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Tatiana Malevich, Antimo Buonocore, Ziad M. Hafed; Rapid stimulus-driven modulation of slow fixational drift eye movements. Journal of Vision 2020;20(11):1322. doi: https://doi.org/10.1167/jov.20.11.1322.
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Stimulus onsets affect microsaccades, but whether slow fixational eye position drifts, normally considered random processes, are also transiently affected is unknown. We recorded eye movements of three rhesus monkeys using scleral search coils, and we analyzed microsaccade-free fixation after visual transients. In a first condition, each monkey fixated a white spot over a gray background. At a random time, the display changed: one entire half (right or left) became black; a half-circle of 0.74-deg radius around the fixated position remained gray, in order to maintain view of the stable fixation spot. The entire gray-and-dark stimulus (centered on gaze position) was presented gaze-contingently, using retinal image stabilization, in order to maintain the “split” view condition for 500-3000 ms. In trials without microsaccades (-100 to 200-350 ms from transient onset), a short-latency drift response occurred at ~50 ms and lasted for ~100 ms. It consisted of an upward velocity pulse (peak speed ~0.5 deg/s) whose horizontal component was also biased by the side of darkness: the upward drift tilted rightward when the darkness was on the right of gaze and leftward when it was on the left. In a control condition (no “split” view stimulus), there was no drift pulse. We then tested two monkeys when a single-frame (~8 ms) white or black full-screen flash occurred. We observed the same upward drift, with a marginally higher peak velocity for the dark flash compared to the white flash. Small localized flashes (1x1 deg squares at 2.1 deg eccentricity either right or left of fixation) were significantly less effective in modulating drift. Our results show that both persistent and transient luminance changes, particularly when they cover relatively big regions, robustly affect fixational eye positions. We hypothesize that upward drifts might reflect impacts of potential overrepresentation of the upper visual field in visual-motor oculomotor structures.
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