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Basal ganglia encompass four to five distinct loops to allow parallel processing of information. Among them, the most intensively studied is the motor loop, which includes two distinct direct and indirect pathways. The direct pathway exerts facilitatory influence upon the motor cortex, whereas the indirect pathway exerts an inhibitory effect. Overall, this dual system provides a center(excitatory)-surround-(inhibitory) mechanism to focus its effect on selected cortical neurons, and several lines of evidence suggest that this center-surround mechanism is used to focus the output on a specific group of muscles required for performing a specific task. This operation is made possible through opening the sensory channel for the expected sensory feed-back afferents during movement. Thus, one of the important functions of basal ganglia seems to be the gating of sensory input for motor control. Dystonia may be caused by a mismatch between sensory input versus motor output, and parkinsonism may be viewed as a disorder of gain control of this sensorimotor integration.

Somatosensory evoked potentials (SEPs) are attenuated or gated during movement. The mechanism for this includes both centrifugal gating of afferent input and competition with other afferents caused by the movement (peripheral gating). Using a paradigm in which the signal for triggering movement is the electric stimulus for SEPs, we studied the gating of SEPs after tibial nerve stimulation prior to foot movement, and compared it with that during counting task. Significant gating was found for P40 component, which distributed centrally and ipsilaterally to the side of the stimulation, whereas the contralateral N40 component showed no changes. Dissociated gating of P40 and N40 indicates multiple generators of these components, in contrast to the previous view of a single generator dipole projecting tangentially. Together with the previous findings in median SEPs, these gating phenomena should represent a general mechanism for sensori-motor integration in preparation for limb movement.