VISUAL ILLUSION AND ACTION

The role of allocentric cues on movement control was investigated in t he present study. Pointing movements directed to the more distant vert ex of closed and open configurations of the Muller-Lyer illusion, as w ell as to the vertex of control lines, were studied in four experiment al 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 execute d 0 sec (no vision 0 sec delay condition) or 5 sec (no-vision 5 sec de lay condition) after the light was switched off. The Muller-Lyer illus ion affected pointing kinematics with respect to the control lines. Su bjects undershot and overshot the vertex location, respectively, of th e closed and open configuration. Correspondingly, the entire kinematic s were changed. The main result was, however, a gradually increasing e ffect of the perceptual illusion when pointing was executed from memor y compared to the full-vision condition. These data are discussed acco rding to the hypothesis that the system underlying visual perception i n the allocentric frame of reference and that involved in motor action can functionally interact. The strength of this interaction depends u pon the efficiency of the egocentric frame of reference by which motor actions are constructed.