Towards understanding the neural underpinnings of associative memory: A tES-EEG study

Abstract

Associative memory (AM) represents an ability to bind unrelated information into meaningful units and encode them as distinct memories. AM has been the function of interest in many non-invasive transcranial electrical stimulation (tES) studies aiming to maximize the potential for memory modulation by varying stimulation loci, frequency, and amplitude. In the current study, we aimed to capture the tES modulation potential of AM performance when tailoring the stimulation protocols to the individual brain rhythms. By matching the stimulation frequency to the frequency of each subject’s AM task-induced electrophysiological (EEG) activity in theta spectrum (4-8 Hz), we developed two types of personalized oscillatory protocols: theta-modulated otDCS and transcranial alternating current stimulation (tACS), which we administered alongside the constant transcranial direct current stimulation (tDCS) and a sham condition in the single-blind cross-over experiment. To comparatively assess the effects of different tES protocols delivered over the posterior parietal cortex, we tested the recognition and recall ability of the 42 healthy young adults on paired associate paradigms after each of four conditions. During AM assessment participant’s EEG activity was recorded. Group-level comparisons of each active tES condition against sham did not show differences in AM task performance either on recognition or cued-recall. However, data showed variability in performance depending on the task and the outcome measures. To explore the potential sources of variability in effect expression, analysis of the function-relevant neurophysiological markers is necessary. Therefore, behavioral results will be accompanied by features of underlying brain activity extracted from the EEG signals. Apart from introducing a novel approach to probing AM with personalized tES, this well-powered, multi-protocol, multi-task, and multi-measure study produced a comprehensive dataset that allows exploration of factors that could uncover different patterns in responsiveness to tES, as well as the insight into how neurophysiological changes reflected on the behavioral level.