Project Summary Age of acquisition effects on sign language development and brain processing Age of first-language age of acquisition, L1 AoA, has lifelong effects on language abilities and brain language processing. Infants born deaf often first experience language at a variety of ages due to an inability to perceive spoken language and not being exposed to sign language until older ages. We use this naturally occurring variation in L1 AoA to model the critical period for language. In a series of studies, ask what the language and neural correlates of the critical period might be. We test the hypothesis the organization of the brain language system is an emergent property arising from interactive effects between language experience and acquisition and post-natal brain growth. We test the hypothesis with two developmental paradigms, retrospective and longitudinal. The retrospective studies test deaf adults whose ASL acquisition is complete. These studies investigate whether L1 AoA effects between the ages of 1-8 are linear with respect to the level of syntactic complexity acquired, the locus and distribution of neurolinguistic processing, and characteristics of the dorsal and ventral white matter tracts. The longitudinal studies test deaf adolescents whose ASL acquisition may be ongoing and whose L1 AoA began in infancy, or in early or late childhood. The adolescents are tested annually for three consecutive years with same experiments as used in the adult study. The language experiments are designed to test the comprehension and production of ASL sentence structure as a function of syntactic complexity. The experimental paradigms are sentence-to-picture matching, elicited imitation, syntactic priming, and spontaneous production. The neurolinguistic experiments use fMRI to investigate sentence and lexical processing. The brain structure studies use DTI to investigate dorsal and ventral white matter tract development. If the brain language system requires linguistic experience during early post-natal brain growth to become fully functional, then levels of syntactic development should correspond with the distribution and location of neurolinguistic processing in the brain as well as with the robustness of white matter tract development in the brain language network.