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 P_s illuminated stroboscopically with a flash frequency f_s induce an apparent movement perception (σ-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 of the eyes corresponds to the following condition: (1) $$\overline {\text{V}} = {\text{k}} \cdot {\text{P}}_{\text{s}} \cdot {\text{f}}_{\text{s}} \cdot [\deg \cdot {\text{s}}^{{\text{ - 1}}} ]$$ k is a constant and equals 1 (or exceptionally 2 or 3). The σ-movement induces a σ-OKN with an average angular speed of its slow phases corresponding to Eq.(l). σ-OKN can be elicited when identical foveal or identical extrafoveal stimulus patterns are applied from flash to flash. A considerable random variablility of the flash sequence does not interrupt the σ-movement and the σ-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 σ-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.