When does actions resist visual illusion? Effector position modulates illusory influences on motor responses

Actors viewed horizontal segments either in isolation or embedded in patterns that produce spatial illusory effects (Kanizsa’s compression illusion and the “dumbbell” version of the Müller-Lyer compression- expansion illusion). They were asked to reproduce the apparent horizontal extent 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 right 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 right endpoint of the segment; (2) closed-loop responses from A at the right endpoint of the segment; (3) open-loop responses from A at the left 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 (1) and (2) demonstrated illusory effects comparable to those measured in standard visual match- ing experiments, whereas results in conditions (3) and (4) showed essentially no illusory effects. Implications for models of visuomotor transformations in peripersonal space are discussed.