The robustness of eye-mouth index as an eye-tracking metric of social attention in toddlers.

Eye-tracking research on social attention in infants and toddlers has included heterogeneous stimuli and analysis techniques. This allows measurement of looking to inner facial features under diverse conditions but restricts across-study comparisons. Eye-mouth index (EMI) is a measure of relative preference for looking to the eyes or mouth, independent of time spent attending to the face. The current study assessed whether EMI was more robust to differences in stimulus type than percent dwell time (PDT) toward the eyes, mouth, and face. Participants were typically developing toddlers aged 18 to 30 months (N = 58). Stimuli were dynamic videos with single and multiple actors. It was hypothesized that stimulus type would affect PDT to the face, eyes, and mouth, but not EMI. Generalized estimating equations demonstrated that all measures including EMI were influenced by stimulus type. Nevertheless, planned contrasts suggested that EMI was more robust than PDT when comparing heterogeneous stimuli. EMI may allow for a more robust comparison of social attention to inner facial features across eye-tracking studies.