An Investigation of Short-Term Plasticity in Human Motor Cortex

Transcranial magnetic stimulation (TMS) produces a transient magnetic field that activates underlying cortical tissue by eliciting an electrical discharge of the neurons in the targeted area. Repetitive TMS (rTMS) uses patterns of repetitive TMS pulses and has been reliably shown to produce changes in the state of cortical excitability outlasting the time of stimulation. One such protocol that has demonstrated states of increased excitability is intermittent theta burst stimulation (iTBS). This method applies high-frequency bursts (50Hz) of pulses every 200 ms in trains of ten bursts. The effects of and differences between rTMS protocols have been investigated since gaining popularity in the 1990’s, however, there are still many unknowns regarding the neurophysiological changes that accompany this plasticity. Much research on these effects takes place in motor cortex due to reliable and quantifiable measures of cortical excitability observed there. Here, I sought to further investigate the effects of iTBS on inter-hemispheric changes in in motor cortex using EEG simultaneously recorded with TMS pulses. That is, I examined if iTBS conducted over right motor cortex would lead to measureable changes in excitability indices of left motor cortex. I quantified changes in right and left motor cortex excitability with measurements of TMS-evoked potentials (TEPs) and motor-evoked potentials (MEPs) elicited through blocks of single pulse TMS immediately following iTBS and 30 minutes post-iTBS. I compared the effects of the iTBS condition to those found in a control condition where a sham version of iTBS was administered. Results indicate differential modulations of cortical TEPs between hemispheres, including an initial enhancement of the P30 in right hemisphere, coinciding with sustained suppression in left hemisphere, and an enhancement of the P190 during left hemisphere stimulation