Visual illusion and action.

The role of allocentric cues on movement control was investigated in the present study. Pointing movements directed to the more distant vertex of closed and open configurations of the Muller-Lyer illusion, as well as to the vertex of control lines, were studied in four experimental conditions. In the first (full-vision condition) subjects saw both stimulus and their hand before and during movement, in the second (non-visual feedback condition) they saw the stimulus, but not their hand during movement. In the two remaining conditions (no-vision conditions) vision of the scene and the hand was precluded. Pointing was executed 0 sec (no vision 0 sec delay condition) or 5 sec (no-vision 5 sec delay condition) after the light was switched off. The Muller-Lyer illusion affected pointing kinematics with respect to the control lines. Subjects undershot and overshot the vertex location, respectively, of the closed and open configuration. Correspondingly, the entire kinematics were changed. The main result was, however, a gradually increasing effect of the perceptual illusion when pointing was executed from memory compared to the full-vision condition. These data are discussed according to the hypothesis that the system underlying visual perception in the allocentric frame of reference and that involved in motor action can functionally interact. The strength of this interaction depends upon the efficiency of the egocentric frame of reference by which motor actions are constructed.