Stochastic processes in the human eye movement control system

Abstract

Experiments have been performed to investigate several of the stochastic processes involved in the control of eye movements, particularly where these processes apply to saccades. Previous models have linearized the eye positioning system, and as a result some of the more important features have been lost in the analysis. The experiments concentrated on four areas. The first of these was temporal prediction by saccades when tracking square wave target motions. This predictor function generates a response saccade approximately 100 msec before the stimulus step. The mean response time, however, never precedes the stimulus by more than 50 msec, and actually occurs after the stimulus when tracking either very short or long period square waves. These changes in mean response time are caused by a shift in the predictive lead at short periods, and by failures of the predictive mechanism at longer periods. This failure results in a delayed response 200 msec after the stimulus. The second area investigated was the non-predictive saccadic response system. The investigation revealed that the sampled data effect previously reported is actually the result of the combination of a time delay and a refractory unit in the response mechanism. The characteristic times of these functions were investigated, along with the ability of the system to alter a response when additional retinal information arrives before the initial response saccade is elicited. Thirdly, the role of spontaneous flicks in steady fixation tasks was investigated utilizing two-dimensional analysis. It was found that the majority of these events are of stochastic origin, whereas previous studies, analyzing only the horizontal components, indicated that the flicks are only used to correct fixation error. A generator for the random flicks can be postulated, and the source of the spontaneous drift motion was identified. Finally, experiments were performed on the perceptual effects of saccades. It was found that the visual system is incapable of detecting small target motions when they occur within ?40 msec of a flick. This suppression of movement detection by the saccades led to a re-evaluation of the efference copy mechanism, and the effect of this reevaluation on eye movement control is discussed.