High-definition transcranial direct current stimulation over the dorsal premotor cortex modulates motor network excitability

The effectiveness of transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) to modulate motor network excitability is highly variable. The dorsal premotor cortex (PMd) may be a promising alternative target, as it provides high-level motor control using multiple pathways, including one independent of M1 and one with direct, influencing connections to M1. This double-blind, placebo-controlled study aimed to differentially excite M1 and PMd using high-definition tDCS (HD-tDCS) with concurrent functional magnetic resonance imaging. Both M1 and PMd HD-tDCS demonstrated high inter-individual variability. However, PMd HD-tDCS led to a slightly greater increase in responders and greater changes in local complexity than M1 HD-tDCS or sham. PMd responsiveness was also positively correlated with the strength of the corticospinal tract originating from PMd. An MRI analysis of individual motor hotspot locations revealed that actual motor hotspots were often located between M1 and PMd, rather than directly over anatomical M1. This heterogeneity in the stimulation site, despite using a canonical definition of the motor hotspot, may contribute to the variability of tDCS results. Altogether, these findings provide new considerations for improving tDCS effects on motor excitability.