Memory Enhancement Using Transcranial Alternating Current Stimulation

The Rey Auditory Verbal Learning (RAVL) test evaluates verbal episodic memory and is designed as a list-learning paradigm in which the subject hears a list of 15 nouns. The participant is asked to recall as many words from the list as possible after each of 5 repetitions of free-recall (total recall), and 15 minutes after an interference trial (long delayed recall). Scores range from 0 to 75 (total recall) and 0 to 15 (long delayed recall), with higher scores meaning a better outcome. An increase in RAVL test (total and long delayed recall) after active but not sham tACS is expected.

The Face-Name Association Task assesses associative episodic memory and is a computer paradigm consisting of an encoding and a retrieval phase of face-name pairs. During the encoding phase, participants are instructed to remember a set of 12 faces and names they are paired. In two additional learning trials, participants are shown the previously learned faces and are asked to recall the correct name associated with each face (inital recognition). This recall of the correct name associated with a learned face is repeated 5 minutes after the learning trials during the retrieval phase (associative recognition). Scores range from 0 to 24 (inital recognition) and 0 to 12 (associative recognition), with higher scores meaning a better outcome. An increase in FNAT score (initial and associative recognition) after active but not sham tACS is expected.

Change in relative power will be used, expecting an increase in relative power in higher alpha, beta and gamma bands and decrease in relative power in theta band after active but not sham tACS.

Change in posterior peak frequency will be used, expecting an increase in posterior peak frequency after active but not sham tACS.

Change in corrected amplitude envelope correlation (AEC-c) will be used, expecting an increase in AEC-c in higher alpha, beta and gamma bands after active but not sham tACS.

Change in phase lag index (PLI) will be used, expecting a decrease in PLI in theta band after active but not sham tACS.

Change in graph theory parameters will be used as a measure of global network structure. Restoration of overall network configuration (expressed by shift of multiple Minimum spanning tree (MST) parameters towards healthy state, e.g. stronger hierarchy) after active but not sham tACS is expected.

Change in graph theory parameters will be used as a measure of global network structure. Preserved small-world configuration (shift of small world index (clustering coefficient/path length) towards less random topology) after active but not sham tACS is expected.

Change in graph theory parameters will be used as a measure of mesoscale network structure (modularory measures) and nodal characteristics (degree and eigenvector centrality). Restoration of hub disruption (stronger modularity index, increase in degree and eigenvector centrality) of precuneus-posterior cingulate and medial temporal lobe after active but not sham tACS is expected.