Differences in high-definition transcranial direct current stimulation over the motor hotspot versus the premotor cortex on motor network excitability

Abstract

The effectiveness of transcranial direct current stimulation (tDCS) placed over the motor hotspot (thought to represent the primary motor cortex (M1)) to modulate motor network excitability is highly variable. The premotor cortex—particularly the dorsal premotor cortex (PMd)—may be a promising alternative target to reliably modulate motor excitability, as it influences motor control across multiple pathways, one independent of M1 and one with direct connections to M1. This double-blind, placebo-controlled preliminary study aimed to differentially excite motor and premotor regions using high-definition tDCS (HD-tDCS) with concurrent functional magnetic resonance imaging (fMRI). HD-tDCS applied over either the motor hotspot or the premotor cortex demonstrated high inter-individual variability in changes on cortical motor excitability. However, HD-tDCS over the premotor cortex led to a higher number of responders and greater changes in local fMRI-based complexity than HD-tDCS over the motor hotspot. Furthermore, an analysis of individual motor hotspot anatomical locations revealed that, in more than half of the participants, the motor hotspot is not located over anatomical M1 boundaries, despite using a canonical definition of the motor hotspot. This heterogeneity in stimulation site may contribute to the variability of tDCS results. Altogether, these preliminary findings provide new considerations to enhance tDCS reliability.

Functional Effects of Bilateral Dorsolateral Prefrontal Cortex Modulation During Sequential Decision-Making: A Functional Near-Infrared Spectroscopy Study With Offline Transcranial Direct Current Stimulation

“…This negative neural–behavioral correlation only in the tDCS group suggests that participants seem to differ in their response to tDCS as discussed above. More specifically, participants with a higher upregulation effect also seem to perform faster in the delayed reward condition of this task (i.e., positive tDCS responders), whereas a lower upregulation seems to be associated with slower task performance (i.e., negative or tDCS non-responders), similar to findings reported by earlier studies (Nejadgholi et al, 2015 ; Russo et al, 2017 ; Di Rosa et al, 2019 ; Lefebvre et al, 2019 ). The neural–behavioral associations based on optimal choice performance did not reach significance, indicating that the observed tDCS-induced hemodynamic upregulation was not accompanied by more successful learning to predict future reward.…”

No effect of anodal tDCS on motor cortical excitability and no evidence for responders in a large double-blind placebo-controlled trial

“…While this modelling study challenges the current interpretation of the APLM latency [11,17,18], it may provide an alternative explanation for the variability in the effect of tDCS between individuals and experiments. New developments, such as advanced tDCS protocols that use magnetic resonance imaging and high-definition tDCS to preferentially target the premotor cortex of individuals may, therefore, improve the reliability of anodal tDCS [39].…”

Classification of EEG Signals Reveals a Focal Aftereffect of 10 Hz Motor Cortex Transcranial Alternating Current Stimulation

“…Support for this notion is found in a focal tDCS study targeting both premotor and M1. Results show larger MEPs when targeting PMC compared with M1, leading the authors to suggest more reliable changes in cortical excitability are produced by PMC tDCS ( Lefebvre et al 2019 ).…”