Facial action unit fusion is a computational technique in computer vision and affective computing that combines spatial, temporal, or feature-level information from individual facial muscle movements to recognize overall facial expressions and subtle micro-expressions. Based on the Facial Action Coding System, which categorizes human facial behavior into discrete, localized muscular motions known as action units, this process integrates data from multiple distinct facial regions or detection channels into a unified representation. By modeling the interactions and co-occurrences among different muscle groups rather than evaluating the face solely as an undifferentiated whole, facial action unit fusion reduces background interference and enhances the sensitivity of automated systems to rapid, fine-grained emotional signals.