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Tom C. A. Freeman; Motion aftereffect following oblique pursuit. Journal of Vision 2004;4(8):738. doi: 10.1167/4.8.738.
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© ARVO (1962-2015); The Authors (2016-present)
Repetitive eye movement gives rise to motion aftereffect (MAE). Most examples use stimulus configurations producing significant retinal motion during adaptation and so emphasise the contribution of retinal motion sensors to the effect. However, pursuit to a target undergoing sawtooth motion in the dark also produces MAE, which suggests extra-retinal input as well. Here we provide further evidence by showing that the direction of MAE following oblique pursuit depends on the relative durations of MAE following vertical and horizontal pursuit. A small fixation target underwent sixty 1s sweeps of sawtooth motion at 12 deg/s in the dark. Observers judged MAE direction and duration in a stationary test dot, using an unseen rod attached to a potentiometer. Leftward and downward MAE was reported following adaptation to rightward (0 deg) and upward (90 deg) pursuit, lasting for about H=13s and V=18s respectively. The duration of MAE following oblique pursuit (45 deg) equalled that found for the downward condition. The final direction, however, was determined by the duration-ratio V/H. In observers with V/H = 1, the final direction reported in the oblique condition was opposite to the pursuit (mean = 218.8 deg, SE = 1.3 deg). In observers with V/H ratio > 2, the final direction was closer to downward (mean = 258.1 deg, SE = 8.9 deg). Asymmetric eye movements could not explain the effect — if anything, the gain of horizontal components was slightly higher than vertical. A second experiment showed reasonable storage of the oblique-pursuit MAE when 30s of darkness was inserted between adapt and test, with the final direction reported closer to downward than oblique. The results suggest that extra-retinal MAE arises from activity in separate horizontal and vertical eye-movement processes that can decay at different rates. The fact that the pursuit-induced MAE stores, however, casts doubt on the suggestion that afternystagmus-suppression is the sole source of extra-retinal MAE.
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