Regional specificity of cathodal transcranial direct current stimulation effects on spatial–numerical associations: Comparison of four stimulation sites

Abstract

Neuromodulation with transcranial direct current stimulation (tDCS) is an increasingly popular research tool to experimentally manipulate cortical areas and probe their causal involvements in behavior, but its replicability and regional specificity are not clear. This registered report investigated cathodal tDCS effects on spatial–numerical associations (i.e., the SNARC effect), the numerical distance effect (NDE), and inhibitory control (i.e., stop‐signal reaction time; SSRT). Healthy adults (N = 160) were randomly assigned to one of five groups to receive sham tDCS or 1 mA cathodal tDCS to one of four stimulation sites (left/right prefrontal cortex [PFC], left/right posterior parietal cortex) with extracephalic return. We replicated that cathodal tDCS over the left PFC reduced the SNARC effect compared to sham tDCS and to tDCS over the left parietal cortex. However, neither NDE nor SSRT were modulated in the main analyses. Post hoc contrasts and exploratory analyses showed that cathodal tDCS over the right PFC had a time‐dependent effect by delayed practice‐related improvements in SSRT. Math anxiety moderated changes in the NDE in the groups receiving tDCS to the right parietal cortex. With few exceptions, the replicability and regional specificity of tDCS effects on behavior were weak and partially moderated by individual differences. Future research needs to characterize the parameter settings for effective neuromodulation.

High‐definition turns timing‐dependent: Different behavioural consequences during and following cathodal high‐definition transcranial direct current stimulation (HD tDCS) in a magnitude classification task

“…Numerical processing and the numerical distance effect in particular, however, should be hosted in parietal regions (Cohen Kadosh et al, 2008;Hubbard et al, 2005) and thus likely not susceptible to prefrontal tDCS. Accordingly, we did not expect that prefrontal tDCS modulated the numerical distance effect, similar to previous studies with cathodal tDCS (Farshad et al, 2024;Schroeder et al, 2016Schroeder et al, , 2017a. Most critically, in order to support a verbal account of the SNARC effect, we expected that HD tDCS of the IFG, but not of the DLPFC, should yield a reduction relative to sham, similar to conventional tDCS.…”

“…Second, considering the intensity of the electric field for HD tDCS, we could not interpret the results regarding a more specific role of IFG or DLPFC for the SNARC effect. At the same time, almost identical tDCS effects in the left and right frontal cortex in another recent study question the assumption that verbal regions are most critical for the emergence of the SNARC (Farshad et al, 2024). Another limitation of the study are differences in baseline performance across groups, although a relatively homogeneous sample was recruited.…”

“…'offline'). Data from conventional tDCS (active and sham) of the left PFC were pooled from another study with the identical design for comparison (Farshad et al, 2024).…”