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Jane H. Sumnall, Tom C. A. Freeman; Pursuit adaptation alters perceived head-centred motion. Journal of Vision 2002;2(7):504. doi: 10.1167/2.7.504.
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Following prolonged unidirectional smooth pursuit, a stationary, fixated test appears to move in the opposite direction to the adapting eye movement. The site of the adaptation is almost certainly extra-retinal and thought to be a consequence of the suppression of pursuit afternystagmus (Chaudhuri, 1991, Vis. Res. 31, 1639–1645). We investigated the effects of pursuit adaptation on perceived motion during an eye movement, since this is a judgement mediated by extra-retinal, eye-velocity signals. In the adaptation phase, observers pursued a small target for 60s as it made 60 unidirectional sweeps across the central 8° of the screen. Each trial in the test phase comprised a single test sweep during which a global motion pattern was displayed for 500ms, followed by 6 top-up adaptation sweeps. The test sweep was either in the same or opposite direction to the adapting eye movement. The global motion pattern was made up of signal and noise dots and observers were instructed to judge the overall direction of the dot pattern with respect to the head. Coherence (% signal dots) was controlled by a staircase which converged on the point at which the dot pattern appeared stationary on the screen. Results showed that at this null point, the retinal motion of the signal dots was opposite to the accompanying pursuit. Compared to a no-adaptation baseline, coherence was set lower following pursuit adaptation in the same direction as the test sweep. Conversely, coherence was set higher than baseline when the pursuit adaptation was opposite to the test sweep. This was true for both horizontal and vertical pursuit conditions. The results suggest that extra-retinal, eye-velocity signals decrease following pursuit adaptation in the same direction as the test sweep, but increase following pursuit adaptation in the opposite direction. This may arise from the need to suppress pursuit afternystagmus during the test sweep.
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