One way to understand changes in plasticity over the course of development (e.g. sensitive period phenomena) is as a result of the process of learning itself. This is readily observed even in simple neural network models. Learning to categorize speech sounds in one's native language can be characterized as a process of tuning the perceptual system to the dimensions along which change is meaningful in that language. One consequence of this process is a diminished sensitivity to foreign language (L2) contrasts that differ along dimensions not contrastive in one's native phonetic inventory, which ultimately can result in failures to learn even after many years of exposure. This specialization is also reflected in the responses of a network of left-lateralized temporal and frontal cortical regions which respond differentially to native speech contrasts relative to unfamiliar L2 sounds and control stimuli not perceived as crossing a categorical boundary. I will present a series of studies combining behavioral, electroencephalographic and fMRI techniques to examine the traces of early experience on the adult speech perception system.
Learning and limits on adult plasticity for speech