Fig. 8.

Time course of activity on trials with similar reaction time. A, Population average responses for T1-choice trials. The responses are aligned to saccade initiation.Color designates the RT of the trials included in the average, which fall within 25 msec of the time indicated (e.g., 400–425 msec). All spikes are included in these averages (n = 54 neurons). Average firing rate was smoothed using a 60 msec running mean. B, The gradual change in spike rate is evident hundreds of milliseconds before the monkey discriminates motion. Trials with long RT were selected for a closer examination of an early portion of motion viewing period, from 200 to 500 msec after motion onset. This epoch corresponds to the beginning of the coherence-dependent response after the stereotyped dip after motion onset and ends at least 200 msec before the saccadic response. Trials are grouped by RT spanning a 100 msec range: 700–799, 800–899, and 900–999 msec, indicated by red, green, andblue, respectively. Points show the average firing rate, calculated in non-overlapping 40 msec bins. A representative error bar (± 1 SEM) is shown for one data point. Lines are weighted least squares fits (Eq. 3B) performed separately for each RT group. Solid lines andfilled symbols correspond to T1 choices; dashed lines and open symbols correspond to T2 choices. The slope (β₂) estimates the change in firing rate as a function of time. C, Response change as a function of time during early motion viewing. Bars represent the slope from the fits in B (error bars represent 95% confidence intervals). The ability to detect linear trends in this epoch implies that the ramp-like responses in Figure 7 do not arise as a consequence of averaging responses that step from an intermediate level of firing to a high or low rate once the decision is formed.