The role of the motor cortex in motor learning

The motor cortex is clearly more than a simple executor of motor commands and is likely involved with different aspects of motor learning. The motor cortex shows considerable plasticity, and both excitability and amount of territory devoted to a muscle or to a specific task can expand or shrink depending oil the amount of use. There are also short-term increases in motor cortex activity when learning new tasks. In the serial reaction time task (SRTT), as demonstrated by transcranial magnetic stimulation (TMS), EEG, and positron emission tomography, the motor cortex is involved in the implicit phase of motor learning and declines in activity during the explicit phase. In learning to increase pinch force and pinch acceleration between index and thumb, the motor evoked potential (MEP) from TMS increases during the early stage of learning, but then declines even though the behavioral change is maintained. In learning a bimanual task, there is a transient increase in EEG coherence between the two hemispheres at the time of the learning. What function this short-term increase in motor cortex activity serves is not certain. It has recently been established that motor learning goes through a phase of consolidation and becomes more secure simply with the passage of time. This was first demonstrated while adapting to making accurate movements in a force field. Neuroimaging studies with these same movements in a force field show a transient increase in motor cortex activity during the learning phase. In our laboratory, we have studied consolidation of the learning to increase pinch force and acceleration. Consolidation is disrupted by 1 Hz repetitive TMS of the motor cortex if done immediately after learning, but not after a rest of 6 hours. This demonstrates a role of the motor cortex in consolidation.