Anticipation and execution of a simple reading task enhance corticospinal excitability

Objective: Electromyographic responses (EMG) evoked in the right hand by transcranial magnetic stimulation (TMS) of the left motor cortex are enhanced during continuous reading. This enhancement is the result of increased excitability of the motor cortex. We proposed that anticipation and reading of single words would also enhance corticospinal excitability. We studied the temporal course of corticospinal excitability changes following left and right hemisphere TMS. Methods: Ten normal volunteers were studied. A warning stimulus (S1) was followed by an imperative stimulus (S2) whereupon a word was presented. Subjects responded by reading the word aloud or reading it silently. In other conditions, no word was displayed and the subjects responded to S2 by saying the word ‘Cat’, pursing their lips, or doing nothing. EMG was recorded over the contralateral hand following a TMS pulse over the motor cortex during and after the S1–S2 period. Results: Enhancement of EMG amplitudes was significantly greater following left hemisphere TMS. The enhancement in the S1–S2 period and that following S2 had a time course similar to several event-related brain potentials. Conclusions: There may be a common mechanism underlying both corticospinal excitability and the contingent negative variation, readiness potential and N400.