Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that allows the manipulation of intrinsic brain oscillations. Numerous studies have applied tACS in the laboratory to enhance cognitive performance. With this systematic review, we aim to provide an overview of frequency-specific tACS effects on a range of cognitive functions in healthy adults. This may help to transfer stimulation protocols to real-world applications. We conducted a systematic literature search on PubMed and Cochrane databases and considered tACS studies in healthy adults (age > 18 years) that focused on cognitive performance. The search yielded n = 109 studies, of which n = 57 met the inclusion criteria. The results indicate that theta-tACS was beneficial for several cognitive functions, including working memory, executive functions, and declarative memory. Gamma-tACS enhanced performance in both auditory and visual perception but it did not change performance in tasks of executive functions. For attention, the results were less consistent but point to an improvement in performance with alpha- or gamma-tACS. We discuss these findings and point to important considerations that would precede a transfer to real-world applications.

Associative memory is one of the first cognitive functions negatively affected by healthy and pathological aging processes. Non-invasive brain stimulation (NIBS) techniques are easily administrable tools to support memory. However, the optimal stimulation parameters inducing a reliable positive effect on older adult’s memory performance remain mostly unclear. In our randomized, double-blind, cross-over study, 28 healthy older adults (16 females; 71.18 + 6.42 years of age) received anodal transcranial direct (tDCS), alternating current in the theta range (tACS), and sham stimulation over the left ventrolateral prefrontal cortex (VLPFC) each once during encoding. We tested associative memory performance with cued recall and recognition tasks after a retention period and again on the following day. Overall, neither tDCS nor tACS showed effects on associative memory performance. Further analysis revealed a significant difference for performance on the cued recall task under tACS compared to sham when accounting for age. Our results suggest that tACS might be more effective to improve associative memory performance than tDCS in higher aged samples.

INTRODUCTION: In people at risk of Alzheimer’s disease (AD), hippocampal hyperactivity relates to worse hippocampus-dependent task performance and promotes AD pathology. Reducing hippocampal hyperactivity promises to improve hippocampus-dependent memory performance. METHODS: 78 participants (69.29±5.87 years old, 60% female) used real-time fMRI neurofeedback twice to downregulate the hippocampus or a control region. We assessed hippocampal activity and memory performance before and after neurofeedback. We classified AD pathology risk using a blood-based biomarker, an established risk-score, or clinical characterisation. RESULTS: During hippocampus, but not during control-region neurofeedback, individuals significantly downregulated hippocampal activity with the effect still detectable one week later. In those at high risk of AD pathology, with all three classifications, stronger hippocampal downregulation resulted in stronger memory improvement, independent of baseline hippocampal hyperactivity. DISCUSSION: Downregulating hippocampal activity using fMRI neurofeedback may be a complementary treatment for people at risk for AD, well before there is hippocampal hyperactivity.