Pursuit Eye Movements Involve a Covert Motor Plan for Manual Tracking

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Abstract
When we make an aiming movement toward a moving visual object, eye–hand coupling is of paramount importance for accurate motor performance. Some studies have suggested that both gaze and manual tracking control systems are driven by a common command signal. However, it has never been demonstrated that a motor plan for the arm is produced even when the object is tracked by the eyes alone. By applying transcranial magnetic stimulation to the motor cortex, we show for the first time that ocular tracking is linked to an overall decrease in the excitability of the motor control system of the relaxed upper limb, as estimated from the amplitude of the motor evoked potentials recorded in contralateral hand and wrist muscles. Furthermore, this reduced excitability is modulated in a manner compatible with a subthreshold neural activation encoding a manual tracking response to the same target pursued by the eyes. In addition, excitability changes are contingent on upper-limb posture, because they are present only with a pronated forearm and not with a supinated hand position. We provide direct evidence that, if the arm is held in a congruent postural configuration, tracking a moving object always entails a coordinated motor plan, which involves both gaze and hand movements. Active inhibitory mechanisms are activated to prevent an overt arm movement, whenever a manual tracking is not requested. Our data provide strong evidence in favor of the existence of a common drive to both eye and hand tracking systems.

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