Transcranial direct current stimulation over parietal cortex and associative memory

Abstract

Background: Memory enhancement is one of the most challenging issues in neurorehabilitation. Memory functions are affected by different neurological conditions as well as by normal aging. Recent data suggest that it may be possible to improve performance on some of the memory tests by physiological modulation of the activity within a neural loop, which plays a crucial role in the formation and maintenance of the associative memory and which consists of the hippocampus and the lateral parietal cortex. Aims: To check the differential effects of facilitatory and inhibitory noninvasive neuromodulation by transcranial direct current stimulation (TDCS) on associative memory. Methods: Nineteen healthy right-handed participants (8 male, age 22–35) were enrolled in a cross-over placebo-controlled study. Following 20-min of either anodal (facilitatory), cathodal (inhibitory) or sham (placebo) TDCS over the left lateral parietal cortex (P3 of 10–20 EEG system), in 3 separate sessions with 5–7 days inter-session intervals, participants completed a face-cued word recall test of associative memory. The test consisted of two blocks: (a) learning block, in which random words were paired with unfamiliar faces, and (b) recall block in which participants were asked to identify the target faces and recall the word previously presented together with each face. In each session, participants also completed a verbal fluency task, as a control task. Results: The repeated measures ANOVA showed a significant main effect of stimulation type (F(2,36) = 3.71, p = .034; partial eta squared = .171). Post-hoc tests revealed that participants performed better on the associative memory task after anodal than cathodal stimulation (p < .05), and the same trend was observed between anodal and sham stimulation. No difference was found between sham and cathodal stimulation. No effect of stimulation on verbal fluency task. Conclusions: Noninvasive brain modulation has the potential to improve performance on tasks requiring associative memory engagement. The effect may prove to be beneficial for cognitive neurorehabilitation.