Individual alpha frequency tACS reduces static functional connectivity across the default mode network

Martín Carrasco-Gómez^1,2,3*Alejandra García-Colomo^2,4*Jesús Cabrera-Álvarez^2,4Alberto del Cerro-León^2,4Carlos J Gomez-Ariza⁵Andres Santos^1,3Fernando Maestú^2,4,6

• ¹Electronic engineering department, ETSIT, Universidad Politécnica de Madrid, Spain, Madrid, Spain
• ²Center for Cognitive and Computational Neuroscience, Complutense University of Madrid, Madrid, Madrid, Spain
• ³CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Instituto de Salud Carlos III, Madrid, Spain, Madrid, Spain
• ⁴Department of Experimental Psychology, Cognitive Processes and Speech Therapy, Faculty of Psychology, Complutense University of Madrid, Madrid, Madrid, Spain
• ⁵Department of Psychology, University of Jaén, Jaén, Andalusia, Spain
• ⁶Health Research Institute of the Hospital Clínico San Carlos (IdISSC), Madrid, Madrid, Spain

IntroductionResearch on the influence of transcranial alternating current stimulation over alpha functional connectivity (FC) is scarce, even when it poses as a potential treatment for various diseases. This study aimed to investigate the effects of individual alpha frequency tACS (IAF-tACS) on FC within the default mode network (DMN) in healthy individuals, particularly following the triple network model.Materials and Methods27 healthy participants were recruited, who underwent a 20-minute IAF-tACS session over parieto-occipital areas and three magnetoencephalography (MEG) recordings: two pre-stimulation and one post-stimulation. Participants were randomly assigned to either the stimulation or sham group. Both dynamic FC (dFC) and static FC (sFC) were evaluated through the leakage corrected amplitude envelope correlation (AEC-c). Statistical analyses compared both Pre-Post FC ratio between groups through ratio t-tests and intragroup FC changes through repeated measures t-tests, with FDR correction applied to account for multiple comparisons. An additional analysis simulated the influence of the cortical folding on the effect of tACS over FC.ResultsIAF-tACS significantly decreased sFC in intra- and inter-DMN links in the stimulation group compared to the sham group, with a special influence over antero-posterior links between hubs of the DMN. Negative correlations were found between AEC-c sFC changes and power alterations in posterior DMN areas, suggesting a complex interaction between cortical folding and electric field direction. On the other hand, dFC increased in both sham and stimulation groups, and no between-group differences were found.ConclusionsAgainst our initial hypothesis, IAF-tACS reduced sFC in the DMN, possibly due to phase disparities introduced by cortical gyrification. These findings suggest that tACS might modulate FC in a more complex manner than previously thought, highlighting the need for further research into the personalized application of neuromodulation techniques, as well as its potential therapeutic implications for conditions like Alzheimer’s disease.