Non-native speech sound learning as a window to individual differences in the neural encoding of speech

Abstract

Non-native speech sound learning in adulthood is a skill marked by enormous individual differences, especially when compared to acquisition of speech sounds in childhood. This has led to the proposal that adult speech sound acquisition might be achieved through different mechanisms (or relying on fundamentally altered neural circuitry) compared to sound learning in infancy and childhood. One possibility we explore in a recent line of work is that individual differences in non-native speech learning are attributable to differences in the structure of brain areas responsible for processing speech, especially the transverse temporal gyrus. Of interest, the morphology of these same regions also predicts individual differences in native language speech perception, suggesting that sound acquisition (whether in childhood or adulthood) is limited by (likely innate) differences in neural architecture supporting sound processing. However, evidence that the best non-native perceivers are also the most finely-tuned native-sound perceivers is limited. Taken together, these results suggest that differences in brain structure are a soft constraint on the precision of acoustic-phonetic encoding, but that native language speech category structure is not the explanation for the difficulty of non-native sound learning.