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Romi Nijhawan, Beena Khurana; The flash-lag effect during voluntary and involuntary limb movements. Journal of Vision 2003;3(9):542. doi: 10.1167/3.9.542.
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The flash-lag effect results from displays in which a flashed object is physically co-localized with a moving object. Despite the alignment of the two objects, the observer perceives the flashed object as lagging behind the moving object. It is not known if analogous effects occur with respect to a moving body part, such as the observer's hand. Here we show that a flash presented in alignment with an invisible rod that the observer voluntarily moves, also appears to lag behind the felt position of the rod. Four observers moved a 220 gm rod in the dark while an LED was flashed at various positions relative to the rod. Observers reported whether they perceived the position of the flash as “ahead”, “behind” or “centered” in relation to the rod. On average, a flash presented 125 ms in advance of the rod position was judged by observers as centered on the rod. In a second preliminary experiment, two new observers grasped a manipulandum. Glue on the manipulandum fixed the observer's hand in position without requiring the observer to exert any force. The invisible manipulandum (and the observer's hand) was then carried by a linear drive, while the flash was presented in various positions. In this “passive movement” condition, observers similarly judged the position of a flashed LED relative to the manipulandum. Once again a significant flash-lag effect was observed. These experiments show that a flash-lag effect occurs relative to the felt position of a limb, both when the limb is moved passively and actively. This suggests that compensation for neural delays in the registration of limb position occurs both when motor and sensory information for limb position is simultaneously available (active movement case), as well as when only sensory information is available (passive movement case).
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