A sensitive period for shibboleths: the long tail and changing goals of speech perception over the course of development.

It is clear that the ability to learn new speech contrasts changes over development, such that learning to categorize speech sounds as native speakers of a language do is more difficult in adulthood than it is earlier in development. There is also a wealth of data concerning changes in the perception of speech sounds during infancy, such that infants quite rapidly progress from language-general to more language-specific perceptual biases. It is often suggested that the perceptual narrowing observed during infancy plays a causal role in the loss of plasticity observed in adulthood, but the relationship between these two phenomena is complicated. Here I consider the relationship between changes in sensitivity to speech sound categorization over the first 2 years of life, when they appear to reorganize quite rapidly, to the "long tail" of development throughout childhood, in the context of understanding the sensitive period for speech perception.