Revisiting alpha-theta cross-frequency dynamics during working memory

Prior Electroencephalography (EEG) research has shown that during working memory delay, alpha (8-14 Hz) and theta (4-8 Hz) oscillations tend to form a 2:1 frequency ratio. According to the Binary Hierarchy Brain Body Oscillation Theory (BHBBOT), a recent model grounded in mathematical analysis, such harmonic (2:1) alpha-theta frequency configurations reflect enhanced connectivity between brain regions generating these rhythms. However, this prediction has not yet been empirically tested. In this study, we leveraged Information Theory and the Theory of Weakly Coupled Oscillators (TWCO) to examine whether the previously observed frequency modulations in alpha and theta rhythms during working memory are accompanied by changes in inter-areal connectivity. Contrary to the BHBBOT predictions, both Information Theory metrics and TWCO parameters showed that connectivity between frontal theta and parietal alpha rhythms was significantly reduced during the working-memory delay period (while the proportion of 2:1 ratios increased). In addition, phase locking value, a standard measure of synchrony, was also significantly reduced during working memory delay and was negatively associated with behavioural performance. In conclusion, our results show that the increased occurrence of 2:1 alpha:theta cross-frequency ratios during working memory reflects functional segregation (rather than integration) between frontal and parietal regions.