Saccades are the rapid eye movements used to change visual fixation. These eye movements are very accurate and end without drift, one of the projects in this lab has studied the ability of the brain to control post-saccadic ocular drift in both eyes, we have found that human subjects, like monkeys, respond to optically- induced post-saccadic slip by developing post-saccadic ocular drift in a compensatory direction. This suggests that after normal saccades there should be no post-saccadic drift. However, normal subjects usually show post-saccadic ocular drift in one or both eyes after every saccade. Attempts to cause monocular adaptation failed, suggesting that the draft after saccades in normal subjects can not be corrected because or the lack of an independent mechanism for each eye. Another study in this lab has focussed on the neural mechanisms of the sensory-to-motor transformation needed to turn visual target information into saccadic eye movements. It has long been known that the superior colliculus (SC) in the brain stem contains both visual and motor maps related to saccadic eye movements. Up until now it has been assumed by all that the colliculus was providing a static command signaling the change of eye position that would get the eye on target. This leaves unresolved the issue of how the command signal is transformed from a location, or cell-code in SC into the frequency/duration code needed by the eye muscles. Based on new experimental analysis of Sc activity patterns, we have formulated a radical new hypothesis of SC function. According to this hypothesis, the SC is the source of the dynamic motor error signal in a local feedback loop controlling saccades. By placing it in the loop, the SC is now shown to be part of the transformation from cell-coded to frequency/duration-coded signals. This radical new hypothesis has far reaching consequences for how we think about the neural control of saccadic our movements.
