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L.C. Osborne, S.G. Lisberger, and W. Bialek
Time course of information about motion direction in visual area MT in primates
University of California, San Francisco
Sensory signals in visual cortical area MT guide the direction and speed of smooth pursuit eye movements on a 100ms timescale. To ask how target direction could be decoded on this time scale, we have calculated the time course of Shannon information about motion direction in individual MT neurons. We recorded from single units in area MT of anesthetized macaques. Stimuli were random dot patterns, stepped from stationary to the preferred speed of the cell for 256ms, in a direction randomly chosen with 15 degree increments. All methods of analysis indicated that the majority of information about motion direction was encoded during an early transient of firing at the onset of the response. If the calculation was based on cumulative spike count, then information rose rapidly in the first 50ms and then saturated (or even peaked) so that continued counting of spikes contributed little additional information. If the calculation was based on spike count in a sliding 50ms window, then information often showed an early peak and was much lower during the response to sustained motion. The early peak could not be attributed to differences in directional tuning bandwidth as a function of time during the response. The enhanced spike rate at motion onset contributes to the effect, but the information carried by each spike is also higher in the initial transient response, because of the temporal structure of correlations in the response variability. Thus almost all of the available directional information could be extracted from the first few spikes of the neuron's response, on a time scale comparable to the initiation of smooth pursuit eye movements.
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