Abstract
When each of many saccades is made to overshoot its target, amplitude gradually decreases in a form of motor learning called saccade adaptation. Overshoot is induced experimentally by a secondary, backwards intrasaccadic target step (ISS) triggered by the primary saccade. Surprisingly, however, no study has compared the effectiveness of different sizes of ISS in driving adaptation by systematically varying ISS amplitude across different sessions. Additionally, very few studies have examined the feasibility of adaptation with relatively small ISSs. In order to best understand saccade adaptation at a fundamental level, we addressed these two points in an experiment using a range of small, fixed ISS values (from 0° to 1° after a 10° primary target step). We found that significant adaptation occurred across subjects with an ISS as small as 0.25°. Interestingly, though only adaptation in response to 0.25° ISSs appeared to be complete (the magnitude of change in saccade amplitude was comparable to size of the ISS), further analysis revealed that a comparable proportion of the ISS was compensated for across conditions. Finally, we found that ISS size alone was sufficient to explain the magnitude of adaptation we observed; additional factors did not significantly improve explanatory power. Overall, our findings suggest that current assumptions regarding the computation of saccadic error may need to be revisited.
Highlights
Because a small, central region of the human retina has the greatest receptor density, the best view of an environmental stimulus is achieved by accurately orienting gaze towards it
This maintenance, restoration, or manipulation of saccade size is termed ‘‘saccade adaptation.’’ Since it was recognized that the adaptive changes achieved by the intrasaccadic step (ISS) paradigm are the same as those resulting from tenectomy [9], the majority of adaptation studies have relied on the ISS paradigm to extensively explore and document the multiplicity of subtle ways that saccade adaptation can display sensitivity [10,11]
We were curious about any differences in this minimum at the subject level: might such differences be related to simple observables such as variability in landing position or the average undershoot of saccades prior to any adaptation? We asked whether subjects responded differently to smaller errors, and, we attempted to determine what potential cause best explained the pattern of adaptive changes we observed
Summary
Because a small (roughly 1u in diameter), central region of the human retina (the fovea) has the greatest receptor density, the best view of an environmental stimulus is achieved by accurately orienting gaze towards it. McLaughlin noted that modification of saccade amplitude can be achieved by an intrasaccadic step (ISS) paradigm, applying experimentally arranged, primary-saccade-triggered secondary target shifts in a series of successive saccade trials [8]. This maintenance, restoration, or manipulation of saccade size is termed ‘‘saccade adaptation.’’ Since it was recognized that the adaptive changes achieved by the ISS paradigm are the same as those resulting from tenectomy [9], the majority of adaptation studies have relied on the ISS paradigm to extensively explore and document the multiplicity of subtle ways that saccade adaptation can display sensitivity [10,11]
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