Lexical learning shapes the development of speech perception until late adolescence

The role of neurobiologically-constrained critical periods for language learning remains controversial. We provide new evidence for critical periods by examining speech sound processing across the lifespan. We tested perceptual acuity for minimal word-word (e.g. bear-pear), and word-pseudoword (e.g. bag-pag) pairs using trial-unique audio-morphed speech tokens. Participants (N=1537) performed a 3-interval, 2-alternative forced-choice perceptual task indicating which of two cartoon characters said a referent word correctly. We adaptively reduced the contrastive acoustic cues in speech tokens to measure the Proportion of Acoustic Difference Required for Identification (PADRI) at 79.4% correct. Results showed effects of age, lexical context, and language experience on perceptual acuity. However, for native-listeners responding to word-word trials, age-related improvements stopped at 16.7 years. This finding suggests a role for continued lexical experience in shaping perceptual acuity for spoken words until late adolescence consistent with interactive models of speech perception and critical periods.