Vision of the hand and environmental context in human prehension

Previous findings on the role of visual contact with the hand in the contro l of reaching and grasping have been contradictory. Some studies have shown that such contact is largely irrelevant, while more recent ones have empha sised its importance. In contrast, information arising from the surrounding environment has received relatively little attention in the study of prehe nsile actions. In order to identify the roles of both sources of informatio n, we made kinematic comparisons between three conditions. In the first, re aching was performed in a dimly lit room and compared with a second conditi on in which reaches in the dark, but with the thumb and first finger illumi nated, were made to a luminous object. This contrast allows the effects of environmental context to be identified. A comparison between the second and a third condition, in which both vision of the hand and the environment wa s removed, but the object was still visually available, enabled the assessm ent of how and when vision of the hand plays a role. Removing environmental cues had effects both early and late in the reach, while vision of the han d was only crucial in the period after peak deceleration. In addition, remo val of both sources of information resulted in larger grip apertures. Diffe rences and similarities between our findings and those of other studies are discussed, as is the ongoing debate about the relative importance of visua l feedback of the hand in the control and co-ordination of prehensile actio ns. We conclude with suggestions for further research based on the set-up u sed in the present study.