Delayed long term impact of working memory training coupled with tRNS on near- and far-transfer cognitive tasks

Abstract

Background. Transcranial random noise stimulation (tRNS) over the parietal cortex can increase behavioral performance with effects lasting beyond the time of stimulation. Multiple-day tRNS protocols can boost visuo-perceptual functions, however, it is unknown whether tRNS over the parietal cortex can also improve working memory (WM) through training, and whether these effects transfer to other cognitive domains. Aim. We aimed to boost WM (N-Back task) training (WMt) with concurrent 20-min high frequency 1mA tRNS, and see whether training effects can transfer to untrained tasks, tested on three near-transfer (within the same cognitive domain: dual N-Back working memory and Forward/Backward Digit Span and Corsi short-term memory) and three far-transfer tasks (in a different domain: Posner attention, Flanker inhibitory control and Raven fluid intelligence tasks). Moreover, we tested the impact of tRNS and frequency of training (one or two sessions/day) at one-month post intervention. Methods. We run 8 sessions: 4 training, two pre- and post-training, and two follow-up sessions. 60 healthy participants were divided into 2 groups: 1) 2-day experiment: 4 sessions (2/day) of WMt + tRNS (subgroup 1) or Sham (subgroup 2) and 2) 4-day experiment: 1 session of WMt + tRNS (subgroup 1) or Sham (subgroupp2) per day. Results. We showed increased performance in untrained tasks in the 2-day experiment condition, when subjects received WMt coupled with tRNS and not after sham. Specifically, training effects transferred to the dual N-Back and the Forward Digit Span tasks. Crucially, these effects were only present at one-month follow-up and not before. While the 4-day experiment only showed a trend towards improvement in the near-transfer Corsi task, however it never reached significance. Conclusions. Results show that WMt coupled with tRNS boosts learning when run on a twice-a-day protocol. These delayed effects might indicate consolidation processes, benefits that need time to transfer to untrained tasks.