# What can neurofeedback and transcranial alternating current stimulation reveal about cross-frequency coupling?

## TL;DR

This paper explores how neurofeedback and tACS can help study brain rhythms called cross-frequency coupling and what factors influence them.

## Contribution

The study systematically reviews how tACS and neurofeedback can be used to investigate cross-frequency coupling and identifies factors affecting its variability.

## Key findings

- tACS and neurofeedback can induce cross-frequency coupling types like phase–amplitude coupling.
- Factors like age, health, and attention state influence cross-frequency coupling variability.
- Adjusting tACS/neurofeedback duration and participant selection may reduce variability in CFC studies.

## Abstract

In recent years, the dynamics and function of cross-frequency coupling (CFC) in electroencephalography (EEG) have emerged as a prevalent area of investigation within the research community. One possible approach in studying CFC is to utilize non-invasive neuromodulation methods such as transcranial alternating current stimulation (tACS) and neurofeedback (NFB). In this study, we address (1) the potential applicability of single and multifrequency tACS and NFB protocols in CFC research; (2) the prevalence of CFC types, such as phase–amplitude or amplitude–amplitude CFC, in tACS and NFB studies; and (3) factors that contribute to inter- and intraindividual variability in CFC and ways to address them potentially. Here we analyzed research studies on CFC, tACS, and neurofeedback. Based on current knowledge, CFC types have been reported in tACS and NFB studies. We hypothesize that direct and indirect effects of tACS and neurofeedback can induce CFC. Several variability factors such as health status, age, fatigue, personality traits, and eyes-closed (EC) vs. eyes-open (EO)condition may influence the CFC types. Modifying the duration of the tACS and neurofeedback intervention and selecting a specific demographic experimental group could reduce these sources of CFC variability. Neurofeedback and tACS appear to be promising tools for studying CFC.