The effects of transcranial direct current stimulation on within- and cross-paradigm transfer following multi-session backward recall training

Abstract

Transcranial direct current stimulation (tDCS) has been shown to enhance the efficacy and generalisation of working memory (WM) training, but there has been little systematic investigation into how coupling task-specific WM training with stimulation impacts more specifically on transfer to untrained tasks. This randomised controlled trial investigated the boundary conditions to transfer by testing whether firstly the benefits of training on backward digit recall (BDR) extend to untrained backward recall tasks and n-back tasks with different materials, and secondly which, if any, form of transfer is enhanced by tDCS. Forty-eight participants were allocated to one of three conditions: BDR training with anodal (10 min, 1mA) or sham tDCS, or visual search training with sham tDCS, applied over the left dorsolateral prefrontal cortex. Transfer was assessed on within- (backward recall with digits, letters, and spatial locations) and cross-paradigm (n-back with digits and letters) transfer tests following three sessions of training and stimulation. On-task training gains were found, with transfer to other backward span but not n-back tasks. There was little evidence that tDCS either enhanced the rate or magnitude of on-task training or transfer gains. These findings indicate that training enhances paradigm-specific processes within WM, but that tDCS does not enhance these gains.

Training executive functions using an adaptive procedure over 21 days (10 training sessions) and an active control group

“…Holmes et al (2019) systematically manipulated the paradigm ( n -back or complex span) and stimuli (verbal or visuo-spatial) used in a WM training programme, and reported no transfer effects between paradigms, even when stimuli were matched. Similarly, Byrne et al (2020) systematically tested the boundary conditions for near-transfer training benefits within and across WM paradigms with different categories of stimuli, and found paradigm-specific improvements following training that did not extend across different WM paradigms.…”

“…Taken together, this research suggests that participants learned to perform particular cognitive tasks (i.e., direct training), but this did not lead to improvement in latent cognitive abilities underling these tasks (e.g., task switching, response inhibition), and therefore contributes to theoretical debates on cognitive training and transfer effects. Transfer effects are thought to occur when the skills learnt in one domain generalise to enhance performance in another domain, and the degree to which this transfer occurs is directly related to the extent of shared features between the trained and untrained task (Byrne et al, 2020; Singley & Anderson, 1989; Woodworth & Thorndike, 1901). This study addressed a gap in the literature by comparing training effects directly within and between different domains of EF (WM, IC, and CF) in a single study; the majority of previous EF training research has focused on understanding the boundaries of training effects in a single domain of EF, most frequently WM.…”

“…On the other hand, the degree to which training effects transfer to untrained tasks or domains remains inconclusive. Some studies have reported near-transfer effects within an EF domain (e.g., Heinzel et al, 2016; Karbach & Kray, 2009; Sandberg et al, 2014), e.g., Heinzel et al (2016) showed improvements in WM when training and assessment was conducted using different paradigms, and Byrne et al (2020) showed improvements in WM when training and assessment used different materials but the same task. Others have shown far-transfer effects between EF domains (e.g., Borella et al, 2010; Dowsett & Livesey, 2000; Karbach & Kray, 2009; Salminen et al, 2012), e.g., Karbach and Kray (2009) reported improvements in WM after training in CF.…”

“…Some studies have reported near transfer effects within an EF domain (e.g. Karbach & Kray, 2009;Sandberg, Banissy, Catmur, & Bird, 2014;Heinzel et al, 2016), for example Heinzel et al (2016) showed improvements in WM when training and assessment was conducted using different paradigms, and Byrne et al (2020) showed improvements in WM when training and assessment used different materials but the same task.…”