Endpoint accuracy in saccades interrupted by stimulation in the omnipause region in monkey.

Electrical stimulation of the omnipause neuron region (OPN) at saccade onset results in interrupted saccades (IS)- eye movements which pause in midflight, resume after a brief period, and end near the target location. Details on the endpoint accuracy of IS do not exist, except for a brief report by Becker et al. (1981). Their analysis emphasized the accuracy of IS relative to the visual target which remained on during the interrupted period. We instead quantified the metric properties of IS relative to nonstimulated saccades during a target flash paradigm. Our results show that IS tend to be slightly hypermetric relative to the nonstimulated saccades to the same target location. The amount of overshoot is not correlated with target eccentricity. Detailed analyses also indicate that the standard deviations of the endpoint in IS are not significantly larger than those for nonstimulated saccades, although there was a much larger variability produced in eye position during the interruption. Both these latter observations support the notion that saccades are controlled by an internal negative feedback system. Also, the size of the remaining motor error during the interrupted period is one factor influencing when an IS resumes, but the variability in this measure is large particularly for smaller motor errors. Recent results have suggested that the resettable neural integrator involved in the feedback loop may be reset after each saccade through an exponential decay process. To probe the properties of the neural integrator, we varied the duration of interruption between the initial and resumed saccades and sought a systematic overshoot in the final eye position with increasing interruption period and variable initial saccade size. Our results showed the neural integrator does not decay during the pause period of interrupted saccades.