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Thérèse Collins, Joshua Wallman; The role of efference copy in saccadic adaptation. Journal of Vision 2011;11(11):496. doi: 10.1167/11.11.496.
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When saccades systematically miss their visual target, their amplitude adjusts, causing the position errors to be progressively reduced. Conventionally, this adaptation is viewed as driven by retinal error (i.e. the distance between primary saccade endpoint and visual target). Implicitly, this view regards the variability in the saccade endpoint as motor noise. Recent work suggests that the oculomotor system is informed about where the eye lands; thus, not all “retinal error” is unexpected. The present study compared two error signals that may drive saccade adaptation: retinal error and prediction error (i.e. the difference between predicted and actual post-saccadic images). Participants made saccades to a visual target on two successive days. The first day, during saccade execution, the target was extinguished if the amplitude was smaller (or, in some experiments, greater) than the running median, thereby modifying the average retinal error participants experienced without moving the target during the saccade. On the second day, targets were extinguished at the start of saccades and turned back on at a position that reproduced the trial-by-trial retinal error recorded on the first day. Despite the retinal error in the first and second sessions having been identical, adaptation proceeded approximately three times as rapidly during the second session, when the predicted target position had been changed, arguing that the eye knows where it lands and where it expects the target to be, and that deviations from this prediction drive saccade adaptation more strongly than retinal error alone.
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