DISSOCIATION OF CENTRALLY AND PERIPHERALLY INDUCED TRANSCRANIAL MAGNETIC STIMULATION EFFECTS IN NONHUMAN PRIMATES.

Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation method that is rapidly growing in popularity for studying causal brain-behavior relationships. However, its dose-dependent centrally-induced neural mechanisms and peripherally-induced sensory co-stimulation effects remain debated. Understanding how TMS stimulation parameters affect brain responses is vital for the rational design of TMS protocols. Studying these mechanisms in humans is challenging due to the limited spatiotemporal resolution of available non-invasive neuroimaging methods. Here, we leverage invasive recordings of local field potentials in a male and a female non-human primate (rhesus macaque) to study TMS mesoscale responses. We demonstrate that early TMS-evoked potentials show a sigmoidal dose-response with stimulation intensity. We further show that stimulation responses are spatially specific. We employ several control conditions to dissociate centrally-induced neural responses from auditory and somatosensory co-activation. These results provide crucial evidence regarding TMS neural effects at the brain circuit level. Our findings are highly relevant for interpreting human TMS studies and biomarker developments for TMS target engagement in clinical applications. Significance Statement Transcranial magnetic stimulation (TMS) is a widely used noninvasive brain stimulation method to stimulate the human brain. To advance its utility for clinical applications a clear understanding of its underlying physiological mechanisms is crucial. Here, we perform invasive electrophysiological recordings in the nonhuman primate brain during TMS, achieving a spatiotemporal precision not available in human EEG experiments. We find that evoked potentials are dose dependent, spatially specific and can be separated from peripheral stimulation effects. This means that TMS evoked responses can indicate a direct physiological stimulation response. Our work has important implications for the interpretation of human TMS-EEG recordings and biomarker development.