Modulating verbal working memory with fronto‐parietal transcranial electric stimulation at theta frequency: Does it work?

• Published in: The European journal of neuroscience Vol. 55; no. 2; pp. 405 - 425
• Main Authors: Biel, Anna Lena, Sterner, Elisabeth, Röll, Lukas, Sauseng, Paul
• Format: Journal Article
• Language: English
• Published: France Wiley Subscription Services, Inc 01.01.2022
• Subjects: Cognition; connectivity; Cortex (frontal); Cortex (parietal); Electric Stimulation; Electrodes; fronto‐parietal network; Humans; Language; Memory, Short-Term - physiology; Parietal Lobe - physiology; Placebos; Short term memory; Synchronization; theta oscillations; Theta rhythms; transcranial alternating current stimulation (tACS); Transcranial Direct Current Stimulation; working memory; transcranial alternating current stimulation (tACS); fronto-parietal network; connectivity; working memory; theta oscillations
• ISSN: 0953-816X; 1460-9568; 1460-9568
• DOI: 10.1111/ejn.15563

Oscillatory theta activity in a fronto‐parietal network has been associated with working memory (WM) processes and may be directly related to WM performance. In their seminal study, Polanía et al. (2012) (de‐)coupled a fronto‐parietal theta‐network by applying transcranial alternating current stimulation (tACS), and showed that anti‐phase tACS led to slower and in‐phase tACS to faster response times in a verbal WM task compared to placebo stimulation. In the literature, this ‘synchronization‐desynchronization’ effect has only been partly replicated, and electric field modelling suggests that it might not be the fronto‐parietal network that is primarily stimulated during in‐phase tACS with a shared return electrode. This provides one possible reason for inconsistency in the literature. In this study, we aimed to reproduce the findings reported by Polanía et al. (2012). We also aimed to investigate whether in‐phase theta tACS with multiple close‐by return electrodes for focal stimulation of the frontal and the parietal cortex will have at least as much of a facilitatory effect as the in‐phase stimulation as indicated by Polania et al. (2012). In a single‐trial distributional analysis, we explored whether mean, variation and right‐skewness of the response time distribution are affected. Against our hypothesis, we found no ‘synchronization‐desynchronization’ effect by fronto‐parietal theta tACS on response times using the same delayed letter discrimination task and stimulation parameters in two experiments, both between‐subjects and within‐subjects. However, we could show that in a more demanding 3‐back task, fronto‐parietal in‐phase and in‐phase focal theta tACS substantially improved task performance compared to placebo stimulation. We used multi‐site transcranial alternating stimulation at theta frequency to either synchronize or desynchronize a fronto‐parietal working memory network in order to modulate verbal working memory performance. Results indicate that a behavioural modulatory effect of synchronizing fronto‐parietal tACS can only be observed in a working memory task of high difficulty, but not so in an easy letter recognition task.