Pupillometry: A Simple and Automatic Way to Explore Implicit Cognitive Processing

Pupil size is a non-invasive and highly sensitive technique used to measure changes in pupil diameter. It not only responds to light but also reflects inner cognitive processes (e.g., attention and emotion perception). Recently, it has been introduced to the traditional cognitive neuroscience field as a useful tool to objectively and sensitively capture the current cognitive state and its temporal dynamics. Importantly, this index is automatic and requires no explicit reports, thus it could be used to investigate the rarely explored realm of implicit cognitive processing. Here, we describe a comprehensive protocol that records pupil responses during the passive viewing of emotional biological motion (BM). Our results reliably reveal the multi-level implicit processing mechanism of BM emotion, as indicated by the fine-grained emotion processing in intact BM and the rapid but rather coarse emotion processing in local BM. Moreover, the emotion modulation effects observed in intact BM are indicative of individual autistic tendencies. We believe this protocol could be adapted to unveil the automatic processing of emotions and other attributes in social signals and further assist the early detection of social-cognitive disorders (e.g., autism). Key features • Pupil measurements could automatically and objectively reflect current cognitive states, which is useful for unfolding the implicit cognitive processes and related time courses. • It requires no explicit reports; thus, with modifications, this procedure could be utilized in a wide range of populations (e.g., infants, patients) and even animals. • Autistic traits strongly correlate with observed emotion-related pupil responses, revealing the potential application of pupil measurements in the detection of social cognitive deficits. • This protocol has been found to reveal the multi-level emotion processing of biological motion, which is further validated by a test-retest replication.