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Nicola Bruno, Paolo Bernardis; When does action resist visual illusion? Effector position modulates relational influences on motor programs. Journal of Vision 2002;2(7):723. doi: 10.1167/2.7.723.
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© ARVO (1962-2015); The Authors (2016-present)
Actors viewed horizontal segments either in isolation or embedded in patterns that produce spatial-relational effects (Kanizsa's compression illusion or the “dumbell” version of the Müller-Lyer compression — expansion illusion). They were asked to reproduce the apparent width of these segments by the amplitude of open- or closed-loop motor responses (after having positioned a finger on position A, choose a position B on the left of A such that apparent width = B − A). A touchmonitor was used to present the displays and to record movement amplitudes and times. In open-loop motor responses, displays were turned off as soon as actors raised their finger from position A. In closed-loop responses, displays could be viewed continuously during the actions. Four conditions were investigated: (1) open-loop responses starting from A at the left endpoint of the segment; (2) open-loop responses from A at the right endpoint of the segment; (3) closed-loop responses from A at the right endpoint of the segment; and (4) open-loop responses from A aligned horizontally with the left endpoint of the segment but displaced vertically below that segment. With both kinds of display, results in conditions (2) and (3) demonstrated relational effects comparable to those measured in standard visual matching experiments, whereas results in conditions (1) and (4) showed essentially no relational effects. Overall, these findings are not consistent with models of visual function rigidly positing either effector-relative coding in the dorsal stream vs. object-relative coding in the ventral stream (Milner & Goodale, 1995) or object-relative coding in motor planning vs. effector-relative coding during on-line motor control (Glover & Dixon, 2001). The availability of effector-relative representations of spatial extents in motor programs may depend on subtle geometrical relationships between the initial position of the effector, the direction of the programmed action, and the orientation of the acted-upon spatial extent.
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