Impact of tDCS on working memory training is enhanced by strategy instructions in individuals with low working memory capacity

Abstract

Interventions to improve working memory, e.g. by combining task rehearsal and non-invasive brain stimulation, are gaining popularity. Many factors, however, affect the outcome of these interventions. We hypothesize that working memory capacity at baseline predicts how an individual performs on a working memory task, by setting limits on the benefit derived from tDCS when combined with strategy instructions; specifically, we hypothesize that individuals with low capacity will benefit the most. Eighty-four participants underwent two sessions of an adaptive working memory task (n-back) on two consecutive days. Participants were split into four independent groups (SHAM vs ACTIVE stimulation and STRATEGY vs no STRATEGY instructions). For the purpose of analysis, individuals were divided based on their baseline working memory capacity. Results support our prediction that the combination of tDCS and strategy instructions is particularly beneficial in low capacity individuals. Our findings contribute to a better understanding of factors affecting the outcome of tDCS when used in conjunction with cognitive training to improve working memory. Moreover, our results have implications for training regimens, e.g., by designing interventions predicated on baseline cognitive abilities, or focusing on strategy development for specific attentional skills.

Figure 1

Description of the two tasks used in the experiment. (A) illustrates the adaptive Nback training task (aNback), whereas (B) depicts the fixed Nback outcome task (fNback).

Figure 2

Schematic of strategy instructions. Participants were asked to assign numbers to positions on the spatial grid, then to create in their mind a target set (grouping) of the positions as numbers as the first ‘n’ items are presented, then compare the new item on the screen with the appropriate recent item (depending on n level) in their memory set, and finally to discard the least recent item of the sequence in mind and update the target set with the new item. Participants in the no- strategy group were introduced to the task and instructed to “do their best trying to get to the highest possible n level”.

Figure 3

Experiment’s timeline.

Figure 4

Correlation plot between standardised aNback (x-axis) and fNback (y-axis) scores at baseline. Points are color-coded according to their capacity group membership.

Figure 5

Performance change ($\mathrm{\Delta }\overline{n}$) with respect to baseline, collapsed across day 1 and day 2 (tDCS sessions) for each group. Means with standard errors are reported for each group and capacity. P-values are marked as follows: * p < 0.05, ** p < 0.01, *** p < 0.001. Changes significantly larger than zero are marked in red.

Figure 6

Increase in mean ‘n’ ($\mathrm{\Delta }\overline{n}$) on the aNBack at the offline POST-ASSESSMENT with respect to BASELINE: means with standard errors are reported for each group and capacity. P-values are marked as follows: * p < 0.05, ** p < 0.01, *** p < 0.001 Improvement significantly larger than zero are marked in red.