Impact of combined transcranial direct-current stimulation (tDCS) and transcranial alternating current stimulation (tACS) on working memory and semantic fluency (Hebrew)

Executive functions, including working memory and semantic fluency, are essential for learning, decision-making, and daily functioning (Diamond, 2013; Friedman & Miyake, 2017). Previous studies have shown that non-invasive brain stimulation using transcranial direct-current stimulation (tDCS) and transcranial alternating-current stimulation (tACS) may improve these functions when each method is examined separately (Fregni et al., 2005; Herrmann et al., 2013), but their combined, continual effect on working memory and semantic fluency has not yet been examined. The aim of the current study was to examine, for the first time to our knowledge, the effect of a sequential protocol (tDCS followed by tACS, within the same session) on verbal working memory and semantic fluency among healthy adults. The study examens the effect of cognitive load level and the persistence of the effect over time. Fifty healthy adults aged 18 - 45 participated in the study, of whom 41 were included in the final analyses. Participants were randomly assigned, under double-blind conditions, either to an experimental group (tDCS+tACS) or a control group (sham stimulation). The intervention included five consecutive daily stimulation sessions of 20 minutes using a bifrontal montage (F3/F4), as well as a follow-up measurement approximately one month after the intervention ended. Working memory was measured using a verbal n-back task at four load levels (Meiron et al., 2021), and semantic fluency was measured using a word-generation test (Kave, 2005). Data were analyzed using linear mixed models (LMM). The findings did not indicate a general effect of stimulation on working memory or semantic fluency beyond the improvement observed over time in both groups (Hypotheses 1 and 2 were not supported). However, an analysis that included cognitive load level revealed a significant interaction between stimulation type and time point in the working memory accuracy measure. In the experimental group, a significant improvement in accuracy was observed following the intervention, which was maintained at the one-month follow-up, whereas no significant change was found in the control group. The improvement was found mainly at the low load level (2- back) and did not increase with rising difficulty level. In the reaction time measure, only an effect of load level was found, with no effect of the intervention. These findings suggest that a combined, sequential tDCS tACS protocol may contribute to a sustained and focused improvement in working memory accuracy, but its effect does not extend to all the measures and functions examined. This study makes a unique contribution as the first to examine a combined, sequential protocol of these two stimulation methods within a single session. The findings highlight the applied potential of non-invasive brain stimulation for improving cognitive functions, alongside the need for follow-up studies examining adapted protocols, a larger number of sessions, and the integration of neurophysiological measures to clarify the underlying mechanisms of the effect.