Effects and Mechanisms of Action of an Individualized Theta Frequency Transcranial Alternating Current Stimulation (ITF-tACS) on a Working Memory Training at Healthy Older Adults With Subjective Cognitive Decline (SCD)

This study uses a randomized, double-blind, two-armed, sham-controlled between-subjects design. It is conducted at the University Hospital for Psychiatry and Psychotherapy Tübingen, Germany. Participants older than 60 years and with subjective cognitive decline (SCD) will be included. Other inclusion and exclusion criteria are displayed under "Eligibility". Participants are recruited via email-newsletters at the University of Tübingen, flyers and newspaper advertisement. Participants will receive financial compensation. A stratified block randomization is applied after the inclusion of the participants and before the baseline-session. The person generating the randomization list is not involved with data collection. Operators do not have access to the randomization list during the data collection.

Sample size is N = 36 with n = 18 participants in each group. As a phase 2 study, the aim of this study is the determination of effect sizes which can be used for further investigations. Thus due to a lack of comparable studies, no power analysis could be conducted. The group size is therefore based on the most frequently found group size (n = 18) observed in a systematic review regarding the effects of tACS targeting the memory in healthy adults.

The study includes seven time points: screening-session, baseline-session (in the end of the first week of data-collection), three training sessions (within the second week of data-collection, each session two days apart), post-session (10-12 days after the baseline-session; in the beginning of the third week of data-collection) and follow-up session (one month after the post-session). Online tACS is applied during the training sessions while the participants will conduct an adaptive spatial n-back task (online stimulation). Resting-state EEG as well as EEGs during the spatial 2-, 3-, 4-back at the baseline- and the post-session are conducted. Two groups are compared: one group with individualized theta-tACS (ITF-tACS) and one with sham-tACS.

Hypothesis H1 contains the effect on the trained task (spatial 2-, 3-, 4-back) at the post-session. The investigators expect a higher performance in the group with ITF-tACS compared to sham-tACS. To test this hypothesis, two ANCOVAs (dependent variable = performance at post-session (Reaction time and d prime, Covariate = performance at baseline-session) will be conducted.

Hypothesis H2.1 contains the effect on the trained task in the training sessions concurrent to the tACS. The investigators expect a higher performance in the group with ITF-tACS compared to sham-tACS in all sessions. To test the hypothesis, a linear mixed model will be conducted.