Natural and drug-induced variations of velocity and duration of human saccadic eye movements: evidence for a control of the neural pulse generator by local feedback.

The present report considers goal directed human saccadic eye movements. It addresses the question how a given perceived target excentricity is transformed into the innervation pattern that creates the saccade to the target. More specifically, it investigates whether this pattern is an appropriately selected preprogram or whether it is continuously controlled by a local feedback loop that compares a non-visual eye position signal to the perceived target excentricity (a visual signal would be too slow). To this end, the relation between the accuracy of saccades aimed at a given target and their velocity and duration was examined. Duration and velocity were found to vary by as much as 60% while the amplitude showed no related variation and had an almost constant accuracy of about 90%. By administrating diazepam, the variabiity of saccade duration and velocity could be further increased, but still the amplitude remained almost constant. These results favour the hypothesis that saccadic innervation is controlled by a local feedback loop.