Naturalistic multimodal emotion data with deep learning can advance the theoretical understanding of emotion.

What are emotions? Despite being a century-old question, emotion scientists have yet to agree on what emotions exactly are. Emotions are diversely conceptualised as innate responses (evolutionary view), mental constructs (constructivist view), cognitive evaluations (appraisal view), or self-organising states (dynamical systems view). This enduring fragmentation likely stems from the limitations of traditional research methods, which often adopt narrow methodological approaches. Methods from artificial intelligence (AI), particularly those leveraging big data and deep learning, offer promising approaches for overcoming these limitations. By integrating data from multimodal markers of emotion, including subjective experiences, contextual factors, brain-bodily physiological signals and expressive behaviours, deep learning algorithms can uncover and map their complex relationships within multidimensional spaces. This multimodal emotion framework has the potential to provide novel, nuanced insights into long-standing questions, such as whether emotion categories are innate or learned and whether emotions exhibit coherence or degeneracy, thereby refining emotion theories. Significant challenges remain, particularly in obtaining comprehensive naturalistic multimodal emotion data, highlighting the need for advances in synchronous measurement of naturalistic multimodal emotion.