The initial horizontal OFRs elicited by two-image movies applied to vertical 3f gratings of various contrasts: dependence of mean R-L peak velocity (upper plots) and latency of the R-L peak (lower plots) on the ISI (data for each of 3 subjects). Positive peak responses denote tracking in the direction of the applied 1.65º steps (error bars, SEs). Peak latencies were measured from the appearance of the 2nd frame using the mean R-L eye velocity traces (so there are no error bars). Filled circles: 4% contrast. Half-filled circles: 12% contrast. Open circles: 36% contrast. Gray symbols: 1/72 responses that failed to exceed 3 SDs and so was selected by hand. A,D: subject FAM (191–234 trials per condition; SDs ranged from 0.36–0.45º/s). B,E: subject JKM (179–245 trials per condition; SDs ranged from 0.54–0.84º/s). C,F: subject KJC (193–243 trials per condition; SDs ranged from 0.49–0.68º/s). Error bars, SE’s.