Alteration of the microsaccadic velocity-amplitude main sequence relationship after visual transients: implications for models of saccade control

摘要

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 three behavioral experiments, we explored microsaccade control by analyzing the impacts of transient visual stimuli on these movements' kinematics. In two male rhesus macaques, we presented peripheral (Experiment 1) or foveal (Experiment 2) visual transients during an otherwise stable fixation period, and we measured microsaccade times, directions, amplitudes, and peak velocities. In both experiments, visual transients resulted in well-known reductions in microsaccade frequency ~100 ms later. Our goal was to investigate whether microsaccade kinematics would additionally be altered for the few movements happening exactly around this inhibition period. We found that microsaccade amplitudes were modulated by the visual transients, and in predictable manners by these transients' geometry; movements directed "towards" the visual transients had larger amplitudes than movements directed "opposite" the visual transients, and this happened even when the transients were foveal and thus near the microsaccade endpoints. 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 amplitude and peak velocity. We generalized these results to larger saccades in Experiment 3, now involving free scanning and a full-screen flash. We hypothesize that visual stimulation during movement preparation affects not only a topographically-organized 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 instantaneous spatial read-out of additional visually-evoked spikes in the "Go" system coding for the visual input's location. Our results demonstrate precise control over individual microscopic saccades, and provide testable hypotheses for mechanisms of saccade control in general.