Circular vection during voluntary suppression of optokinetic reflex

Optokinetic circular vection (CV) was investigated in 12 subjects using an optokinetic pattern rotating at 15 degrees /s, 30 degrees /s, or 60 degrees /s, and four viewing conditions: FOL, subjects attentively followed details of pattern; STA, subjects stared at the pattern; SUP, subjects suppressed their optokinetic reflex (OKR) voluntarily (this was facilitated by a white, featureless band at eye level which separated the pattern in an upper and lower half); FIX, subjects suppressed OKR by fixating at a stationary fixation point (FP). To quantify CV, subjects pressed a signal button each time they felt rotated by a further 90 degrees; OKR was recorded by electro-oculography. Voluntary suppression of OKR was achieved during 2-70% of stimulus duration. Total apparent self-displacement (cumulated 90 degrees indications) was smallest during FOL, increasing gradually in the order FOL < STA < SUP < FIX (all inequalities significant); CV latency decreased in the same order. Slow eye velocity was identical during FOL and STA, and was reduced by 70-30% during SUP. We conclude from these results: (1) the effect of eye movements on CV depends on whether these are intentional (FOL) or not (STA); (2) the increase in CV during voluntary OKR suppression without FP suggests that afferent motion cues (retinal slip) are processed with larger gain than efferent motion cues (eye movement); hence (3) the enhancement of CV during fixation of FP is not, or not solely, the result of the apparent motion of the FP counter to the direction of pattern movement (Duncker illusion).