Theta burst stimulation of the visual cortex does not alter contrast sensitivity under optically induced blur
This study investigated whether a single session of repetitive transcranial magnetic stimulation (TMS), specifically theta burst stimulation (TBS), acutely modulates visual function under conditions of induced optical defocus. Twelve adults with normal or corrected-to-normal vision participated in a pre-registered, within-subject, double-blind, sham-controlled study. Participants first adapted to an experimentally induced + 1.50D optical blur for 10 minutes. Following this, stimulation was applied to the primary visual cortex according to a pre-randomised allocation. Stimulation consisted of continuous TBS (cTBS), intermittent TBS (iTBS), or sham stimulation, with each condition applied to all participants over separate visits. Visual acuity (VA) and contrast discrimination thresholds were measured under the induced blur condition immediately before and after stimulation. Repeated measures ANOVA revealed no significant interaction between stimulation condition and time for either contrast discrimination threshold (F(2,22) = 0.03, p = 0.96, partial ηp² = 0.003) or VA (F(2,22) = 0.12, p = 0.88, partial ηp² = 0.011). Although a significant main effect of time was observed for VA (F(1,11) = 6.30, p = 0.029, partial ηp² = 0.364), reflecting an improvement across all conditions, there were no differential effects attributable to active cTBS or iTBS compared to sham. Consequently, a single session of TBS applied to the visual cortex did not result in a measurable modulation of visual performance under acute optical defocus in adults with typical vision. Rapid homeostatic mechanisms activated during short-term blur adaptation may reduce the susceptibility of the visual cortex to the acute effects of TBS.