Inhibition for gain modulation in the motor system.

Signatures of inhibition within the cortico-spinal pathway are frequently observed during action preparation in humans. Popular theoretical and computational models highlight a critical role for inhibition as the suppressor of motor system output, e.g., to withhold undesired action tendencies or to stop ongoing movements. However, inhibition frequently serves a modulatory role in non-motor systems. For example, in vision and somatosensory systems, inhibition can adjust the relationships between input and output, a computation referred to as gain modulation. Inhibition may modulate gain within the motor system as well. Changes in cortico-spinal inhibition observed during human behavior can reflect adjustments in motor system gain and may be sensitive to latent behavioral states. This review summarizes roles for inhibition in gain modulation, drawing principally on evidence from non-motor systems, and examines the hypothesis that homologous functions operate in the animal and human motor systems to facilitate action preparation.