Execution and evaluation of eye movements: from muscles to medial frontal cortex

Abstract

I have studied the neurophysiological bases of saccade execution and evaluation within the oculomotor system of Macaca mulatta and Macaca radiata. To better understand saccade execution, I recorded the electromyogram (EMG) from extraocular muscles and the occurrence of microsaccades while monkeys canceled responses in the saccade stop-signal task. I found reduction in extraocular EMG and in the occurrence of microsaccades when monkeys canceled eye movements. These results are unanticipated by a well-accepted, new theory of saccade execution. I conclude that the theory's proposed mechanism of fixation is incomplete. To study saccade evaluation, I recorded activity from single neurons, small neural ensembles, and large groups of neurons in and overlying an area of frontal medial cortex called the supplementary eye field (SEF) while monkeys viewed light flashes or made correct and errant responses during the saccade stop-signal task. I found 1) that SEF displays functional microcircuitry very similar to that described in early sensory cortex even though anatomy differs drastically between these areas, 2) that monkeys display electrophysiological correlates of error detection homologous to the error related negativity (ERN) and error related positivity (Pe) recorded in humans, and 3) that current flow in SEF contributes to, but is not the sole determinant of the ERN. This work highlights differences between the oculomotor and spinal motor systems, suggests the need for improvements on current models of fixation, and provides novel insight on the neural basis of performance monitoring.