Velocity step responses of the human gaze pursuit system. Experiments with sigma-movement

By means of d.c.-electrooculography or the electromagnetic search coil technique, horizontal and vertical eye position signals were recorded in subjects (head fixed) attentively pursuing a rotating dot circle (dot distance Ps = 0.8 or 1.0 deg). In addition, circular eye pursuit movements were evoked by sigma-movement seen when the stationary dot circle was illuminated stroboscopically (flash frequency fs). The rotation velocity Vs or the velocity of sigma-movement (Ps X fs) was changed in positive or negative steps, leading to acceleration or deceleration of pursuit eye velocity. This step response of eye velocity could be well described by a linear second-order differential equation with an additional dead time of about 80-100 msec. When gaze position error signals were larger than 0.3-0.5 deg, correcting saccades were superimposed on the step response of the smooth pursuit system. Voluntary saccades across the rotating or apparently rotating circle did not lead to any impairment in successive pursuit eye movements or an interruption in the sigma-movement. The gaze tracking command signals (smooth pursuit and saccadic responses) are evidently related to the stimulus movement in the extrapersonal space and not to retinal movement signals.