Objective: To explore effects of low-frequency repetitive transcranial magnetic stimulation (rTMS) of the primary motor cortex (M1) on motor excitability 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 individual resting motor threshold. The effects of rTMS on motor excitability were assessed by monitoring changes in individual resting motor threshold and input-output curves of motor evoked potentials (MEPs) in the flexor pollicis 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 abductions of the fifth finger before and after rTMS.
Results: rTMS produced a significant increase in resting motor threshold and a significant suppression of MEP input-output curves that persisted for 30 min. The suppressing effect was restricted to the hand motor representation which was the prime target of the stimulation procedure, and there were no significant effects on the biceps representation. Peak force and peak acceleration were not affected while the motor representations of muscles involved in the behavioral measurements were significantly suppressed by rTMS.
Conclusions: Low-frequency rTMS of M1 transiently depresses motor excitability 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.