Interindividual variability of the modulatory effects of repetitive transcranial magnetic stimulation on cortical excitability

Repetitive transcranial magnetic stimulation (rTMS) appears to have effects on cortical excitability that extend beyond the train of rTMS itself. Thes e effects may be inhibitory or facilitatory and appear to depend on the fre quency, intensity, duration and intertrain interval of the rTMS. Many studi es assume facilitatory effects of high-frequency rTMS and inhibitory effect s of low-frequency rTMS. Nevertheless, the interindividual variability of t his modulation of cortical excitability by rTMS has not been systematically investigated. In this study, we applied 240 pulses of rTMS at 90% of the s ubjects' motor threshold to their motor cortex at different frequencies (1, 10, 15 and 20 Hz) and examined the effects on motor evoked potentials (fre quency tuning curve). Although the averaged group data showed a frequency-d ependent increase in cortical excitability, each subject had a different pa ttern of frequency tuning curve, i.e. a different modulatory effect on cort ical excitability at different rTMS frequencies. The interindividual variab ility of these modulatory effects was still high, though less so, when the number of rTMS pulses was increased to 1600. These findings illustrate the degree of variability of the rTMS effects in the human brain.