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Bonnie Lawrence, Marisa Carrasco; Differential effects of covert and overt orienting on microsaccade rate. Journal of Vision 2014;14(10):641. doi: 10.1167/14.10.641.
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© ARVO (1962-2015); The Authors (2016-present)
Goal. Previous research has demonstrated that microsaccade rate decreases prior to the onset of a saccade, consistent with lateral inhibitory interactions between fixation and movement neurons at the level of the superior colliculus. We examined whether such suppression is linked specifically to the planning and execution of a saccade of known metrics, or is (i) generalized to conditions where saccade metrics are not completely specified and, (ii) to conditions where observers are planning to covertly shift attention. Methods. We analyzed monocular eye position in thousands of trials across multiple observers in separately blocked covert and overt trials of a discrimination task. Specifically, we examined a fixation interval that was bookended by the onset of a standard stimulus and a directional cue signaling the location of a test stimulus –both covert and overt blocks- and the execution of a saccade –overt blocks only. At the end of the trial, observers performed a discrimination task. Given that the fixation interval was identical for both conditions, any difference in microsaccade rate can be attributed to differences in "task set" between covert and overt blocks. Results. We observed the characteristic biphasic signature of microsaccade suppression (~100-200 ms) and enhancement (~250-350 ms) following the standard stimulus onset for both covert and overt trials. Interestingly, microsaccade rate was suppressed on overt trials relative to covert trials throughout the fixation interval and prior to the saccade. Conclusion. These results reveal that overt and covert "task sets" differentially influenced microsaccade rate, and thus that suppression is not a necessary precursor for preparing to discriminate a target. This finding is consistent with top down influences on microsaccades; it suggests that the "task set" brought about by the preparatory overt and covert states modulates the activity of fixation and movement neurons in the superior colliculus.
Meeting abstract presented at VSS 2014
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