Target selection in eye-hand coordination: Do we reach to where we look or do we look to where we reach?

Abstract

During a goal-directed movement of the hand to a visual target the controlling nervous system depends on information provided by the visual system. This suggests that a coupling between these two systems is crucial. In a choice condition with two or more equivalent objects present at the same time the question arises whether we (a) reach for the object we have selected to look at or (b) look to the object we have selected to grasp. Therefore, we examined the preference of human subjects selecting the left or the right target and its correlation to the action to be performed (eye-, arm- or coordinated eye-arm movement) as well as the horizontal position of the target. Two targets were presented at the same distance to the left and right of a fixation point and the stimulus onset asynchrony (SOA) was adjusted until both targets were selected equally often. This balanced SOA was then taken as a quantitative measure of selection preference. We compared these preferences at three horizontal positions for the different movement types (eye, arm, both). The preferences of the 'arm' and 'coordinated eye-arm' movement types were correlated more strongly than the preferences of the other movement types. Thus, we look to where we have already selected to grasp. These findings provide evidence that in a coordinated movement of eyes and arm the control of gaze is a means to an end, namely a tool to conduct the arm movement properly.