Transcranial electric stimulation over posterior parietal cortex and short-term associative memory – Differential effects of constant vs. theta frequency oscillatory stimulation

Abstract

Transcranial electric stimulation (tES) techniques are promising tools for neuromodulation of memory functions. However, previous findings on their effectiveness are inconsistent, and there have been only a few studies directly contrasting different types of stimulation and examining their differential effects on memory performance. The study objective was the comparative assessment of the online effects of three types of tES over the left posterior parietal cortex (PPC) on the enhancement of short-term associative memory. The three tES types were constant anodal tDCS (1.5mA), and two types of personalized theta frequency (4-8Hz) oscillatory stimulation protocols - oscillatory tDCS (otDCS, 1.5mA ± 0.5mA) and tACS (0 ± 1mA). Participants (N = 40) took part in the sham-controlled cross-over experiment where they performed parallel forms of the short-term associative memory task while receiving different stimulation types in four counterbalanced experimental sessions (tDCS, otDCS, tACS, and sham). The stimuli within the short-term associative memory task consisted of digit-color association sequences in which single-digits (0-9) were presented sequentially on the cards of different colors. Participants were instructed to try to remember the digit-color associations presented in each sequence. The length of sequences varied between three (low-demand) to five stimuli (high-demand). At the end of each sequence, the participants were presented with one of the previously seen colored cards, and they needed to recall the digit that was presented on a given card. Planned contrasts within repeated-measures ANOVA showed that relative to sham constant anodal tDCS improved short-term associative memory for both low- and high-demand sequences, while two oscillatory protocols improved memory performance in high-demand sequences only. The results indicate that different stimulation protocols potentially affect different cognitive processes. Namely, the effects of constant anodal tDCS on cognitive performance seem to be mediated by the facilitation of low-level attention processes, while the effects of both otDCS and tACS appear to affect processes that are more central to the associative binding.