Distinct differences in cortical reactivity of motor and prefrontal cortices to magnetic stimulation

Objective : The stimulus intensity of prefrontal transcranial magnetic stimulation (TMS) is usually determined with respect to the motor threshold (MT). However, the association between the excitability of the prefrontal and motor cortices is unknown. Methods : Magnetic pulses to the left motor and prefrontal cortices were delivered at the MT of the right abductor digiti minimi muscle for 9 subjects and at 4 different stimulus intensities (60, 80, 100, and 120% of MT) for two subjects. Simultaneously, EEG was recorded with 60 scalp electrodes. Results : Global mean field amplitudes of the TMS-evoked responses were significantly (32%) smaller after prefrontal than after motor cortex TMS, but they correlated positively. Conclusions : The reactivity to TMS is different between the motor and prefrontal cortices. However, an association between these reactivities suggests that MT may be used for determining the stimulus intensity of prefrontal TMS.