The effect of transcranial magnetic stimulation and peripheral nerve stimulation on corticomuscular coherence in humans

Cortex and muscle show coupled oscillations in the 15-35 Hz frequency band during voluntary movements. To obtain evidence of the neuronal network responsible for this rhythmicity we investigated the effect of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation on the coupling between eletcroencephalographic (EEG) activity recorded from the scalp over the motor cortex and electromyographic (EMG) activity recorded from the tibialis anterior (TA) muscle in 15 healthy human subjects. TMS over the leg area at intensities between 0.95 and 1.1 x threshold for a motor evoked potential (MEP) in the TA increased corticomuscular coherence in the 15-35 Hz frequency band. This effect lasted on average for 300 ms, but could last up to 600-800 ms in some subjects. Stimulation of motor nerves from the ankle muscles suppressed corticomuscular coherence in the 15-35 Hz frequency range between leg area EEG and TA EMG for a period up to 600-800 ms. In addition, increased coherence around 10 Hz was observed for a period up to 250 ms after the stimulation. Stimulation of motor nerves in the arm and motor nerves from the ankle muscles in the other leg had no effect. The findings indicate that TMS has direct access to the neuronal circuitry in the motor cortex, which generates the corticomuscular coherence. This effect was caused either by direct activation of corticospinal cells or by activation of local neuronal circuitries in the motor cortex. The effects of peripheral nerve stimulation suggest that an alternative rhythm generator may entrain the cortical cells into a lower 10 Hz rhythm and disrupt the 15-35 Hz rhythm.