Modulation of oscillatory activity in response to very low intensity transcranial magnetic stimulation in the human primary motor cortex

Abstract Transcranial Magnetic Stimulation (TMS) is widely used to probe and modulate human brain function, yet the neural effects of stimulation delivered at very low intensities remain unclear. Here, we show that very low intensity TMS can alter ongoing oscillatory activity in the human primary motor cortex. In healthy participants, we combined TMS with electroencephalography (EEG) and assessed neural responses to single pulses and rhythmic stimulation in the motor cortex. Conventional high intensity stimulation produced robust local evoked responses and synchronized beta-frequency oscillations in the directly TMS-targeted cortical area. Low intensity rhythmic stimulation, despite not evoking measurable responses, modified local oscillatory activity in a manner consistent with phase-dependent enhancement of ongoing rhythms. These findings suggest that cortical oscillations can be influenced by magnetic fields substantially weaker than those typically employed to date in human studies. Very low intensity stimulation may therefore offer a route towards portable, energy-efficient technologies for investigating and modulating brain networks.