Sigma-movement and optokinetic nystagmus elicited by stroboscopically illuminated stereopatterns

Sigma-movement is an apparent movement seen when a stationary periodic visual pattern of the period PS is illuminated stroboscopically at the flash frequency fS and smooth gaze pursuit eye movements are performed across the pattern at an angular velocity Ve = PS X fS deg X S-1. Sigma-movement leads to an optokinetic nystagmus (Sigma OKN) which in turn sustains Sigma-movement perception. (1) Sigma-movement was also seen in an apparent three-dimensional periodic stripe pattern generated by two periodic monocular stimulus patterns with a certain degree of horizontal binocular disparity. (2) Sigma-movement perception and Sigma-OKN were also elicited by a stroboscopically illuminated, stationary, random dot stereostripe pattern. The periodicity PS of this pattern is generated on the cyclopean retina (Julesz 1971). The equation described above was also valid. When the time delay delta t between left eye and right eye flashes was varied, the apparent depth of the random dot stereostripe pattern decreased with increasing delta t, but the Sigma-effects were not affected. (3) Sigma-movement illusion and Sigma-pursuit movements can also be induced when real three-dimensional objects composed of periodic components are stroboscopically illuminated and adequate gaze or eye pursuit movements are induced. Sigma-movement is related to gaze movement and is therefore elicitable by eye, head or body movements. (4) Sigma-movement is presumably caused by the interaction of efference copy signals (generated in a cortical gaze pursuit system) and afferent visual signals. The present data indicate that neuronal mechanisms for this interaction are located--at least in part--at or beyond the level of binocular fusion and stereopsis.