Non-Native Speech Sound Learning: Studies of Sleep, Brain, and Behavior
Speech perception is subject to critical/sensitive period effects, such that acquisition of non-native (L2) speech sounds is far more difficult in adulthood than in childhood. Although adults can be trained to perceive differences among speech sounds that are not part of their native language, success is (1) variable across individuals, (2) variable across specific sounds to be learned, and (3) training may or may not generalize to untrained instances. Any theory of L2 speech perception must explain these three phenomena. Accounts of the L2 speech learning process have drawn from traditions in linguistics, psychology, and neuroscience, yet a full description of the barriers to perceptual learning of L2 sounds remains elusive. New evidence from our lab suggests that training on non-native speech produces plastic effects in the brain regions involved in native-language perception, and that consolidation during sleep plays a large role in the degree to which training is maintained and generalizes to new talkers. Further, similar mechanisms may be at play when listeners learn to perceive non-standard tokens in the context of accented speech. Taken together, these findings suggest that speech perception is more plastic than critical period accounts would predict and that individual variability in brain structure and sleep behavior may predict some of the variability in ultimate L2 sound acquisition success.