Transcranial direct current stimulation of the prefrontal cortex modulates working memory performance: combined behavioural and electrophysiological evidence

Tino Zaehle, Pascale Sandmann, Jeremy D Thorne, Lutz Jäncke, Christoph S Herrmann, Tino Zaehle, Pascale Sandmann, Jeremy D Thorne, Lutz Jäncke, Christoph S Herrmann

Abstract

Background: Transcranial direct current stimulation (tDCS) is a technique that can systematically modify behaviour by inducing changes in the underlying brain function. In order to better understand the neuromodulatory effect of tDCS, the present study examined the impact of tDCS on performance in a working memory (WM) task and its underlying neural activity. In two experimental sessions, participants performed a letter two-back WM task after sham and either anodal or cathodal tDCS over the left dorsolateral prefrontal cortex (DLPFC).

Results: Results showed that tDCS modulated WM performance by altering the underlying oscillatory brain activity in a polarity-specific way. We observed an increase in WM performance and amplified oscillatory power in the theta and alpha bands after anodal tDCS whereas cathodal tDCS interfered with WM performance and decreased oscillatory power in the theta and alpha bands under posterior electrode sides.

Conclusions: The present study demonstrates that tDCS can alter WM performance by modulating the underlying neural oscillations. This result can be considered an important step towards a better understanding of the mechanisms involved in tDCS-induced modulations of WM performance, which is of particular importance, given the proposal to use electrical brain stimulation for the therapeutic treatment of memory deficits in clinical settings.

Figures

Figure 1
Figure 1
Behavioural Data: Plots show performance in the WM task after anodal and cathodal tDCS in relation to preceding sham stimulation as well as the performance of the separate control group. Top: The d' increased more after anodal than after cathodal stimulation with an intermediate effect for the control group, demonstrating a polarity effect of the tDCS-induced behavioural improvement. Bottom: Participants responded generally faster from sham to tDCS measure and pre-to-post measure in the control group for Hits (left) and False Alarms (right), respectively. Asterisks indicate statistical significance (P < 0.05).
Figure 2
Figure 2
Event-related potentials: Grand average event-related potentials (ERPs) are shown for each condition at the occipito-parietal ROI (above) and at channel Pz (below). Topographies of overall grand averages (computed across all participants and conditions) are shown at P1, N1, and P3 latencies.
Figure 3
Figure 3
DC stimulation effect on oscillatory brain activity: Event-related spectral perturbation (ERSP) time-frequency plots are given separately for the two sham conditions (A: sham anodal; D: sham cathodal) preceding the two active stimulation conditions (B: anodal, E: cathodal). Differences between the conditions were computed by subtracting the sham ERSPs from the ERSPs of the active conditions (C: active anodal - sham anodal; F: active cathodal - sham cathodal) and by subtracting the cathodal ERSPs from anodal ERSPs (G: sham anodal - sham cathodal; H: active anodal - active cathodal). Subplot J illustrates the overall contrast between the differences obtained for the anodal and cathodal conditions (subplot C - subplot F) and for the active and sham conditions (subplot G - subplot H). White contours indicate significant differences between the conditions (P < 0.05, corrected for multiple comparisons). Note that the statistical analyses revealed no significant differences between the two sham conditions.
Figure 4
Figure 4
Experimental design. Top: After mounting EEG and tDCS electrodes, the experiment started with a 15-minute Sham-tDCS. For the next 10 minutes, participants performed a letter two-back working memory (WM) task while EEG was recorded. Subsequently, a 15 minute active-tDCS (anodal or cathodal) was applied, followed by a further 10-minute WM task. EEG recording was stopped during the stimulation periods. Each participant performed one anodal and one cathodal tDCS session separated by at least one day. The session order was counterbalanced across participants. An additional control group underwent one experimental session performing the similar WM task twice but without receiving a treatment during the 15 minute break in between. Bottom: Schematic description of the letter two-back working memory task.

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