Fig. 7.

Time course of LIP activity in the RT-direction-discrimination task. A, Average response from 54 LIP neurons. Responses are grouped by motion strength and choice as indicated by color and line type. The responses are aligned to two events in the trial. On the left, responses are aligned to the onset of stimulus motion. Response averages in this portion of the graph are drawn to the median RT for each motion strength and exclude any activity within 100 msec of eye movement initiation. On the right, responses are aligned to initiation of the eye movement response. Response averages in this portion of the graph show the buildup and decline in activity at the end of the decision process. They exclude any activity within 200 msec of motion onset. The average firing rate was smoothed using a 60 msec running mean. Arrows indicate the epochs used to compare spike rate as a function of motion strength in the next panels.Arrows a and b mark the 40 msec epoch ending at the median RT for 51.2% motion trials (370–410 msec after stimulus onset); arrows c and d mark the 40 msec epoch ending 30 msec before saccade initiation. Only correct choices are included in these graphs for motion coherences >0%. B, Effect of motion strength on firing rate during decision formation. Response averages were obtained from 54 neurons in the 40 msec epochs corresponding to arrows a and b above. When motion was toward the RF (solid line; epoch a), the spike rate increased linearly as a function of motion strength. When motion was away from the RF (dashed line; epochb), the spike rate decreased as a function of motion strength. Note that, for the 0% coherence stimulus, there was no net direction of motion, but the activity was greater when the monkey chose the T1 direction. Symbols represent weighted means ± SEM. Lines are weighted least squares fits to Equation 2A(*p < 0.05; H₀: β₂ = 0 ). C, Effect of motion strength on firing rate at the end of the decision process. Response averages were obtained from 54 neurons in the 40 msec epochs corresponding to arrows c and d. The large response preceding eye movements to the RF (solid line,filled circles; arrow c) did not depend on the strength of motion. Responses preceding eye movements away from the RF were more attenuated with stronger motion stimuli (dashed line; arrow d). Use of weighted means in Band C introduces small discrepancies from averages indicated by arrows in A.