200/200 words Abstract 1The effectiveness of transcranial direct current stimulation (tDCS) placed over the motor hotspot 2 (thought to represent the primary motor cortex (M1)) to modulate motor network excitability is 3 highly variable. The premotor cortex-particularly the dorsal premotor cortex (PMd)-may be a 4 promising alternative target to more effectively modulate motor excitability, as it influences motor 5 control across multiple pathways, one independent of M1 and one with direct, modulating 6 connections to M1. This double-blind, placebo-controlled study aimed to differentially excite 7 motor and premotor regions using high-definition tDCS (HD-tDCS) with concurrent functional 8 magnetic resonance imaging (fMRI). HD-tDCS applied over either the motor hotspot or the 9 premotor cortex demonstrated high inter-individual variability in changes on cortical motor 10 excitability. However, HD-tDCS over the premotor cortex led to a higher number of responders 11 and greater changes in local fMRI-based complexity than HD-tDCS over the motor hotspot.
12Furthermore, an analysis of individual motor hotspot anatomical locations revealed that, in more 13 than half of the participants, the motor hotspot is not located over anatomical M1 boundaries, 14 despite using a canonical definition of the motor hotspot. This heterogeneity in stimulation site may 15 contribute to the variability of tDCS results. Altogether, these findings provide new considerations 16 to enhance tDCS reliability. 17 18 4of tDCS applied over the primary motor cortex (M1), functionally localized as the motor hotspot 3 5 4,5 in each participant, have been demonstrated in post-stroke motor recovery 6-10 . However, a major 6 issue that prevents tDCS from being widely adopted in clinical practice is the high inter-individual 7 variability shown in motor behavioral or cortical physiological changes following M1 8 stimulation 11,12 . The reasons for this inter-individual variability have not been fully elucidated 13 .
9One potential solution to reduce the inter-individual variability is to use alternative stimulation 10 sites and/or more precise tDCS montages. In particular, stimulating another entry point into the 11 motor network may lead to more reliable changes following tDCS. The premotor cortex, and 12 particularly the dorsal premotor cortex (PMd), is a promising alternative neural target for 13 modulating motor excitability as it is the second major origin of the corticospinal tract (CST), with 14 30% of CST projections 14,15 . Non-human primates studies have shown that PMd provides high-15 level control of the upper limb motor commands using a downstream pathway that is independent 16 of M1 14,16-18 . Moreover, neural inputs to the hand and forelimb representation in M1 mostly 17 originate from PMd 19,20 , creating a cortico-cortical circuitry that has influences on motor network 18 excitability 21-24 . Finally, where these cortico-cortical connections exist, the onset of movement-19 related activity occurs sooner in the premotor cortex than in M...