Cross-frequency coupling of frontal theta and posterior alpha is unrelated to the fidelity of visual long-term memory encoding.

Given that the encoding of information into visual long-term memory relies on multiple spatially distinct areas in the human brain, encoding into visual memory is a cognitive process likely to rely on networks formed via large-scale coupled neuronal oscillations. Previous research suggests that decreases in occipital alpha power and increases in mid-frontal theta power individually contribute to the encoding of retrievable representations in visual long-term memory. The present study asks whether these oscillations form a coupled network that operates during long-term memory encoding. Here we show that neither amplitude-amplitude coupling nor phase-amplitude coupling between frontal theta and posterior alpha are correlated with observers' subsequent memory when storing pictures in visual long-term memory. Correlations between alpha and theta power were stronger during eyes-open, resting-state periods when no task was performed than immediately following the presentation of a to-be-remembered picture. Finally, we also found that the strength of theta-alpha coupling was not modified by temporal lobe anodal transcranial direct current stimulation, despite observers showing enhanced memory for pictures behaviorally. Collectively, these findings indicate that posterior alpha and frontal theta activity are not simply component parts of a larger scale coupled network underlying visual memory.