Abstract
Large moving patterns give rise to nystagmus eye movements. We find that following a prolonged period of nystagmus a pronounced motion aftereffect (MAE) is experienced. Naive observers viewed two vertically translating striped patterns (16.5x25 deg) separated horizontally by 10 deg. Stripes consisted of dimly illuminated dots shown on a black background in a completely darkened room. Six conditions were investigated. In two eye-movement conditions, observers maintained gaze on a central blank area. Stimuli moved upward or downward at 10 deg/s. Following 60s stimulation the adapting pattern was replaced by a small spot, which observers fixated and then indicated when the MAE ceased. The duration was recorded. To investigate whether the MAE was based on adaptation of retinal motion sensors, four eye-stationary conditions were presented containing similar adapting motion but viewed with a static fixation point. Upward and downward conditions were combined with translation speeds of 2 and 4 deg/s, thus approximating the retinal motion experienced in the eye-movement conditions if slow-phase gains of nystagmus were 0.8 and 0.6, respectively. No observer reported any MAE in the eye-stationary conditions. Conversely, all reported strong MAE opposite to the adapting motion in the eye-movement conditions. This was confirmed by the recorded durations (eye moving upward, mean duration = 25.5s; downward, mean = 9.1s; eye-stationary, mean = 3.5s). The marked asymmetry between the eye-movement conditions may be the result of greater slow-phase gains found during the upward adaptation phase. The results suggest the MAE is extra-retinal in origin because the stationary test stimulus was displayed in a region of the retina previously unstimulated and also keeping the eyes stationary during adaptation produced no MAE. The MAE probably results from an extra-retinal signal created by the need to suppress afternystagmus when fixating the stationary test.