Cortical Oscillations and Plasticity Induced by Repetitive Transcranial Magnetic Stimulation

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique that is able to modulate cortical activity beyond the stimulation period. The residual aftereffects are akin to the plasticity mechanism of the brain and suggest the potential use of rTMS for therapy. In parallel, there is evidence that altered oscillatory brain rhythms and network dynamics may lead to symptoms of neuropsychiatric disorders. However, the rTMS interference upon cortical and network oscillatory activity remains relatively unknown. Despite this uncertainty, rTMS continues to be used to alleviate symptoms of neuropsychiatric disorders. By combining rTMS and electroencephalography (EEG), the thesis explored the local and network cortical plasticity in healthy humans through the characteristics of oscillatory brain rhythms. We investigated cortical and network oscillatory activity following simple rTMS protocols and continuous theta burst stimulation (cTBS) to the primary motor cortex. The measurements of rTMS-induced aftereffects were quantified by the direct electrophysiology index of EEG and the indirect behavioural measures of motor evoked potentials (MEPs). The results of the experiments showed that rTMS was able to transiently modulate cortical brain rhythms, especially low frequency theta oscillations. The significance of this finding is the possible involvement of independent cortical theta generators besides mu and beta generators over the motor network with different reactivity to rTMS protocols. However, long-term potentiation/depression (LTP-/LTD)-like mechanisms may not be the only mechanisms that drive the rTMS aftereffects as shown by the dissociation between EEG and MEPs cortical output. Here, we explore alternative explanations that drive the EEG oscillatory modulations post rTMS. The significant of this work is the ability of rTMS to transiently modify the internal state of the brain by altering brain oscillations particularly low-frequency brain rhythms. This finding offers exciting possibilities for future clinical trials to explore the use of non-invasive brain stimulation to reverse abnormal synchronisation in neuropsychiatric disorders.