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J. Rhys Davies, Tom C.A. Freeman; Simultaneously adapting retinal motion and smooth pursuit eye movement in orthogonal directions. Journal of Vision 2008;8(6):670. doi: https://doi.org/10.1167/8.6.670.
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Adapting to retinal motion yields a motion aftereffect (MAE). An ‘extra-retinal’ MAE can be obtained when adapting to repetitive pursuit. When the adaptation simultaneously combines retinal motion and pursuit, the resulting direction of the MAE suggests the two types of aftereffect combine vectorially. An alternative is that the direction of the aftereffect is opposite to the motion perceived during adaptation. To differentiate between these two hypotheses, we measured perceived direction of motion during adaptation and during MAE. Stimuli consisted of moving random dot patterns presented centrally (about the pursuit target) or peripherally (10° eccentric to the pursuit target). All stimuli were presented in the dark on a black background. The pursuit target executed a sawtooth wave (period 1 s) consisting of constant upward motion (P = 4°/s) and abrupt return. The retinal motion of the dot pattern was horizontal (R = 4°/s). Adaptation consisted of R only, P only or R+P combined. Experiment 1 showed that central and peripheral adaptation produced similar MAE directions. Experiment 2 investigated central adaptation and found that perceived direction during adaptation could not predict the perceived direction during test. Similar results were found in Experiment 3 which investigated adaptation and test in the periphery. As a final test of the vectorial-sum hypothesis, Experiment 4 alternated brief sequences of R and P within a single adaptation. We found that MAE direction was equivalent to the sum of the component MAEs. Overall, these results suggest that MAE following simultaneous adaptation is the vectorial combination of adapted retinal and extra-retinal motion signals.
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