Movement initiation-related signals differ across the caudal cingulate motor area, supplementary motor area, and primary motor cortex for contralateral but less so for ipsilateral movements in monkeys.

The frontal cortex contains multiple motor areas, including the caudal cingulate motor area (CMAc), supplementary motor area (SMA), and primary motor cortex (M1). Although neurons in these areas exhibit activity related to both contralateral and ipsilateral limb movements, whether their contributions to movement initiation differ across areas and effectors remains unclear. Here, we recorded single-neuron activity from the CMAc, SMA, and M1 in monkeys performing a button-press task with either hand and quantified trial-by-trial correlations between spike counts and reaction times to assess initiation-related signals. For contralateral movements, both the proportion of neurons with significant correlations and the magnitude of the correlation increased from the CMAc through the SMA to M1. Among neurons showing significant correlations during contralateral movements, correlation strength remained greater in M1 than in the SMA and CMAc, whereas no difference was observed between the SMA and CMAc. In contrast, for ipsilateral movements, areal differences were weak or absent, with no differences in correlation strength across areas among neurons showing significant correlations during ipsilateral movements. These findings indicate that contralateral movement initiation is associated with both increased neuronal recruitment and stronger single-neuron signaling in M1 than in the CMAc and SMA, whereas ipsilateral movement initiation is associated with more comparable initiation-related signaling across the three motor areas.