The behavioural consequences of dissociating the spatial directions of eye and arm movements

Many of our daily movements use visual information to guide our arms toward objects of interest. Typically, these visually guided movements involve first focusing our gaze on the intended target and then reaching toward the direction of our gaze. The literature on eye-hand coordination provides a great deal of evidence that circuitry in the brain exists which can couple eye and arm movements. Moving both of these effectors towards a common spatial direction may be a default setting used by the brain to simplify the planning of movements. We tested this idea in 20 subjects using two experimental tasks. In a "Standard" condition, the eyes and a cursor were guided to the same spatial location by moving the arm (on a touchpad) and the eyes in the same direction. In a "Dissociated" condition, the eye and cursor were again guided to the same spatial location but the arm was required to move in a direction opposite to the eyes to successfully achieve this goal. In this study, we observed that dissociating the directions of eye and arm movement significantly changed the kinematic properties of both effectors including the latency and peak velocity of eye movements and the curvature of hand-path trajectories. Thus, forcing the brain to plan simultaneous eye and arm movements in different directions alters some of the basic (and often stereotyped) characteristics of motor responses. We suggest that interference with the function of a neural network that couples gaze and reach to congruent spatial locations underlies these kinematic alterations.