Effects of low-frequency transcranial magnetic stimulation on motor excitability and basic motor behavior

Objective: To explore effects of low-frequency repetitive transcranial magn etic stimulation (rTMS) of the primary motor cortex (M1) on motor excitabil ity and basic motor behavior in humans. Design and Methods: Seven normal volunteers underwent 1 Hz rTMS of the hand representation of the right M1 for 15 min at an intensity of 115% of the i ndividual resting motor threshold. The effects of rTMS on motor excitabilit y were assessed by monitoring changes in individual resting motor threshold and input-output curves of motor evoked potentials (MEPs) in the flexor po llicis brevis, first dorsal interosseus, abductor digiti minimi and biceps brachii muscles. Changes in basic motor behavior were studied by measuring maximal and mean peak force and peak accelerations of thumb flexions and ab ductions of the fifth finger before and after rTMS. Results: rTMS produced a significant increase in resting motor threshold an d a significant suppression of MEP input-output curves that persisted for 3 0 min. The suppressing effect was restricted to the hand motor representati on which was the prime target of the stimulation procedure, and there were no significant effects on the biceps representation. Peak force and peak ac celeration were not affected while the motor representations of muscles inv olved in the behavioral measurements were significantly suppressed by rTMS. Conclusions: Low-frequency rTMS of M1 transiently depresses motor excitabil ity but this does not affect basic motor behavior. This is relevant for the therapeutic use of low-frequency rTMS in disorders with abnormal cortical excitability. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.