ABSTRACT The proposed project will lead to an understanding of how common caregiver interaction strategies facilitate moment-to-moment changes in reciprocal communication and how child characteristics are associated with this relationship. Acquiring spoken language by age 5 is a key predictor of lifelong communication outcomes and an estimated one-third of autistic children who have not met this benchmark, also commonly classified as minimally verbal, experience significant difficulties that impact daily living and quality of life long-term. Because reciprocal communication sets the stage for successful social interactions and language learning, promoting reciprocity is an important goal for both development and intervention in nonspeaking children. While many intervention approaches leverage parent-child synchrony and turn-taking to support communication, our understanding of how specific behaviors influence reciprocity is primarily limited to summary-level measures and often uses third-party subjective ratings. In contrast, the key innovation in this proposal is investigating interaction dynamics on two timescales – at the interaction level and at the moment-to-moment level. Analyzing the interaction at the moment-to-moment level allows for a more direct temporal link between commonly used caregiver strategies and changes in reciprocity than summary-level measures can provide. The proposed study will utilize a cohort observational design and a sequential analysis approach to address two specific aims: to evaluate the influence of caregiver strategies on moment-to-moment changes in reciprocal interaction in nonspeaking autistic children and to determine the impact of individual child characteristics on the relationship between caregiver behavior and reciprocal interchanges. These aims will test the working hypothesis that caregiver imitation and multimodal communication are more likely to elicit communication interchanges with their child than expected by chance, and that child receptive language is associated with the relationship between these strategies and occurrence of interchanges. Multimodal communication and imitation strategies are commonly employed by caregivers and recent work has shown that they lead to increased engagement in younger autistic children. This highly quantitative approach builds on prior work by addressing the influence of timing and coordination during interaction, which are known to be impacted in autism. By analyzing the play interactions of 4–6-year-old children, we will gain more insight into the nature of reciprocal communication dynamics at a critical juncture in children's communication development – during the shift from preverbal to minimally verbal status. This knowledge will move the field toward identifying specific mechanisms of social interaction in nonspeaking children and refine intervention strategies for this population. Future work can extend this approach to increase our understanding of how careg...