Abstract
Noninvasive neurostimulation methods such as transcranial direct current stimulation (tDCS) can elicit long-lasting, polarity-dependent changes in neocortical excitability. In a previous concurrent tDCS-fMRI study of overt picture naming, we reported significant behavioural and regionally specific neural facilitation effects in left inferior frontal cortex (IFC) with anodal tDCS applied to left frontal cortex (Holland et al., 2011). Although distributed connectivity effects of anodal tDCS have been modelled at rest, the mechanism by which ‘on-line’ tDCS may modulate neuronal connectivity during a task-state remains unclear. Here, we used Dynamic Causal Modelling (DCM) to determine: (i) how neural connectivity within the frontal speech network is modulated during anodal tDCS; and, (ii) how individual variability in behavioural response to anodal tDCS relates to changes in effective connectivity strength. Results showed that compared to sham, anodal tDCS elicited stronger feedback from inferior frontal sulcus (IFS) to ventral premotor (VPM) accompanied by weaker self-connections within VPM, consistent with processes of neuronal adaptation. During anodal tDCS individual variability in the feedforward connection strength from IFS to VPM positively correlated with the degree of facilitation in naming behaviour. These results provide an essential step towards understanding the mechanism of ‘online’ tDCS paired with a cognitive task. They also identify left IFS as a ‘top-down’ hub and driver for speech change.
Highlights
Transcranial direct current stimulation is a non-invasive brain stimulation method, which can be used to modulate spontaneous cortical activity in the human brain in a polarity-dependent way (Nitsche and Paulus, 2000; Nitsche and Paulus, 2001)
Consistent with predictions of neuronal adaptation, we previously demonstrated that anodal Transcranial direct current stimulation (tDCS) applied to the left inferior frontal cortex during overt picture naming concurrent with functional magnetic resonance imaging (fMRI) had a regionally specific neural priming effect on the BOLD signal in left inferior frontal sulcus (IFS) and left ventral premotor cortex (VPM)
There was no correlation between the Euclidean distance between the centre of the two spherical volumes of interest (VOI) calculated for each subject and these results (VPM self-connections: r = − 0.56, df = 10, p = 0.879; VPM-IFS: r = 0.297, df = 10, p = 0.405)
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation method, which can be used to modulate spontaneous cortical activity in the human brain in a polarity-dependent way (Nitsche and Paulus, 2000; Nitsche and Paulus, 2001). Resulting model parameters from the DCM were used to provide a measure of both the strength and direction of neuronal interactions between pre-specified left frontal regions known to be important for speech production (Penny et al, 2004; Penny et al, 2010; Friston, 2011). Using this approach our data provide novel insights into the underlying neuronal dynamics of anodal tDCS that operate on the naming network
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