Microsaccades occur during gaze fixation in order to correct for miniscule foveal motor errors. The mechanisms governing such fine oculomotor control are still not fully understood. In this study, we explored microsaccade control by analyzing the impacts of transient visual stimuli on these movements' kinematics. We found that such kinematics can be altered in systematic ways depending on the timing and spatial geometry of visual transients relative to the movement goals. In two male rhesus macaques, we presented peripheral or foveal visual transients during an otherwise stable period of fixation. Such transients resulted in well-known reductions in microsaccade frequency, and our goal was to investigate whether microsaccade kinematics would additionally be altered. We found that both microsaccade timing and amplitude were modulated by the visual transients, and in predictable manners by these transients' timing and geometry. Interestingly, modulations in the peak velocity of the same movements were not proportional to the observed amplitude modulations, suggesting a violation of the well-known "main sequence" relationship between microsaccade amplitude and peak velocity. We hypothesize that visual stimulation during movement preparation affects not only the saccadic "Go" system driving eye movements, but also a "Pause" system inhibiting them. If the "Pause" system happens to be already turned off despite the new visual input, movement kinematics can be altered by the read-out of additional visually-evoked spikes in the "Go" system coding for the flash location. Our results demonstrate precise control over individual microscopic saccades, and provide testable hypotheses for mechanisms of saccade control in general.
- Saccadic inhibition
- Superior colliculus
- Omnipause neurons
- Copyright © 2016, Journal of Neurophysiology