Accuracy of corrective saccade in the dark depends on the latency of the primary saccade

Abstract

Purpose: To clarify how and when corrective movements are generated after primary saccades in the absence of visual feedback.

Methods: Ten healthy human subjects were tested with a visually-triggered saccade task. By varying the duration of the target presentation, we manipulated the error signals used by the saccade system to bring the eyes on target to be either extraretinal feedback (efference copy or proprioceptive feedback) or visual feedback from target. The target remained on (ON), was switched off at the onset of the primary saccade (OFF_onset), or was switched off after 100 ms of viewing (OFF_100ms).

Results: Whether or not the target remained visible after it jumped to a new location did not affect the overall characteristics (amplitude, peak velocity, duration and latency) of the primary saccade. Neither the time the target was on before the saccade nor the presence of any possible visual feedback during the saccade affected the accuracy of primary saccades. However, the distributions of the position errors after the primary saccades with and without a secondary saccade were significantly different. Analysis of the secondary saccades revealed that the tolerance range of the position error is small (−2% ± 2%) when there is a visual error signal at the end of the primary saccade but increases to −6% ± 6% if the target disappears before the primary saccade. Moreover, when the primary saccades undershoot the target by an amount that reaches a certain threshold (on average, 10% here), a secondary saccade is very likely to occur regardless of whether or not visual feedback is present. The mean latencies of the secondary saccades in conditions OFF_100ms and OFF_onset were each different from that in condition ON but not from each other, suggesting that the secondary saccades are generated later when visual error information is absent. Moreover, the corrective saccade compensated for 70% of primary saccade error if a retinal error signal was available at the end of the primary saccade, but only 50% if not. In the absence of visual feedback, the goodness of correction positively correlated with the latency of primary saccade but not with the latency of corrective saccade.

Conclusions: Taken together, our results suggest that, while corrective saccades do respond to retinal error if it is available, they are also to some extent pre-programmed with primary saccades that have relatively longer reaction times.

Keywords: 525 eye movements: saccades and pursuits