Motor cortex excitability in patients with cerebellar degeneration

Objectives: To study motor cortex (M1) excitability and the effect of subthreshold transcranial magnetic stimulation (TMS) in patients with cerebellar degeneration and normals performing a reaction time (RT) task. Methods: Time to wrist flexion after a visual go-signal was measured. TMS was always delivered at 90% of resting motor evoked potential (MEP) threshold. In one experiment, test TMS was delivered at various intervals after the go-signal. In half the trials priming TMS was also given with the go-signal. A second experiment examined the effect on RT of M1 and occipital priming stimulation alone. Results: M1 excitability, measured as the likelihood of producing MEPs in the wrist flexor muscles, increased immediately after the go-signal in the patients and stayed high until movement. In controls, excitability rose gradually. This difference was largely eliminated by priming TMS. RT was longer in the patient group, but improved with priming TMS. Occipital priming produced less effect on RT than M1 stimulation in both controls (P=0.008) and patients (P=0.0004). Conclusions: M1 excitability prior to movement in an RT task increases abnormally early in cerebellar patients. This may reflect compensation for deficient thalamocortical drive. Subthreshold TMS can partially normalize the prolonged RT and abnormal excitability rise in cerebellar patients.