Abstract
Despite strong evidence to the contrary in the literature, microsaccades are overwhelmingly described as involuntary eye movements. Here we show in both human subjects and monkeys that individual microsaccades of any direction can easily be triggered: (1) on demand, based on an arbitrary instruction, (2) without any special training, (3) without visual guidance by a stimulus, and (4) in a spatially and temporally accurate manner. Subjects voluntarily generated instructed “memory-guided” microsaccades readily, and similarly to how they made normal visually-guided ones. In two monkeys, we also observed midbrain superior colliculus neurons that exhibit movement-related activity bursts exclusively for memory-guided microsaccades, but not for similarly-sized visually-guided movements. Our results demonstrate behavioral and neural evidence for voluntary control over individual microsaccades, supporting recently discovered functional contributions of individual microsaccade generation to visual performance alterations and covert visual selection, as well as observations that microsaccades optimize eye position during high acuity visually-guided behavior.
Highlights
Despite strong evidence to the contrary in the literature, microsaccades are overwhelmingly described as involuntary eye movements
Likelihoods of microsaccade direction, amplitude, and frequency are systematically modulated under a variety of conditions[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17], and microsaccade generation is causally affected by activity in the midbrain superior colliculus (SC)[18,19,20,21] and the cortical frontal eye fields (FEF)[22], both involved in voluntary eye movement control[23]
Our task required voluntary eye movement generation based on an arbitrary instruction that we enforced by task design and without any visual guidance[36,37]; corrective re-fixation saccades were visually guided
Summary
Despite strong evidence to the contrary in the literature, microsaccades are overwhelmingly described as involuntary eye movements. Note how the reaction time of the corrective, visually guided microsaccade (from target reappearance) was comparable to that of the voluntarily generated memory-guided microsaccade (from fixation spot disappearance), and that the latter movement was directionally accurate (directed towards the remembered foveal flash location).
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