Smooth pursuit eye movements and optokinetic nystagmus elicited by intermittently illuminated stationary patterns.

Stationary periodic visual patterns (row of equally spaced dots or black-white stripes) of the period Ps illuminated stroboscopically with a flash frequency fs induce an apparent movement perception (sigma-movement) when slow eye movements are performed across the periodic pattern. The movement appears in the direction of the eye movements when the angular speed VE of the eyes corresponds to the following condition: Ve = k . Ps . fs [deg . s-1] (1) k is a constant and equals 1 (or exceptionally 2 or 3). The sigma-movement induces a sigma-OKN with an average angular speed of its slow phases corresponding to Eq.(1). sigma-OKN can be elicited when identical foveal or identical extrafoveal stimulus patterns are applied from flash to flash. A considerable random variability of the flash sequence does not interrupt the sigma-movement and the sigma-OKN. Both phenomena can also be elicited by a stimulus pattern with its periodicity hidden in spatial noise and this periodic pattern only becomes visible during the eye movements. It is argued that the sigma-phenomena are caused by efference copy signals of the gaze control system, which interact with the afferent signals (displacement of visual stimuli on the retina) at different levels of the afferent visual system. One interaction is supposed at a cortical level where the extrapersonal visual space is represented.