Effects of Transcranial Alternating Current Stimulation on Cognitive Functions in Healthy Young and Older Adults

Abstract

Recently, transcranial alternating current stimulation (tACS) has emerged as a tool to enhance human cognitive processes. Here, we provide a brief summary of the rationale behind tACS-induced effects on task-relevant brain oscillations and associated cognitive functions and review previous studies in young subjects that have applied tACS in cognitive paradigms. Additionally, we present pilot data where we administered theta-tACS (6 Hz) over the temporoparietal cortex and a supraorbital reference for 20 min during implicit language learning in healthy young (mean/SD age: 22/2) and older (mean/SD age: 66/4) adults, in a sham-controlled crossover design. Linear mixed models revealed significantly increased retrieval accuracy following tACS-accompanied associative learning, after controlling for session order and learning success. These data provide the first implementation of tACS during cognitive performance in older adults and support recent studies suggesting that tACS in the theta frequency range may serve as a tool to enhance cognition, possibly through direct modulation of task-relevant brain oscillations. So far, studies have been heterogeneous in their designs, leaving a number of issues to be addressed in future research, including the setup of electrodes and optimal stimulation frequencies to be employed, as well as the interaction with age and underlying brain pathologies in specific patient populations.

Figure 1

(a) Sample auditory (presented via headphones) and visual (presented on the screen) stimuli of the experiment. (b) Duration and composition of each trial (120 in total in each block). (c) Illustration of the electrode positions and experimental design. The two stimulation electrodes were placed over the left temporoparietal area (CP5; 5 × 7 cm²) and the right supraorbital area (10 × 10 cm²). L1–L5, learning blocks 1–5. R, retrieval testing.

Figure 2

(a) Retrieval performance in the transfer task which directly followed the learning blocks. Model-based estimates are depicted. N = 22 subjects/44 measures. ^∗ p < .05. (b) Learning performance in the five learning blocks. Blue rectangles/rhombs for sham condition in young/older adults, red circles/triangles for tACS condition in young/older adults. N = 22 subjects/220 measures. Means and 95% CI.