Multi-locus transcranial magnetic stimulation with pulse-width modulation

Background: In transcranial magnetic stimulation (TMS), a pulse of current driven through a coil on the scalp induces an electric field on the cortex, eliciting neuronal activation. Multi-locus TMS (mTMS) uses multiple coils to generate various electric field patterns, enabling electronic control of stimulation location and orientation. However, changing the stimulation target with mTMS can take up to a few seconds when the driving voltages are adjusted for each coil. Objective: To investigate whether multi-coil TMS combined with pulse-width modulation (PWM) in the microsecond scale could be used to overcome the issue of rapid pulse delivery to different cortical targets. Methods: We devised a methodology to generate and drive PWM approximations of given reference pulses. We compared resting motor thresholds (RMTs) and motor evoked potentials (MEPs) between trapezoidal and PWM pulses in several multi-coil targeting scenarios. Results: The cortical electric field of PWM pulses had high temporal complexity, but the pulse types showed no statistically significant differences when stimulating with 2- or 3-coil combinations. With PWM pulses using five coils, the RMT increased by 9 % (p = 0.06), and MEP amplitudes decreased by 20 % with stimulation doses over 120 % RMT (p < 0.05). Conclusions: Motor responses with trapezoidal and PWM pulses were equivalent when using two or three coils concurrently, but some differences were found with five coils. Using PWM with multi-coil TMS device to control the stimulation targets in millisecond-scale is feasible, although more research is needed to understand the neurophysiological effects of increasing coil count.