Repetitive transcranial magnetic stimulation (rTMS)
appears to have effects on cortical excitability that
extend beyond the train of rTMS itself. These effects
may be inhibitory or facilitatory and appear to depend
on the frequency, intensity, duration and intertrain
interval of the rTMS. Many studies assume
facilitatory effects of high-frequency rTMS and
inhibitory effects of low-frequency rTMS.
Nevertheless, the interindividual variability of this
modulation of cortical excitability by rTMS has not
been systematically investigated. In this study, we
applied 240 pulses of rTMS at 90% of the subjects'
motor threshold to their motor cortex at different
frequencies (1, 10, 15 and 20 Hz) and examined the
effects on motor evoked potentials (frequency tuning
curve). Although the averaged group data showed a
frequency-dependent increase in cortical excitability,
each subject had a different pattern of frequency
tuning curve, i.e. a different modulatory effect on
cortical excitability at different rTMS frequencies.
The interindividual variability of these modulatory
effects was still high, though less so, when the number
of rTMS pulses was increased to 1,600. These
findings illustrate the degree of variability of the
rTMS effects in the human brain.