Transcranial direct current stimulation (tDCS) over parietal cortex improves associative memory

Abstract

Associative memory plays a key role in everyday functioning, but it declines with normal ageing as well as due to various pathological states and conditions, thus impairing quality of life. Associative memory enhancement via neurostimulation over frontal areas resulted in limited success, while posterior stimulation sites seemed to be more promising. We hypothesized that anodal transcranial direct current stimulation (tDCS) of parietal areas would lead to higher performance in associative memory due to high connectivity between posterior parietal cortex (PPC) and hippocampus. Forty-two healthy adults participated in two sham-controlled cross-over experiments. Anodal electrode (20 min, 1.5 mA) was placed over P3 in Experiment 1 and over P4 in Experiment 2. During tDCS participants played a simple computer game. After each stimulation session, participants completed parallel forms of an associative memory task (Experiment 1: face-word memory; Experiment 2: object-location memory) and a control task (verbal fluency). In both experiments, associative memory was improved after anodal stimulation compared to sham stimulation, while no differences were observed in the control task. Additionally, memory performance was higher in the second than in the first trial, but the increase in performance between the two trials did not differ between stimulation conditions. It can be concluded that a single-session anodal tDCS over posterior parietal cortex can improve associative memory performance. The specificity, robustness, and reproducibility of the effect suggest that PPC is a promising target for brain stimulation aiming to enhance memory functions.

Keywords: Associative memory; Cognitive enhancement; Face-word memory; Object-location memory; Posterior parietal cortex (PPC); Transcranial direct-current stimulation (tDCS).