When to act, or not to act: that's the SMA's question

Traditionally, the Supplementary Motor Area (SMA) is linked to motor control, including voluntary movement selection and response inhibition. However, it is also implicated in timing, whether a timed motor response is required or not. We synthesize recent neuroimaging, electrophysiological and stimulation studies of timing and action control and reconcile these distinct literatures by proposing a functional gradient for voluntary action within SMA: preSMA controls the voluntary selection and inhibition of stimulus-specified action whereas SMA proper instigates spontaneous, self-generated action. In addition, developmental and learning studies indicate that we develop and refine our sense of time through action, potentially explaining why even non-motor, perceptual forms of timing recruit SMA. We suggest that SMA is engaged through action control to build up a sensory representation of time.