Learning new sounds of speech: reallocation of neural substrates.

Functional magnetic resonance imaging (fMRI) was used to investigate changes in brain activity related to phonetic learning. Ten monolingual English-speaking subjects were scanned while performing an identification task both before and after five sessions of training with a Hindi dental-retroflex nonnative contrast. Behaviorally, training resulted in an improvement in the ability to identify the nonnative contrast. Imaging results suggest that the successful learning of a nonnative phonetic contrast results in the recruitment of the same areas that are involved during the processing of native contrasts, including the left superior temporal gyrus, insula-frontal operculum, and inferior frontal gyrus. Additionally, results of correlational analyses between behavioral improvement and the blood-oxygenation-level-dependent (BOLD) signal obtained during the posttraining Hindi task suggest that the degree of success in learning is accompanied by more efficient neural processing in classical frontal speech regions, and by a reduction of deactivation relative to a noise baseline condition in left parietotemporal speech regions.