GRASPING AN ILLUSION

In the present study we attempted to determine the nature of the visua l analysis that is performed on an object in order to grasp it. We req uired eight healthy subjects to reach and grasp a wooden bar which was superimposed over the shaft of the Muller-Lyer illusion. Vision of bo th the hand and the bar was allowed. Three different bar lengths were used. Two additional control tasks in which the subjects were required to reproduce the length of the shafts were carried out. The results s howed that hand shaping while grasping the bar was influenced by the i llusion configurations on which it was superimposed. However, this eff ect was smaller than that observed in the two tasks of length reproduc tion. These results support the notion that visual analysis performed on the object of a grasp movement is global and takes into account the object itself, as well as its relationships with surrounding cues. We propose, as suggested previously for reaching movements (Gentilucci, M, et nl., Neuropsychologia, 1996, 34, 369-376), two partially indepen dent stages during visuo-motor integration for grasping an object. In the first stage, the object is coded inside an object-centred frame of reference. In the second stage it is transposed in an egocentric fram e of reference, in which the spatial relations between object and agen t are computed. In this second stage the influence of cues surrounding the target is minimized. (C) 1997 Elsevier Science Ltd. All rights re served.