PROJECT SUMMARY ? Original Submission Primary language impairment (PLI) begins early in life and affects 6-8% of children. Although language intervention is maximally effective the earlier it is delivered, normative variation in language acquisition across toddlerhood (here 24-36 months) impedes accurate identification of PLI prior to late preschool age. The proposed study introduces a novel, theoretically- grounded, neurodevelopmental framework designed to generate a sensitive and specific model to identify PLI as early as possible. Our developmentally- sensitive, translational approach introduces multiple innovations including: (1) characterizing the developmental patterning of toddler emergent language beginning at 24 mos. using state-of-the-art methods, within a large community sample; (2) incorporating EEG/ERP neural biomarkers of language into PLI risk assessment; (3) using a novel paradigm to assess the protective effects of both behavioral and neural synchronization within parent-child language transactions; and (4) consideration of irritability, a robust developmental marker of early mental health risk, to enhance identification of those language delayed toddlers at highest risk for persistence. For the proposed When to Worry about Language Study (W2W-L), we capitalize on our funded study of 350 infants (50% irritable and 50% non-irritable) (R01MH107652, Wakschlag, PI) and enrich it via recruitment of a sub-sample of 200 late talking toddlers. This will yield a large and diverse sample of 550 24-month-olds. Our key predictors will be toddler emergent language patterns (24-36 months), their neural biomarkers and synchrony within the transactional language environment. Our central outcome is primary language impairment (PLI) status at preschool age (54 mos., when PLI can be reliably evaluated), assessed via clinical gold standard expressive and receptive language abilities. SPECIFIC AIMS: AIM 1a. Evaluate accuracy of PLI prediction based on multi-component measures of language including intensive longitudinal assessments of toddler developmental precursors of key language functions at older ages, neural biomarkers. We will assess neural and linguistic processing via quantitative EEG during parent-child interaction and ERPs to speech sounds as well as during eye tracking tasks and 1b. Evaluate feasibility of creating an algorithm for early identification of PLI that can be applied in clinical practice, using cross-validation and machine learning. AIM 2: Test the hypothesis that parent-child dyadic synchrony buffers PLI risk For the first time, we combine behavioral and novel social EEG measures of parent-child synchrony during natural interaction and a custom-designed word learning task to directly test how observed (behavioral) and neural (EEG) dyadic synchrony impact word learning. AIM 3: Test whether consideration of toddler irritability enhances PLI prediction. In sum, PLI confers sustained negative effects on a variety of personal-social and academic outcomes. Pinpointing children at highest risk for PLI is critical for reducing the public health burden of PLI for children, families, and the systems supporting them, and enhancing targeted allocation of resources.