Anodal tDCS modulates specific processing codes during conflict monitoring associated with superior and middle frontal cortices

Adelhöfer, Nico; Stock, Ann-Kathrin; Beste, Christian · 2021 · Crossref

DOI: 10.1007/s00429-021-02245-4

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Summary

This study investigates the specificity of anodal transcranial direct current stimulation (atDCS) effects on conflict monitoring processes. While the N2 event-related potential (ERP) is a established neurophysiological correlate of conflict monitoring, it likely represents a mixture of distinct cognitive subprocesses, including stimulus processing, response selection, and stimulus-response translation. The authors hypothesized that atDCS applied to superior frontal areas would selectively modulate specific subprocesses—specifically those involved in mapping stimuli to responses—rather than affecting all aspects of conflict monitoring uniformly. To test this, 21 healthy adults participated in a single-blind, crossover study involving a modified Simon task while undergoing EEG recording. Participants performed the task under two conditions: active atDCS (2 mA for 16 minutes over the vertex to target superior frontal regions, including the supplementary motor area) and sham stimulation. The task included congruent and incongruent trials, as well as parallel and crossed hand positions to assess the role of proprioceptive information. The researchers employed Residue Iteration Decomposition (RIDE) to dissociate the EEG signal into stimulus-related (S-cluster), response-related (R-cluster), and central stimulus-response translation (C-cluster) components. Source localization was performed using standardized low-resolution brain electromagnetic tomography (sLORETA). Behavioral results indicated that atDCS significantly increased reaction times during incongruent trials compared to sham stimulation, but had no significant effect on congruent trials. This detrimental effect on speed was not accompanied by changes in accuracy, ruling out a speed-accuracy trade-off. Crucially, the neurophysiological analysis revealed that atDCS effects were highly specific. The stimulation modulated the C-cluster of the N2 component, which reflects stimulus-response translation processes, but did not modulate the S-cluster (stimulus-related) or R-cluster (purely response-related) components. Furthermore, atDCS did not affect processing related to proprioceptive hand position information, despite these processes occurring in similar brain regions. Source localization identified that the modulation of the C-cluster was driven by activity changes in the superior frontal cortex (BA6), middle frontal gyrus (BA9), and medial frontal areas (BA32). The findings demonstrate that atDCS does not broadly enhance or inhibit cognitive control but rather modulates specific processing codes within the conflict monitoring network. Specifically, it affects the translation of stimulus information into response selection without altering pure stimulus or response processing. This specificity suggests that neuromodulatory interventions like tDCS interact with distinct functional subprocesses, supporting theoretical models that dissociate these components within the N2 ERP. The results imply that future interventions targeting cognitive control should consider the specific subprocesses involved, as broad stimulation may yield mixed or task-specific behavioral outcomes.

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• executive function