TIME-COURSE OF CORTICOSPINAL EXCITABILITY IN REACTION-TIME AND SELF-PACED MOVEMENTS

We used transcranial magnetic stimulation (TMS) to study the time cour se of corticospinal excitability before and after brisk thumb abductio n movements, either in a simple reaction time (RT) paradigm or self-pa ced. Premovement increase in corticospinal excitability began about 20 msec earlier for self-paced compared with simple RT movements. For bo th simple RT and self-paced movements after electromyographic (EMG) of fset, there was a first period of increased excitability from 0 to 100 msec, followed by a second period from 100 to 160 msec. Corticospinal excitability was decreased from about 500 to 1,000 msec after EMG off set for both types of movements. Our results show that motor preparati on that begins 1.5 to 2 seconds before self-paced movement is not asso ciated with increased corticospinal excitability, The first phase of i ncreased corticospinal excitability after EMG offset may be due to act ivity of motor cortex neuron subthreshold for activating spinal motor neurons, and the second phase may reflect a subthreshold second agonis t burst. The period of decreased corticospinal excitability after move ment corresponds to the onset of event-related synchronization (ERS) o f electroencephalographic signals in the 20-Hz band, and supports the hypothesis that ERS may be related to an inactive, idling state of the motor cortex.