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Anna Ma-Wyatt, Suzanne P. McKee; The last moment for a change in pointing direction. Journal of Vision 2005;5(8):130. doi: 10.1167/5.8.130.
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© ARVO (1962-2015); The Authors (2016-present)
Past evidence has shown that new visual information can induce online changes to a reaching trajectory, even during rapid movements (e.g. Saunders & Knill, 2004). Perturbations have typically been used to estimate the time required to incorporate feedback. We investigated the effect of new visual information on motor outcome. We made small visual perturbations of target location and measured endpoint accuracy. The perturbations could occur at different times during the reach. The target, a high contrast white dot subtending 0.5°, appeared initially at an eccentricity of 8° and then disappeared for 67ms. On half the trials, the target reappeared at the same location and for the other half, it was shifted laterally by an amount (∼0.5°) that was easily visible on every trial. The direction of the shift was varied across blocks. The time of perturbation was measured from the subject's initiation of a pointing trial by a key press. The perturbation could occur early in the reach (110ms after trial onset), at an intermediate time (180ms) or late in the reach (250ms) in separate blocks of trials. Subjects made a rapid point to the target and were given negative feedback if their response was too slow (>500ms); average movement time across subjects was 430ms. For early or intermediate perturbations during the movement, subjects were able to adjust their pointing trajectory so that there was a substantial difference between the mean of their points to the perturbed and static targets. For a late perturbation, subjects were unable to change their motor plan, because there was no difference between means for the perturbed and static targets. Subjects adopted an interesting strategy to cope with late perturbations. They pointed to the centroid of the two possible locations, thereby ‘hedging their bets’ about the likely location of the target. We conclude that subjects need about 250 msec to complete a shift in their planned hand trajectory in response to a visual perturbation.
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