Rotational field TMS: Comparison with conventional TMS based on motor evoked potentials and thresholds in the hand and leg motor cortices

Abstract

Background: Transcranial magnetic stimulation (TMS) is a rapidly expanding technology utilized in research and neuropsychiatric treatments. Yet, conventional TMS configurations affect primarily neurons that are aligned parallel to the induced electric field by a fixed coil, making the activation orientationspecific. A novel method termed rotational field TMS (rfTMS), where two orthogonal coils are operated with a 90 phase shift, produces rotation of the electric field vector over almost a complete cycle, and may stimulate larger portion of the neuronal population within a given brain area. Objective: To compare the physiological effects of rfTMS and conventional unidirectional TMS (udTMS) in the motor cortex. Methods: Hand and leg resting motor thresholds (rMT), and motor evoked potential (MEP) amplitudes and latencies (at 120% of rMT), were measured using a dual-coil array based on the H7-coil, in 8 healthy volunteers following stimulation at different orientations of either udTMS or rfTMS. Results: For both target areas rfTMS produced significantly lower rMTs and much higher MEPs than those induced by udTMS, for comparable induced electric field amplitude. Both hand and leg rMTs were orientation-dependent. Conclusions: rfTMS induces stronger physiologic effects in targeted brain regions at significantly lower intensities. Importantly, given the activation of a much larger population of neurons within a certain brain area, repeated application of rfTMS may induce different neuroplastic effects in neural networks, opening novel research and clinical opportunities.

Responses of Model Cortical Neurons to Temporal Interference Stimulation and Related Transcranial Alternating Current Stimulation Modalities

“…The rotation of the E-field could be contributing to TIS stimulation. For example, rotational field TMS, applied using two perpendicular figure-of-eight TMS coils driven with the same current waveforms with a 90° phase shift, demonstrated lower thresholds and stronger evoked responses (Rotem et al, 2014;Roth et al, 2019Roth et al, , 2020. The rotational E-field is hypothesized to sweep across the neural population and simultaneously activate neurons with different directional sensitivity.…”

TMS with fast and accurate electronic control: Measuring the orientation sensitivity of corticomotor pathways

“…We have also developed an mTMS system with a large general-purpose 5-coil transducer [14]. Previous studies have also combined two figure-of-eight coils to produce a rotating E-field in a single TMS pulse capable of reducing the orientation sensitivity of neuronal excitation [17,18]. In this study, we developed a compact 2-coil transducer tailored for controlling the orientation of the peak induced E-field to allow high-resolution mappings of the neuronal sensitivity to the TMS pulse orientation.…”

Effect of stimulus orientation and intensity on short-interval intracortical inhibition (SICI) and facilitation (SICF): A multi-channel transcranial magnetic stimulation study

“…Thus, it was concluded that excitatory and inhibitory neuronal mechanisms exhibit a critical sensitivity to the stimulus orientation. The observations of Souza et al [20] and Ziemann et al [8] were based on the CS and TS intensities set relative to the resting motor threshold (RMT) only in the AM orientation and not in the PM orientation, in which the RMT can be about 30% higher than in the AM orientation [24]. Therefore, it remains unclear whether the neuronal mechanisms engaged in the inhibitory and facilitatory phenomena are truly affected by the stimulus orientation rather than a decrease in the relative stimulation intensity.…”