Stochastic resonance effects reveal the neural mechanisms of transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a popular method for studying causal relationships between neural activity and behavior. However, its mode of action remains controversial, and so far there is no framework to explain its wide range of facilitatory and inhibitory behavioral effects. While some theoretical accounts suggest that TMS suppresses neuronal processing, other competing accounts propose that the effects of TMS result from the addition of noise to neuronal processing. Here we exploited the stochastic resonance phenomenon to distinguish these theoretical accounts and determine how TMS affects neuronal processing. Specifically, we showed that online TMS can induce stochastic resonance in the human brain. At low intensity, TMS facilitated the detection of weak motion signals, but with higher TMS intensities and stronger motion signals, we found only impairment in detection. These findings suggest that TMS acts by adding noise to neuronal processing, at least in an online TMS protocol. Importantly, such stochastic resonance effects may also explain why TMS parameters that under normal circumstances impair behavior can induce behavioral facilitations when the stimulated area is in an adapted or suppressed state.