Polarity-specific effects of motor transcranial direct current stimulation on fMRI resting state networks

Abstract

Transcranial direct current stimulation (tDCS) has been used to modify motor performance in healthy and patient populations. However, our understanding of the large-scale neuroplastic changes that support such behavioural effects is limited. Here, we used both seed-based and independent component analyses (ICA) approaches to probe tDCS-induced modifications in resting state activity with the aim of establishing the effects of tDCS applied to the primary motor cortex (M1) on both motor and non-motor networks within the brain. Subjects participated in three separate sessions, during which resting fMRI scans were acquired before and after 10min of 1mA anodal, cathodal, or sham tDCS. Cathodal tDCS increased the inter-hemispheric coherence of resting fMRI signal between the left and right supplementary motor area (SMA), and between the left and right hand areas of M1. A similar trend was documented for the premotor cortex (PMC). Increased functional connectivity following cathodal tDCS was apparent within the ICA-generated motor and default mode networks. Additionally, the overall strength of the default mode network was increased. Neither anodal nor sham tDCS produced significant changes in resting state connectivity. This work indicates that cathodal tDCS to M1 affects the motor network at rest. In addition, the effects of cathodal tDCS on the default mode network support the hypothesis that diminished top-down control may contribute to the impaired motor performance induced by cathodal tDCS.

Keywords: Default mode; Functional MRI; Independent component analysis; Motor; Resting state connectivity; Transcranial direct current stimulation.

Fig. 1

Inter-hemispheric coherence within key motor network nodes. Cathodal stimulation increases inter-hemispheric coherence between specific regions within the motor network. (A) Significantly increased coherence was identified between the hand areas of the left and right M1 (M1_Hand) and (B) the left and right SMA. (C) A trend towards an increase in inter-hemispheric coherence after cathodal tDCS was also observed in the PMC. (D) No significant changes in inter-hemispheric coherence were observed for M1 as a whole. Error bars: ± 1 SEM.

Fig. 2

The resting state networks. Melodic ICA generated resting state networks from our data. Group mean (A) motor, (B) default mode, (C) medial visual, (D) lateral visual, (E) executive, (F) auditory, (G) right dorsal visual stream, and (H) left dorsal visual stream networks.

Fig. 3

Changes in network strength with tDCS. Cathodal tDCS increased the strength of the default mode network. (A) A significant (p < 0.05) increase in functional connectivity in the DMN was observed following cathodal, but not anodal or sham, tDCS. (B) The strength of the motor network was not significantly changed over time by tDCS. Error bars: ± 1 SEM.

Fig. 4

Exploratory voxel-wise analysis of cathodal tDCS-induced changes in network strength. Cathodal stimulation modifies connectivity between specific areas of the motor and default mode networks. (A) Regions of increased functional connectivity within the motor network after cathodal tDCS. A single large cluster is identified. (B) Regions of increased functional connectivity within the DMN after cathodal tDCS. A further exploratory analysis investigated changes in functional connectivity within these identified regions for the motor network (C) and the default mode network (D). No significant modulation of functional connectivity was identified within these regions after anodal or sham stimulation. Bar graphs represent the amount of resting activity in the clusters shown. Error bars: ± 1 SEM.