Frontal regions involved in learning of motor skill--A functional NIRS study.

To investigate cerebral mechanisms underlying learning of motor skill, we assessed serial changes of cortical activation patterns during a pursuit rotor (PR) task in 18 right-handed, healthy subjects using a functional near-infrared spectroscopy (fNIRS) system. Subjects performed the task with the right hand for 30 s alternated with 30-s rest for 8 repetitions (cycle1 to 8). Gains in motor skill were evaluated by time for keeping the stylus on the target (max 30 s), surface EMG patterns and trajectories of the arm. Performance improved with repetitions of the task cycles (12.9/17.1/19.3/20.0/21.1/22.2/23.6/23.9 s on average) and reached plateau at the 7th cycle. Reciprocal EMG patterns and steady trajectories were associated with acquisition of the motor skill. Task-related increases of oxygenated hemoglobin (oxyHb) were observed in the channels covering the sensorimotor cortex (SMC), premotor and prefrontal regions. There were also task-related decreases of deoxygenated hemoglobin (deoxyHb) in these areas although the changes were smaller compared with those of oxyHb. The center of task-related increases of oxyHb was initially located in the presupplementary motor area (preSMA) and shifted caudally to the supplementary motor area (SMA) with cycle repetitions. The ratios of oxyHb changes in preSMA to SMA significantly decreased with task repetitions. DeoxyHb changes confirmed the activation patterns. These data suggest that preSMA plays an important role in the early phase of motor learning while the SMA might be more involved in the late learning phase of the motor skill.