Immediate effect of quadri-pulse stimulation on human brain microstructures

Several studies have implied that human brain microstructures can change immediately after a behavioral training. However, since widespread regions are involved in behavioral training, it remains unclear whether the microstructure in the living human brain changes immediately after the change in activity of a specific brain area. Hence, we aimed to examine whether the microstructures in the human brain change after the increase and decrease in the specific brain activity by repetitive transcranial magnetic stimulation, namely quadri-pulse stimulation (QPS). Right-handed healthy adults underwent both the excitatory (QPS5) and inhibitory (QPS50) QPS protocols over the left M1. Before and after QPS, diffusion MRI and resting-state fMRI scans were collected to detect any microstructural (fractional anisotropy [FA] and mean diffusivity [MD] values) and functional (functional connectivity between the bilateral M1) changes after QPS5 and QPS50. As a result, we observed no statistically significant change in FA or MD values after either QPS5 or QPS50 in cerebral cortex. This suggests that the brain activity change in widespread area is required to induce microstructural change immediately.