Impact of low frequency transcranial magnetic stimulation on event-related brain potentials

Contradictory findings exist concerning the inhibitory function of low frequency repetitive transcranial magnetic stimulation (rTMS). Therefore, the study examines the impact of different duration of low frequency rTMS on ERPs. In 17 subjects, auditory ERPs were measured before and after 1 Hz rTMS delivered over the left prefrontal cortex during 10 min (600 pulses) and 15 min (900 pulses). Results showed that 15 min of 1 Hz rTMS induced a significant increase of P300 latency. There was no effect for early ERP components (N100, P200 and N200). This study confirms and extends that 1 Hz rTMS produces a real inhibitory effect only when the duration of the stimulation is about 15 min. The data suggest that rTMS modifies the speed of cognitive processing rather than the energetical aspect of information processing, and that cortical inhibition induced by the magnetic stimulation affects principally the controlled cognitive processes and not the automatic ones.