Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that holds promise for modulating cognitive functions. By applying weak alternating currents to the scalp, tACS can influence neuronal excitability and potentially enhance or impair specific cognitive processes. The exact mechanisms and optimal parameters for tACS remain under investigation. This study aimed to explore the effects of online and offline tACS at different frequency bands on various cognitive domains. The research is crucial for understanding the potential of tACS as a therapeutic tool for cognitive enhancement and for refining the parameters used in future research. Understanding the effectiveness of tACS across different cognitive domains is critical for developing targeted interventions for various neurological and psychiatric conditions where cognitive deficits are present. Furthermore, characterizing the side effects associated with tACS is crucial for determining its safety and feasibility in clinical applications. This study addresses these research needs by systematically investigating the effects of both online and offline tACS across a range of frequencies and cognitive domains, while meticulously documenting any reported side effects.

A comprehensive review of the existing literature on tACS and its cognitive effects is not explicitly provided within the given data. However, the numerous cited studies (Braun 2017, Brignani 2013, Deng 2019, etc.) suggest that the authors likely reviewed prior work demonstrating both promising and inconclusive findings on the efficacy of tACS on cognition. The literature review would have likely explored different tACS protocols, targeted brain regions, and cognitive outcomes. It would have set the stage by highlighting knowledge gaps and the rationale for this particular study design. The inclusion of so many past studies suggests a significant amount of prior research informing the current investigation.

The methodology involved both online and offline tACS paradigms. Participants underwent tACS at various frequency bands (delta, theta, alpha, beta, gamma) targeting specific brain regions (prefrontal cortex, posterior parietal cortex, temporal cortex, and multiple regions). Cognitive performance was assessed using standardized tests measuring executive function/complex attention, perceptual-motor function, and learning and memory. Online tACS involved the application of stimulation while the cognitive tasks were performed, while offline tACS involved stimulation prior to the cognitive tests. The number of participants varied across studies and experimental conditions, ranging from 10 to 96. Side effects were recorded for each participant, ranging from tingling and itching to more serious effects like headaches and difficulty concentrating. Standardized mean differences (SMD) and standard errors (SE) were calculated to quantify the effect of tACS on cognitive performance, along with Z-scores and p-values to determine statistical significance. Precise details on specific cognitive tests used, stimulation parameters (intensity, duration), and experimental procedures are not fully available in the provided data.

The results are presented in supplementary tables showing the effects of tACS on cognitive performance for different brain regions and cognitive domains. The findings are summarized using standardized mean differences (SMD), standard errors (SE), Z-scores, and p-values. Several frequency bands and brain regions showed statistically significant effects on specific cognitive domains. For example, in the gamma frequency band, stimulation of the temporal cortex resulted in a significant improvement in a cognitive measure (p=0.001). Other frequency bands (delta, theta, alpha, beta) also showed varying effects on cognitive performance depending on the brain region targeted and the specific cognitive domain. Specific details about the magnitude and direction of effects (improvements or impairments) are not fully interpretable without access to the complete data tables and statistical analysis from the original paper. A considerable number of studies showed no significant effects or had missing data on side effects, indicating variability in the efficacy of tACS depending on various parameters.

The findings suggest that tACS, when applied with specific parameters (frequency and targeted brain region), may have a modest effect on cognitive performance. The variability across studies highlights the importance of refining tACS protocols to maximize efficacy and minimize unwanted side effects. The observed variability might be attributed to factors such as individual differences in brain structure and function, the specific cognitive tasks used, or the tACS stimulation parameters. Future studies should focus on optimizing tACS parameters and investigate the underlying neurobiological mechanisms to improve the precision and efficacy of this stimulation technique. Further research is needed to establish the clinical relevance of tACS for cognitive enhancement or remediation in specific populations.

This research investigated the impact of online and offline tACS on cognitive functions across multiple frequency bands and targeted brain areas. The results demonstrated variable effects on cognitive performance, with some frequencies and brain regions showing significant effects on certain cognitive domains. However, significant variability and inconsistent results highlight the need for further investigation to optimize tACS protocols and understand the underlying mechanisms of action. Future research should explore individual differences, optimize stimulation parameters, and explore potential interactions with other therapeutic modalities.

The study's limitations include the variability in sample sizes across different studies and experiments. The exact nature of the cognitive tests used is not fully detailed, making direct comparison between studies challenging. Incomplete data on side effects in some studies limits the comprehensive assessment of tACS safety. The lack of a detailed literature review within the given data prevents a full understanding of the research context. The variability in results and methodology across the cited studies hinder the ability to draw broad conclusions about the effectiveness of tACS for cognitive enhancement.