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

Transcranial Magnetic Stimulation 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 magnetic pulses can alter ongoing oscillatory activity in the human primary motor cortex. In healthy participants, we combined transcranial magnetic stimulation with electroencephalography to assess neural responses to single pulses and rhythmic stimulation in the motor cortex. Conventional high intensity stimulation produced robust evoked responses and synchronized beta-frequency oscillations. Low-intensity rhythmic stimulation, despite generating much weaker direct 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 used in human studies. Low-intensity stimulation may therefore offer a route towards portable, energy-efficient technologies for investigating and modulating brain networks.