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Roy Amit, Shlomit Greenberg; Inter-dependency of microsaccades and its modulation by visual context. Journal of Vision 2015;15(12):1276. doi: 10.1167/15.12.1276.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Our eyes are in constant motion. Even when fixating, our eyes produce small saccades called microsaccades. Following a visual stimulus onset, microsaccade rate follows a consistent modulation of a decrease and then increase. However, much less is known on the effect of ocular events on the occurrence of consecutive microsaccades. It was previously suggested that the process triggering microsaccades is a Poisson process, and therefore the inter-microsaccade intervals (IMSIs) follow an exponential distribution. The purpose of the current research was to test this hypothesis in different visual contexts and to compare the effects of visual and ocular events on microsaccade occurrences. Methods: In two experiments we recorded the gaze position of subjects while they were either maintaining fixation or free-viewing a scene. We manipulated the visual scene to be: A) Complete darkness, gray screen or natural scene (experiment 1); B) Checkerboard with low or high spatial frequency (experiment 2). In addition we tested microsaccade rate relative to the abrupt appearance of a visual stimulus. Results: In both experiments microsaccade occurrences were inter-dependent. The distribution of IMSIs demonstrated a good fit to an Ex-Gaussian rather than exponential distribution. Furthermore, with increasing complexity of stimulation (natural scene vs. gray screen vs. darkness and also with high vs. low spatial frequency) the rise of the Ex-Gaussian was later, i.e. there was a larger “refractory period” between microsaccades. Moreover, the distribution of the IMSIs was similar to the distribution of the intervals between stimulus onsets and microsaccades. Conclusion: Microsaccades are interdependent and are not driven by a Poisson process as previously thought. The IMSIs distribution is modulated by the amount of retinal image displacement caused by each microsaccade. Our findings suggest that the same process can explain both the modulation of microsaccade-rate by visual events and its modulation by precedent microsaccades.
Meeting abstract presented at VSS 2015
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