A FUNCTIONAL GRADIENT AMONG THE CAUDAL CINGULATE MOTOR, SUPPLEMENTARY MOTOR, AND PRIMARY MOTOR AREA UNDERLYING CONTRALATERAL AND IPSILATERAL HAND MOVEMENT INITIATION IN MONKEYS

There are three motor areas located at the most caudal levels of the frontal cortex: the primary motor cortex (M1), supplementary motor area (SMA), and caudal cingulate motor area (CMAc). These areas are anatomically close to the output stage of motor control, yet their contributions to the initiation of hand movements remain unclear. In the present study, we examined the relationship between neuronal spike counts and reaction times while macaque monkeys (Macaca fuscata) performed button-pressing movements with either the contralateral or ipsilateral hand. For contralateral hand movements, the correlation between neuronal spike counts measured in the period immediately following Go signal onset and reaction times showed a clear gradient across the three areas. Specifically, the mean correlation between neuronal spike counts and reaction times was the strongest in M1 neurons, intermediate in the SMA neurons, and weakest in the CMAc neurons. Consistent with this pattern, the proportion of neurons whose activity correlated with reaction times was highest in M1 (27%), followed by the SMA (20%) and the CMAc (12%). Similarly, for ipsilateral hand movements, the mean correlation between neuronal spike counts and reaction times was stronger in M1 and the SMA than in the CMAc. The proportions of neurons showing such correlations also differed among the areas, with 17% in M1, 15% in the SMA, and 12% in the CMAc. These findings indicate the presence of a functional gradient in the initiation of hand movements, extending from the CMAc through the SMA to M1, for both contralateral and ipsilateral hands.