Immediate effect of quadri-pulse stimulation on human brain microstructures and functions.

It remains unclear whether repetitive stimulation of a single brain area immediately alters brain microstructure. Thus, we investigated the immediate changes in human brain microstructures following repetitive extrinsic excitation of the left primary motor cortex (M1) through quadri-pulse stimulation (QPS). Sixteen right-handed healthy adults underwent excitatory (QPS5) and inhibitory (QPS50) QPS. Diffusion magnetic resonance imaging (MRI) and resting-state functional MRI were conducted before and after QPS to detect microstructural and functional changes, respectively. No significant alterations in microstructural indices after QPS5 or QPS50 were observed in the cerebral cortex. The functional connectivity (FC) between the bilateral M1 was significantly decreased after QPS5, while it was not significantly modulated after QPS50. Microstructural changes exhibited no significant correlation with this FC change in any region after QPS5 or QPS50. Although no significant FC change was observed following QPS50, these results may suggest that repetitive stimulation of a single brain area can be insufficient to induce immediate microstructural alterations. This would be supported by demonstrating the lack of microstructural changes after QPS together with changes in cortical excitability of the stimulated region.