest that broadly tuned null direction suppression (motion opponency) plays a fundamental role in computing pattern motion direction in MT.
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