High‐definition transcranial direct current stimulation modulates performance and alpha/beta parieto‐frontal connectivity serving fluid intelligence

Abstract

Fluid intelligence (Gƒ) includes logical reasoning abilities and is an essential component of normative cognition. Despite the broad consensus that parieto‐prefrontal connectivity is critical for Gƒ (e.g. the parieto‐frontal integration theory of intelligence, P‐FIT), the dynamics of such functional connectivity during logical reasoning remains poorly understood. Further, given the known importance of these brain regions for Gƒ, numerous studies have targeted one or both of these areas with non‐invasive stimulation with the goal of improving Gƒ, but to date there remains little consensus on the overall stimulation‐related effects. To examine this, we applied high‐definition direct current anodal stimulation to the left and right dorsolateral prefrontal cortex (DLPFC) of 24 healthy adults for 20 min in three separate sessions (sham, left, and right active). Following stimulation, participants completed a logical reasoning task during magnetoencephalography (MEG). Significant neural responses at the sensor‐level were imaged using a beamformer, and peak task‐induced activity was subjected to dynamic functional connectivity analyses to evaluate the impact of distinct stimulation montages on network activity. We found that participants responded faster following right DLPFC stimulation vs. sham. Moreover, our neural findings followed a similar trajectory of effects such that left parieto‐frontal connectivity decreased following right and left DLPFC stimulation compared to sham, with connectivity following right stimulation being significantly correlated with the faster reaction times. Importantly, our findings are consistent with P‐FIT, as well as the neural efficiency hypothesis (NEH) of intelligence. In sum, this study provides evidence for beneficial effects of right DLPFC stimulation on logical reasoning. Key points Logical reasoning is an indispensable component of fluid intelligence and involves multispectral oscillatory activity in parietal and frontal regions. Parieto‐frontal integration is well characterized in logical reasoning; however, its direct neural quantification and neuromodulation by brain stimulation remain poorly understood. High‐definition transcranial direct current stimulation of dorsolateral prefrontal cortex (DLPFC) had modulatory effects on task performance and neural interactions serving logical reasoning, with right stimulation showing beneficial effects. Right DLPFC stimulation led to a decrease in the response time (i.e. better task performance) and left parieto‐frontal connectivity with a marginal positive association between behavioural and neural metrics. Other modes of targeted stimulation of DLPFC (e.g. frequency‐specific) can be employed in future studies.

Exploring the effectiveness of transcranial direct current stimulation in enhancing cognitive outcomes: the problem of heterogeneity

“…Considering the current lack of evidence, particularly using HD-tDCS, which may offer more targeted stimulation, there is still substantial ground to make up regarding the most effective NIBS approaches for improving clinical outcomes across healthy and unhealthy populations. The important results of Arif et al (2021) reinforce the notion that cognitive functions are modulated by several brain regions and neural networks. Consequently, more adequately powered NIBS studies, with combined brain imaging methods, are necessary in understanding how specific brain regions can be stimulated to induce cognitive improvements.…”

“…This article also explored some methodological aspects which should be considered in future studies to strengthen the preliminary results by Arif et al . (2021) and reduce study heterogeneity across NIBS studies. Considering the current lack of evidence, particularly using HD‐tDCS, which may offer more targeted stimulation, there is still substantial ground to make up regarding the most effective NIBS approaches for improving clinical outcomes across healthy and unhealthy populations.…”

“…Moreover, as a follow-up to these study results, it would be interesting to investigate any sex-related differences in cognitive outcomes, particularly as anodal stimulation may cause stronger excitability responses in women compared to men (Thair et al 2017). It can be noted that 38% of the study population in Arif et al (2021) were women, and so the effects of tDCS and hormone levels during different phases of the menstrual cycle of women, and the…”

“…Despite the novel and intriguing findings presented, there are still large inconsistencies across tDCS and cognitive function studies. Therefore, some methodological design aspects and statistical analysis techniques may prove useful to reduce heterogeneity, which should serve to build on the results of Arif et al (2021). For instance, Luque-Casado et al (2020) used sub-group cluster analysis to address the high inter-individual variability commonly known to affect tDCS outcomes, which delineates 'responder' and 'non-responder' sub-groups.…”

“…Interestingly, in Arif et al . (2021) the offline HD‐tDCS exposure occurred on average 55 min prior to the logical reasoning task, which was indicated to fall within the time window for offline ‘priming’ effects without cortical excitability dissipation. The neuromodulatory aftereffects induced by tDCS on cortical excitability appear to be fairly stable over prolonged time courses, but the nature and magnitude of these effects are dissimilar across individuals insomuch that endogenous fluctuations in neural activity may confound results (Buch et al .…”