The Effects of Age of Acquisition and Proficiency on the Neural Correlates of Categorical Perception of Non-native Speech

Age of Acquisition (AoA) and second language (L2) proficiency have been shown to influence bilingual neural recruitment and neuroanatomy, but previous literature shows inconsistencies. The current studies used multiple regression analyses to understand the influence of AoA and L2 proficiency on neural processing for categorical perception in Spanish-English bilinguals during a speech identification task. Functional data showed that AoA and L2 proficiency differentially recruited areas previously associated with speech processing. Increased L2 proficiency was associated with increased activity in bilateral inferior frontal gyrus and middle frontal gyrus as well as right superior temporal gyrus, middle temporal gyrus and angular gyrus. AoA was associated with a separate region of MFG. The data suggest that increased proficiency is associated with higher-level strategies such as attentional mechanisms and semantic processing to aid in a perceptual task. Study 2 focused on the influence of AoA and L2 proficiency on neuroanatomy. Structure based morphometry and multiple regression analyses were used to determine the relationship of AoA, L2 proficiency and L2 use and brain structure in speech processing areas. Significant relationships were found in left MTG, left supramarginal gyrus and right angular gyrus. The results suggest that L2 proficiency and AoA are associated with structural measures in speech processing areas, those associated with higher-level processing. The studies combine to provide a better understanding of the variability of AoA and L2 proficiency in bilinguals and how it impacts speech processing through recruitment of different neural regions that may underlie different strategies to complete a speech perception task.