PROJECT SUMMARY Human infants are understood to begin life with a complex of brain mechanisms and sensitivities to environmental and social factors that, together, appear to contribute to our species' unique ability to learn language. However, we are only beginning to understand the nature and development of these brain mechanisms and sensitivities, especially as they contribute to the central question posed here: How does the infant discover the finite set of phonetic units in their native language from the infinite combinations of sensory stimuli around them? One hypothesis proposes that infants are born with sensitivities to specific rhythmic-temporal patterning at the nucleus of human language phonology in both spoken and signed language, which permits segmentation and categorization of the continuously varying linguistic stream. We suggest that the superior temporal gyrus (STG) is a key neural site that governs this capacity and is the brain mechanism that enables infants' sensitivity to the rhythmic-temporal patterning from which it will build all the words and sentences of its native languages. While it has been suggested that babies are born with sensitivity to rhythmic-temporal patterns in maximal contrasts at around 1 to 1.5 Hz, the precise frequencies to which babies are biologically attracted to remain unknown. In addition, we do not know if this sensitivity is linked to only the pure timing of the signal (the temporal-general property), or also requires the alternation of maximally contrastive units present in both signed and speech phonology (the phonology-specific property). We use integrated functional Near Infrared Spectroscopy (fNIRS) and Tobii eye tracking to examine deaf and hearing infants' response to sign phonetic-syllabic units and moving point-light scenes presented at different frequencies (.5, 1.5, 3 Hz) at a key developmental age, 5-6 months to adjudicate whether infants are sensitive to the temporal-general property or also to the phonology-specific property within rhythmic-temporal patterning. [[The proposed study addresses if humans are born with neural tissue dedicated to acquisition of phonology, and clarifies on a modality-free level how this tissue may interact with auditory or visual functions.]] Discovering the properties of rhythmic-temporal patterning to which babies are biologically attracted will advance our knowledge about how babies discover the core parts of their languages. The use of signed language stimuli allows us to determine whether infants are sensitive to general temporal patterns or to specific linguistic phonetic contrasts, [[advancing our knowledge of]] language acquisition universals. We will gain insight into how experience-dependent brain changes provide infants with the neural circuitry necessary for learning [[the phonology of their]] language(s), allowing us to support clinicians in identifying infants at risk for phonology-based language and reading disorders. Finally, this first-ever developmental neurobiological investigation of signed language perception will permit new understanding of the importance of early visual language experience for learning and reading outcomes in deaf children, and indeed, all children.