Kira N Noad¹, Daniel Kaiser^1,2,3,4; ¹Neural Computation Group, Department of Mathematics and Computer Science, Physics, Geography, Justus Liebig University Giessen, 35392 Giessen, Germany, ²Center for Applied Computer Science and Data Science (ZAD), Justus Liebig University Giessen, 35392 Giessen, Germany, ³Center for Mind, Brain and Behavior (CMBB), Philipps University Marburg, Justus Liebig University Giessen, and Technical University Darmstadt, 35032 Marburg, Germany, ⁴Cluster of Excellence “The Adaptive Mind”, Justus Liebig University Giessen, Philipps University Marburg, and Technical University Darmstadt, 35394 Giessen, Germany

Naturalistic stimuli, such as movies, evoke reliable neural responses across individuals in fMRI. These shared neural dynamics vary with experience, with individuals familiar with a movie sharing more similar fMRI response dynamics. However, it remains unclear how time-varying rhythmic brain signals synchronise between individuals during natural viewing, and whether these brain rhythms are modulated by familiarity. To address this, we used a natural viewing paradigm in which participants viewed clips from the TV show Game of Thrones (GoT) while brain responses were recorded using EEG. Time-varying power in each frequency band (alpha, beta, delta, gamma, theta) across the whole GoT movie was compared between participants using intersubject correlation analysis. There was significant similarity between participants in all EEG frequency bands, demonstrating that naturalistic viewing aligns brain rhythms across individuals. Next, we explored whether familiarity modulated brain rhythms by comparing the intersubject correlations in participants who were familiar with GoT to participants who were unfamiliar with GoT. We found that the familiar group exhibited greater intersubject synchronisation in time-varying beta rhythms, compared to the unfamiliar group. Finally, we explored whether the time course of these rhythmic responses related to the time course of BOLD fMRI responses during viewing of the same GoT movie. Here, we found correlations for familiar participants between beta power and BOLD responses in a network of brain regions previously found to be involved in coding familiarity. No comparable associations were found for unfamiliar participants. Together, these findings demonstrate that brain rhythms synchronise across individuals during natural viewing. They further suggest that familiarity enhances representations across a network of familiarity-coding regions through synchronized beta-frequency dynamics.