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Abstract:
Background: Despite its rapidly expanding application and fast-rising popularity, transcranial magnetic stimulation (TMS) is poorly understood physiologically. The lack of knowledge on TMS physiology, together with the absence of an experimental platform on which various human TMS applications can be studied, developed and refined in vivo at the neuronal level, block the exciting scientific and
therapeutic potential of this non-invasive brain stimulation tool.
Methods: We developed a novel experimental method that offers the direct in vivo electrophysiological access to TMS-evoked neuronal activities in the brain. The method, compatible with standard TMS stimulators and coils, attenuates a variety of TMS-induced artifacts in
extracellular electrophysiology recordings and allows the recording to resume 0.8 – 1 ms after a variety of Tesla-level strong single or repetitive TMS stimulus. Furthermore, using rodents, a common laboratory animal model, we successfully replicated single-pulse TMS
as is routinely used in humans and unveiled TMS-evoked neurons spiking activities in the layer II/III and V of the rodent primary motor cortex.
Results: The suprathreshold monophasic TMS stimulus reliably evoked unilateral activation of the forelimb muscles, evidenced by muscle unit action potentials recorded in the m. biceps brachii (onset latency 11 ms post-TMS). On the neuronal level, the cortical evoked multi-unit activity displayed 5 distinct phases: early excitation (< 6 ms), second excitation (8-26 ms), inhibition (33-172 ms) and rebound excitation (199-238 ms), all of which reveal striking relations to various well-known phenomena in human TMS ranging from intracortical facilitation to late cortical disinhibition.
Conclusions: The data obtained with our method depicted, for the first time, the neuronal response pattern of the classical single-pulse TMS that is widely used in research and clinical works. By bridging the gap between neurons and behaviors, the advance presented here facilitates a new level of insight into the TMS-brain interaction and is vital for developing and utilizing this non-invasive tool to
purposefully explore and effectively treat the human brain.
