Figure 1. Torsional eye movements evoked by an optokinetic stimulus.
(A) An example of the experimental block depicting the stimulus in the upper row and the associated torsional eye position beneath. A block consisted of four phases. ‘Fixation in darkness’ phase I was a fixation period (start from zero), followed by a optic flow phase I phase in which a large field visual stimulus rotating counterclockwise was presented on the monitor (start marked with a red line). The subsequent ‘fixation in darkness’ phase II and ‘optic flow’ phase II were identical to ‘fixation in darkness’ phase I and ‘optic flow’ phase I, respectively. Note the build-up of eye torsion during the stimulus presentation. (B) Enlarged torsional eye position from (A) showing torsional optokinetic nystagmus consisting of fast phases in green towards the zero torsional eye position and slow phases in red which follow the direction of the rotating stimulus. Notice that the eye torsional position did not return completely back to zero, i.e. it showed a hysteresis effect.