Repetitive paired-pulse transcranial magnetic stimulation over the visual cortex alters visual recovery function

Background: Some repetitive transcranial magnetic stimulation (rTMS) techniques fail to facilitate cortical excitability in the human visual cortex. A more effective and facilitatory method is needed to increase the feasibility of rTMS to explore visual cortex function. Objective: The present study aimed to develop a novel tool for modulating the visual cortex excitability and examined the influences of repetitive transcranial magnetic paired-pulse stimulation (rPPS) on the visual cortex. Methods: Optimal interstimulus intervals (ISIs) were determined to assess recovery function of visual evoked potentials (VEPs). Paired stimuli from checkerboard pattern reversals were presented at 11 ISIs from 50-200 ms. Each session consisted of control (S1) and paired (S1 + S2) epochs to extract the S2 response. The recovery function was calculated as the ratio of S2/S1 amplitudes. Subsequently, rPPS was utilized with a 1.5 ms ISI over the visual cortex at the stimulus intensity of the visual masking effect. Amplitudes and suppression ratios of the paired VEPs were compared before and after rPPS. The effect of single pulse TMS was also evaluated. Results: Paired VEPs resulted in suppressive effects at ISIs up to 200 ms, with an optimal ISI of 90 ms due to small variability and moderate inhibitory effects. There was no significant effect of rPPS on N75-P100 with paired VEPs. Following rPPS, however, P100-N145 inhibition decreased up to 10 min. The single pulse protocol did not result in these effects. Conclusions: Modulation of VEP recovery by rPPS suggested that rPPS exhibited a disinhibitory effect on the visual cortex.