Complexity of neural outputs elicited by transcranial magnetic stimulation.

Complex neural activity of the motor cortex is posited to serve as the foundation for a large repertoire of activation patterns crucial for executing movements. As transcranial magnetic stimulation (TMS) predominantly activates monosynaptic fast-conducting corticospinal projections, which are involved in dexterous movement control, complexity of neural outputs elicited by TMS may reflect an underlying repertoire of activation patterns crucial for executing dexterous movements. We proposed to quantify dimensionality of multi-muscle motor-evoked potentials (MEPs) through dimensionality reduction as an integrated measure to reflect complexity of neural outputs elicited by TMS. For its validation, we conducted two experiments focusing on different stimulus parameters: stimulus intensity (experiment 1: n = 40) and size of motor mapping (experiment 2: n = 35). The present findings demonstrated that lower intensities resulted in higher complexity and vice versa but no effects of different sizes of motor mapping on complexity. Analyses incorporating disparities in MEP amplitude across different muscles supported that complexity was effectively captured through dimensionality reduction. Notably, complexity was minimally influenced by the selection of muscles and was associated with individual differences in motor cortex anatomy. We performed two fingers alternating tapping as one index of dexterous movements, and results demonstrated its association with complexity of neural outputs elicited by TMS: higher complexity corresponded to better dexterous performance. The proposed complexity measure might reflect neural processes aligned with the principle of motor abundance. The framework complements well-established MEP analyses and offers a novel perspective for investigating the motor system.