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

- SMA is implicated in response selection and inhibition.
- SMA is implicated in perceptual and motor timing.
- SMA amplitude codes for stimulus duration and for error monitoring.
- SMA mediates both stimulus-specified and spontaneous aspects of voluntary action.
- SMA is engaged through action control to develop our sense of time.

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.