Understanding the Effects of Transcranial Electrical Stimulation in Numerical Cognition: A Systematic Review for Clinical Translation

Abstract

Atypical development of numerical cognition (dyscalculia) may increase the onset of neuropsychiatric symptoms, especially when untreated, and it may have long-term detrimental social consequences. However, evidence-based treatments are still lacking. Despite plenty of studies investigating the effects of transcranial electrical stimulation (tES) on numerical cognition, a systematized synthesis of results is still lacking. In the present systematic review (PROSPERO ID: CRD42021271139), we found that the majority of reports (20 out of 26) showed the effectiveness of tES in improving both number (80%) and arithmetic (76%) processing. In particular, anodal tDCS (regardless of lateralization) over parietal regions, bilateral tDCS (regardless of polarity/lateralization) over frontal regions, and tRNS (regardless of brain regions) strongly enhance number processing. While bilateral tDCS and tRNS over parietal and frontal regions and left anodal tDCS over frontal regions consistently improve arithmetic skills. In addition, tACS seems to be more effective than tDCS at ameliorating arithmetic learning. Despite the variability of methods and paucity of clinical studies, tES seems to be a promising brain-based treatment to enhance numerical cognition. Recommendations for clinical translation, future directions, and limitations are outlined.

Keywords: arithmetic processing; cognitive training; dyscalculia; interventions; non-invasive brain stimulation; number processing.

Figure 1

Neurobiological bases of numerical cognition. The neurocognitive bases are described as interactions of multiple brain networks—especially the fronto-parietal network—that supports domain-specific mechanisms (blue color coding) and domain-general processes (orange color coding). Legend: DLPFC = dorsolateral prefrontal cortex, VLPFC = ventrolateral prefrontal cortex, PSPL = posterior superior parietal lobe, IPS = intraparietal sulcus, SMG = supramarginal gyrus, AG = angular gyrus, FG = fusiform gyrus, HC = hippocampus. Reproduced from reference [18], 10.1038/s41539-021-00099-3, under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/ (accessed on 29 March 2022).

Figure 2

Stimulation waveforms for transcranial direct current stimulation (tDCS) (anodal and cathodal), transcranial alternating current stimulation (tACS), and transcranial random noise stimulation (tRNS). Reproduced from reference [59], https://doi.org/10.1007/s10545-018-0181-4, under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/ (accessed on 29 March 2022).

Figure 3

Search flow PRISMA diagram.

Figure 4

Summary of tES effects. Panel (A) shows the tES effects across all reviewed studies based on non-symbolic and symbolic number processes and arithmetic processes. Panel (B) shows the comparisons between tDCS and tRNS effects across all studies addressing non-symbolic and symbolic number processes. Panel (C) shows the comparisons between tDCS and tRNS effects across all studies addressing arithmetic processes. The effects of HD-tDCS were included in the tDCS effects. The effects of tACS were also not considered since only one study was found. Legend: tES = transcranial electrical stimulation; tDCS = transcranial direct current stimulation, tRNS = transcranial random noise stimulation.