Our eyes are never still, but tend to "freeze" in response to stimulus onset. This effect is termed “oculomotor inhibition” (OMI); its magnitude and time course depend on the stimulus parameters, attention, and expectation. We previously showed that the time course and duration of microsaccade and spontaneous eye-blink inhibition provide an involuntary measure of low-level visual properties such as contrast sensitivity during fixation. We investigated whether this stimulus-dependent inhibition also occurs during smooth pursuit, for both the catch-up saccades and the pursuit itself. Observers followed a target with continuous back-and-forth horizontal motion while a Gabor patch was briefly flashed centrally with varied spatial frequency and contrast. Catch-up saccades of the size of microsaccades had a similar pattern of inhibition as microsaccades during fixation, with stronger inhibition onset and faster inhibition release for more salient stimuli. Moreover, a similar stimulus dependency of inhibition was shown for pursuit latencies and peak velocity. Additionally, microsaccade latencies at inhibition release, peak pursuit velocities, and latencies at minimum pursuit velocity were correlated with contrast sensitivity. We demonstrated the generality of OMI to smooth pursuit for both microsaccades and the pursuit itself and its close relation to the low-level processes that define saliency, such as contrast sensitivity.
It is now well established that the movement of the eyes, which occurs constantly even during fixation, tends to “freeze” in response to perceptual events, with a magnitude and time course that depends on the stimulus properties, attention, and anticipation. This “freeze” or oculomotor inhibition (OMI) was found for microsaccades, blinks, smooth-pursuit, and catch-up saccades; yet remains unclear whether it also applies to ocular drift. Since video-based eye-trackers are known to produce positional artifacts, we used here data from a high-speed and precision retinal imaging eye-tracker (FreezEye Tracker, FET). The observers (n = 15) watched a series of flashed Gabor patches, with different spatial frequency and contrast while their eyes were tracked. We analyzed the data by removing the saccades, aligning the traces, and computing four drift measures relative to the stimulus onset: (1) velocity, (2) area, (3) diffusion, and (4) heat map. We found that all measures produced a highly significant modulation over time. The overall drift velocity, area, and diffusion followed the microsaccade inhibition pattern, whereas the heat map peaks showed the opposite pattern. The drift inhibition was also stimulus dependent, showing shorter onset and prolonged release estimates for more salient stimuli, matching the microsaccade OMI for the onset but showing the opposite for the release. Overall, the results confirm that the oculomotor inhibition effect can be generalized for ocular drift, but its opposite stimulus dependency for inhibition release and the shifted time course may suggest a complex interdependency between drift and saccades.
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