Background

Transcranial direct current stimulation (tDCS) has been found to improve working memory (WM) performance in healthy participants following a single session. However, results are mixed and the overall effect size is small. Interpretation of these results is confounded by heterogeneous study designs, including differences in tDCS dose (current intensity) and sham conditions used.

Aims

We systematically investigated the effect of tDCS dose on working memory using behavioural and neurophysiological outcomes.

Methods

In a single-blind parallel group design, 100 participants were randomised across five groups to receive 15 min of bifrontal tDCS at different current intensities (2 mA, 1 mA, and three sham tDCS conditions at 0.034 mA, 0.016 mA, or 0 mA). EEG activity was acquired while participants performed a WM task prior to, during, and following tDCS. Response time, accuracy and an event-related EEG component (P3) were evaluated.

Results

We found no significant differences in response time or performance accuracy between current intensities. The P3 amplitude was significantly lower in the 0 mA condition compared to the 0.034 mA, 1 mA and 2 mA tDCS conditions. Changes in WM accuracy were moderately correlated with changes in frontal P3 amplitude (channel Fz) following tDCS compared to baseline levels (r = 0.34).

Conclusions

Working memory was not significantly altered by tDCS, regardless of dose. The P3 amplitude showed that stimulation at 1 mA, 2 mA and a sham condition (0.034 mA) had biological effects, with the largest effect size for 1 mA stimulation. These findings indicate higher sensitivity of neurophysiological outcomes to tDCS and suggests that sham stimulation previously considered inactive may alter neuronal function.