Mapping cortical excitability in the human dorsolateral prefrontal cortex

Background: Repetitive transcranial magnetic stimulation (rTMS) to the dorsolateral prefrontal cortex (dlPFC) is an effective treatment for depression, but the neural response to rTMS remains unclear. TMS with electroencephalography (TMS-EEG) can probe these neural effects, but variation in TMS-evoked potentials (TEPs) across the dlPFC are not well characterized and often obscured by muscle artifact. Mapping TEPs and artifacts across dlPFC targets is needed to identify high fidelity subregions that can be used for rTMS treatment monitoring.

Objective: To characterize ‘early TEPs’ anatomically and temporally close (20-50 ms) to the TMS pulse and associated muscle artifacts (<20 ms) across the dlPFC. We hypothesized that TMS location and angle would affect these early TEPs and that TEP size would be inversely related to muscle artifact. We sought to identify an optimal TMS target / angle for the group and asked if individualization would be beneficial.

Methods: In 16 healthy participants, we applied single-pulse TMS to six targets within the dlPFC at two coil angles and measured EEG responses.

Results: Early TEPs were sensitive to stimulation location, with posterior and medial targets yielding larger early TEPs. Regions with high early TEP amplitude had less muscle artifact, and vice versa. The best group-level target yielded 102% larger TEP responses compared to other standard targets. Optimal TMS target differed across subjects, suggesting that a personalized targeting approach could boost the early TEP by additional 36%.

Conclusions: The early TEPs can be probed without significant muscle-related confounds in posterior-medial regions of the dlPFC. A personalized targeting approach may further enhance the signal quality of the early TEP.