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.

Transcutaneous Auricular Vagus Nerve Stimulation Improves Spatial Working Memory in Healthy Young Adults

“…The improved selective attention induced by VNS ( Sun et al, 2017 ) might have a larger effect on the spatial attention network and contributed to the difference in the improvement of taVNS on spatial and digit WM performance. Besides, the researches in tDCS found that the modulation of electric field on WM depends on the baseline performance (e.g., Assecondi et al, 2021 ). The individuals or tasks with lower baseline outcome were more likely to have a higher improvement.…”

Event-Related Potentials as Markers of Efficacy for Combined Working Memory Training and Transcranial Direct Current Stimulation Regimens: A Proof-of-Concept Study

“…According to the magnification-compensation theory, individuals who are already using their resources optimally have less room for improvement, therefore will improve less, expecting a negative correlation between initial performance and gains from cognitive training ( Lövdén et al, 2012 ). Finally, considering our previous findings ( Assecondi, 2021 ), a failure to find an effect of stimulation could also be ascribed to the impossibility, given the small sample size, to focus on individuals with low working memory capacity.…”

“…Before starting the first n-back (at baseline, T0) participants were provided with instructions on how to perform the task, depicted in Figure 5 . Instructions follow a general “memorize,” “compare,” and “update” strategy ( Laine et al, 2018 ; Assecondi, 2021 ), applicable to n-back tasks, as we found that efficient strategy use can modulate tDCS effectiveness and can reduce noise due to interindividual differences ( Lövdén et al, 2012 ).…”

“…The non-invasive direct current stimulation (tDCS) targets the right dorsolateral prefrontal cortex (DLPFC) as it has been shown that tDCS to the DLPFC can modulate working memory in young adults ( Au et al, 2016 ; Assecondi, 2021 ). A bipolar setup (Starstim system, Neuroelectrics Inc., Barcelona, Spain) included two round Ag/AgCl electrodes (area = 3.14 cm 2 ) filled with conductive gel (SIGNAGEL ® ) and placed on F4 (active electrode) and Fp1 (return electrode).…”

“…Moreover, individual differences may influence the outcome of these interventions ( Kosslyn et al, 2002 ), consistent with the notion that cognitive and electric brain stimulation, are both state-dependent techniques, i.e., their effect depends on the current substrate’s state. Individuals starting the training with limited cognitive resources respond better to stimulation than their more equipped counterpart ( Tseng et al, 2012 ; Hsu et al, 2014 ; Benwell et al, 2015 ; Wang et al, 2018 ; Matysiak et al, 2019 ; Assecondi, 2021 ; Krebs et al, 2021 ), although opposite results have been reported ( Berryhill and Jones, 2012 ; Jones et al, 2015 ).…”