Temporal characteristics of error signals driving saccadic gain adaptation in the macaque monkey.

Saccadic gain (saccade amplitude/target amplitude) can be reduced gradually by repeatedly stepping the target backward during the saccade. The gain reduction produced by this paradigm is thought to be driven by an error signal created by the backstep. We investigated the effects of varying the timing of this error signal relative to the end of the saccade by using two different paradigms in macaques. In the brief backstep paradigm, the target was stepped backward 30% during the saccade but extinguished after different durations. For very short backstep durations (32 ms), little gain reduction occurred. As backstep duration increased, the amount of gain reduction also increased. When backstep duration reached 80 ms, the amount of gain reduction was just under that achieved during the conventional adaptation paradigm in which the backstep remained visible for 1000-1200 ms. In the delayed backstep paradigm, as the saccade occurred, we extinguished the target and then, after a delay, illuminated it for 1 s at the backstep location. In most experiments with short delay times of 16-64 ms, the saccadic gain reduction reached that achieved during conventional adaptation. At delays of 112-208 ms, the amount of gain reduction decreased to approximately 75% of that reached during conventional adaptation. With still longer delays, the amount of gain reduction decreased more gradually. At delays of 750 ms, average gain reduction was 10%. By delays of 1.5 s, gain reduction had fallen essentially to zero. Taken together, these data suggest that the error signal must be present for a limited time ( approximately 80-100 ms) after the saccade to produce the most robust saccadic gain adaptation. However, errors present as long as 750 ms after the saccade still can produce a significant gain reduction.