Role of visual context and oculomotor conditions in pointing accuracy.

Localizing a target in the extrapersonal space may rely on two types of spatial coordinate systems: egocentric or exocentric. Two experiments investigated the role of these systems in the accuracy of goal-directed movements. The accuracy of pointing movements performed without visual feedback of the hand was measured in two conditions of target presentation (darkness or within a visually structured background), and in two conditions of eye-hand coordination (eye fixed on a fixation point, or with a foveation saccade). The results showed (1) that pointing accuracy increased in the presence of a visual background, and (2) enhancement of this beneficial effect by a steady retinal image of target and background, that is without a foveation saccade. An object that has to be reached in the prehension space can be localized in two ways: (1) in relation to the body (egocentric), induced by the absence of a structured visual environment, (2) in relation to the external space (exocentric), favored by the presence of a visually structured background. We investigated the importance of the localizing mode in the accuracy of reaching movements, and how the two modes can be optimized. We found better reaching accuracy with exocentric than with egocentric localization, particularly when the retinal image of the goal and its environment was stabilized in the absence of eye movements.