IS THERE DISSOCIATION OF PERCEPTUAL AND MOTOR-RESPONSES TO FIGURAL ILLUSIONS

Open-loop reaching for locations within figural illusions was measured in three experiments. The experiments differed with respect to whethe r subjects were provided a visible target toward which to direct their reaching or were required to form a mental representation of the inte nded target. In the first experiment, subjects' reaching errors for ve rtices of a Muller-Lyer figure were similar to those for a nonillusory control stimulus. In experiment 2, subjects' errors while reaching to the imaginary bisector of the Judd illusion were consistent with the presence of an illusion of bisector location. However, when a bisector line was added to the Judd figure, reaching errors were similar to th ose obtained with a control figure. In experiment 3, subjects' open-lo op reaching at the perceived midpoint of a triangle was biased toward its illusory perceptual midpoint. When a mark was placed at the midpoi nt between a vertex and the opposite side, reaching errors were simila r to those obtained with a control figure. The results of the experime nts indicate that hand-eye coordination is biased in the direction of illusions of bisector location only when no target is present at the i ntended goal of the reaching response and subjects are required instea d to form a mental image of the target. Under these conditions, reachi ng responses appear to utilize the spatial map of the visual system, a nd are influenced by figural illusions of bisector location. The prese nt data can be understood without invoking the notion of visual- motor dissociation.