Mild traumatic brain injury alters function in the dorsolateral prefrontal cortex: a TMS-EEG study

Numerous studies highlight alterations in motor cortical neurophysiology after mild traumatic brain injury (mTBI). In contrast, understanding of effects in cognitive brain areas, which potentially underpin more impactful effects of injury, remains limited. The current study addressed this limitation by using electroencephalography to index responses to transcranial magnetic stimulation (TMS-EEG) applied over the dorsolateral prefrontal cortex (DLPFC). In 18 mTBI patients and 22 healthy controls, TMS-EEG was used to index cortical inhibition and long-term depression (LTD)-like plasticity within DLPFC. These were assessed using paired-pulse TMS (short-[SICI] and long-[LICI] interval intracortical inhibition) and continuous theta burst stimulation (cTBS), respectively. Functional effects of injury were assessed via the N-back and Trail Making Test. Responses were quantified in both temporal (i.e., TMS-evoked potentials; TEPs) and spectral (i.e., TMS-related oscillations; TROs) domains. For TEPs, measures of inhibition and plasticity both tended to be reduced by mTBI. In contrast, TRO data suggested a contradictory increased response of patients to both paired-pulse stimulation and cTBS that was specific to high-frequency bands. In addition, while TMT performance was reduced in the mTBI group, this was unrelated to neurophysiological effects of injury. These findings highlight novel effects of mTBI on DLPFC neurophysiology and emphasise the importance of considering both evoked and induced activity when examining neurophysiological effects of mTBI.