Age of acquisition and proficiency in a second language independently influence the perception of non-native speech.

Sensorimotor processing in children and higher-cognitive processing in adults could determine how non-native phonemes are acquired. This study investigates how age-of-acquisition (AOA) and proficiency-level (PL) predict native-like perception of statistically dissociated L2 categories, i.e., within-category and between-category. In a similarity task, participants rated the level of similarity between pairs of English syllables from 1 (similar) to 4 (dissimilar). Early L2 acquisition predicts accurate within-categorization and high proficiency in late L2 acquisition predicts improved between-categorization. Our results suggest that the manner in which bilinguals learn to categorize non-native sounds depends on the cognitive processes available at the age of L2 exposure.