Patrick kelly^1,2, Jefferson Ortega¹, Keanan Joyner¹, Silvia Bunge¹, David Whitney¹; ¹UC Berkeley, ²UCLA

Naturalistic emotional videos elicit shared patterns of brain activity, but it is unclear what this synchrony reflects: merely shared attention to the same sensory input, or shared internal computations about a character’s evolving emotional state. Here we test whether inter-subject synchronization in fronto–temporo-parietal networks indexes shared emotional meaning—whether people whose brains are more aligned with the group are also more accurate at tracking a character’s moment-to-moment emotional state. We used functional near-infrared spectroscopy (fNIRS) during a dynamic emotion perception task, Inferential Emotion Tracking (Chen & Whitney, 2019). Participants (N = 33) continuously rated the valence and arousal of a target character in 28 clips while fNIRS measured oxygenated hemoglobin in bilateral prefrontal (DLPFC, VLPFC, OFC, RLPFC) and temporo-parietal regions (STS, TPJ). We computed region-of-interest (ROI) inter-subject correlations (ISCs) by correlating each participant’s activation pattern with the leave-one-out group average. All twelve ROIs showed significant positive ISC (mean r ≈ .16–.26, p < 0.001). Using inter-subject representational similarity analysis (IS-RSA), we then asked whether ISC strength predicted emotion-tracking performance. Overall ISC robustly predicted Inferential Emotion Tracking accuracy: high performers’ brain activity converged on similar patterns, whereas low performers’ activity was more idiosyncratic (Spearman r = .47, 95% CI [.20, .60], permutation p < 0.001). Finally, penalized regression identified which regions carried these behaviorally relevant shared representations. ISC in bilateral VLPFC, left TPJ, left STS, and left OFC showed the largest weights, with additional contributions from right OFC and right TPJ. Effects were strongly lateralized: higher ISC in right frontal and left temporoparietal cortex was associated with better performance, whereas higher ISC in left frontal and right temporoparietal cortex was associated with worse performance. fNIRS ISC thus provides a sensitive, lateralized marker of shared emotional meaning during naturalistic emotion perception.