Facilitation of supplementary motor area activity modulates the sense of effort: a theta burst stimulation study

The sense of effort, defined as the phenomenological experience of invested physical resources in the task, is a critical component of motor control. The supplementary motor area (SMA) has been implicated in generating this sense, yet it remains unclear how modulating SMA activity affects unconscious motor output when individuals intend to exert the same level of effort. The present study aimed to investigate how inhibiting and facilitating SMA activity influences the subjective sense of effort and associated motor output. Twelve healthy volunteers received both facilitatory and inhibitory theta burst transcranial magnetic stimulation (TBS) applied to the SMA. We assessed corticospinal excitability, submaximal effort force production in an index finger abduction task (at a subjective effort level of 35% without visual feedback), and force-matching performance before and after the TBS protocols. Results showed that facilitatory TBS led to a significant reduction in the submaximal index finger abduction force, which was accompanied by decreased EMG activity. In contrast, inhibitory TBS did not produce significant changes in submaximal force. Importantly, TBS applied to the SMA did not significantly alter motor-evoked potentials, indicating that corticospinal excitability remained unchanged. These findings suggest that enhanced SMA activity increases the effort required to produce a given force. The underlying mechanism likely involves changes in efference copy signals, as the effect occurred without altering the corticospinal excitability. In addition, force-matching performance was improved at 30 min after facilitatory TBS applied to the SMA. This delayed behavioral improvement may suggest a time-dependent modulation of sensorimotor processing, possibly involving the SMA and posterior parietal cortex, although network-level mechanisms remain speculative. This study provides causal evidence for the SMA's role in processing effort-related signals and contributes to a growing understanding of the cortical mechanisms underlying the sense of effort.