VISUAL DISTRACTORS DIFFERENTIALLY INTERFERE WITH THE REACHING AND GRASPING COMPONENTS OF PREHENSION MOVEMENTS

In the present study we addressed the issue of how an object is visual ly isolated from surrounding cues when a reaching-grasping (prehension ) movement towards it is planned. Subjects were required to reach and grasp an object presented either alone or with a distracter. In five e xperiments, different degrees of elaboration of the distracter were in duced by varying: (1) the position of the distracter (central or perip heral); (2) the time when the distracter was suppressed (immediately o r delayed, with respect to stimulus presentation); and (3) the type of distracter analysis (implicit or explicit). In addition, we tested wh ether the possible effects of the distracter on reaching-grasping were due to the use of an allocentric reference centered on it. This was o btained by comparing the effects of the distracter with those of a sti mulus, the target of a placing movement successive to the reaching-gra sping. The results of the five experiments can be summarized as follow s. The necessary condition for an interference effect on both the reac hing and the grasping components was the central presentation of the d istracter. When the information on the distracter could be immediately suppressed, an interference effect was observed only on the grasp com ponent. In the case of delayed suppression, an effect was found on the reaching component. Finally, when an overt analysis of the distracter was required, the interference effect disappeared. Two main conclusio ns have been drawn from the results of the present study. First, compa rison between properties of the target and surrounding cues is perform ed by two independent processes for reaching and grasping an object. T he process for the grasp relies more on allocentric cues than that for the reach. Second, when surrounding stimuli are automatically analyze d during visual search of the target, the process of visuo-motor trans formation can incorporate their features into the target. In contrast, overt analysis of surrounding stimuli is performed separately from th at of the target. Finally, the data of the present study are discussed in support of the premotor theory of attention.