Origin of facilitation in repetitive, 1.5ms interval, paired pulse transcranial magnetic stimulation (rPPS) of the human motor cortex.

OBJECTIVE: Repetitive paired-pulse TMS (rPPS) given at an interstimulus interval (ISI) of 1.5 ms has been reported to induce a lasting motor evoked potential (MEP) facilitation. This after-effect was considered to be a cortical event because F-waves were not affected by the same rPPS. To confirm its cortical facilitation, we compared the after-effects of rPPS on MEPs to single pulse TMS over the motor cortex (motor cortical MEPs) with those to brainstem stimulation (brainstem MEPs).

METHODS: Subjects were 10 healthy volunteers. Suprathreshold paired-pulse TMS at an ISI of 1.5 ms was applied to the motor cortex for 30 min at a rate of 0.2 Hz. After intervention, we measured motor cortical MEPs for 30 min. We also studied brainstem MEPs in five subjects.

RESULTS: Motor cortical MEPs were facilitated to about 190% of baseline (p<0.001) for 10 min post rPPS intervention and returned to the baseline at 10-15 min post intervention. Brainstem MEPs were not affected by the intervention.

CONCLUSIONS: The facilitation of MEPs after rPPS at an interval of 1.5 ms occurs at the motor cortex.

SIGNIFICANCE: rPPS at an interval of 1.5 ms is an effective method for increasing motor cortical excitability.