Theta Burst Stimulation-induced Metaplasticity: Changing Tactile Perception and Physiology in Primary Somatosensory Cortex

I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public iii ABSTRACT Theta-burst stimulation (TBS) over human primary motor cortex evokes plasticity and metaplasticity, the latter contributing to the homeostatic balance of excitation and inhibition. Our knowledge of TBS-induced effects on neighboring primary somatosensory cortex (SI) is limited and it is unknown whether TBS is capable of inducing metaplasticity within human SI. Sixteen right-handed participants (6 females, mean age 23) received six different TBS protocols delivered over SI in separate sessions. TBS protocols were delivered at 30 Hz (612 pulses) and included continuous TBS (cTBS), intermittent TBS (iTBS), cTBS followed by cTBS, iTBS followed by iTBS, cTBS followed by iTBS and iTBS followed by cTBS. Dependent measures included the amplitude of the first and second somatosensory evoked potential (SEP) following median nerve stimulation and their paired-pulse ratio (PPR), and temporal order judgment (TOJ) performed on digits 2 and 3. Dependent measures were obtained before and at 5, 25, 50 and 90 minutes following stimulation. Results indicate similar effects following cTBS and iTBS; increased amplitude of the second SEP with marginal changes to PPR, and elevated TOJ thresholds. CTBS-cTBS and iTBS-iTBS both demonstrated metaplasticity via measures of TOJ but with disparities in their direction and timing of effects. I conclude that 30 Hz cTBS and iTBS protocols delivered in isolation induce similar plasticity in SI as measured by decreases in SI intracortical inhibition and impairments in TOJ performance, and when applied in multiples, produce metaplasticity effects on TOJ performance. iv ACKNOWLEDGEMENTS