Transcranial Electrical Stimulation for Associative Memory Enhancement: State-of-the-Art from Basic to Clinical Research

Abstract

Associative memory (AM) is the ability to bind new information into complex memory representations. Noninvasive brain stimulation (NIBS), especially transcranial electric stimulation (tES), has gained increased interest in research of associative memory (AM) and its impairments. To provide an overview of the current state of knowledge, we conducted a systematic review following PRISMA guidelines covering basic and clinical research. Out of 374 identified records, 41 studies were analyzed—twenty-nine in healthy young adults, six in the aging population, three comparing older and younger adults, as well as two studies on people with MCI, and one in people with Alzheimer’s dementia. Studies using transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS) as well as oscillatory (otDCS) and high-definition protocols (HD-tDCS, HD-tACS) have been included. The results showed methodological heterogeneity in terms of study design, stimulation type, and parameters, as well as outcome measures. Overall, the results show that tES is a promising method for AM enhancement, especially if the stimulation is applied over the parietal cortex and the effects are assessed in cued recall paradigms.

High-density theta oscillatory-modulated tDCS (theta HD-otDCS) over the parietal cortex for targeted memory enhancement

“…reaction time, correct rejection, false memories) 35,36 and one study even reported inhibitory effects 52 . This is in contrast with the results of the standard tES studies which generally tend to favor PPC as the cortical region where stimulation provides beneficial effects on AM 51 . Taken together, these data could imply that, although spatial optimization is certainly a relevant factor, it might not be sufficient to lead to strong AM facilitation.…”

“…Stimulating prefrontal cortex (PFC) 34,50 repeatedly failed to produce facilitatory effects. This is in line with the results of the previous research regarding standard tES montages which do not support PFC as an optimal cortical target in associative memory studies (reviewed in 51 ). On the other hand, HD studies stimulating parietal regions yielded variable results.…”

“…Additionally, the results of this study address an issue raised in previous literature overviews 51,64 questioning whether the memory process during which the stimulation is delivered could be a potential confounding variable in tES studies. Our results based on the direct comparison between stimulation effects during encoding and retrieval in this experiment shows that it might not be the case.…”

“…As previously stated, PPC is considered to be an optimal stimulation target for episodic memory enhancement 51 . However, studies using standard tDCS montage with anode over PPC and cathode on a contralateral cheek could not definitely conclude that the observed facilitatory effects were caused by PPC activation 21,22,35,52 .…”

The Effects of Transcranial Direct Current Stimulation in Patients with Mild Cognitive Impairment

“…The optimal number of sessions and the time period between them remain to be established in future studies. In healthy people, experiments showed that the effect of a single session of stimulation can be observed after a day; meanwhile, after that period, the aftereffects are negligible [68].…”