The present study used magnetoencephalography (MEG) to examine perceptual learning of American English /r/ and /l/ categories by Japanese adults who had limited English exposure. A training software program was developed based on the principles of infant phonetic learning, featuring systematic acoustic exaggeration, multi-talker variability, visible articulation, and adaptive listening. The program was designed to help Japanese listeners utilize an acoustic dimension relevant for phonemic categorization of /r-l/ in English. Although training did not produce native-like phonetic boundary along the /r-l/ synthetic continuum in the second language learners, success was seen in highly significant identification improvement over twelve training sessions and transfer of learning to novel stimuli. Consistent with behavioral results, pre-post MEG measures showed not only enhanced neural sensitivity to the /r-l/ distinction in the left-hemisphere mismatch field (MMF) response but also bilateral decreases in equivalent current dipole (ECD) cluster and duration measures for stimulus coding in the inferior parietal region. The learning-induced increases in neural sensitivity and efficiency were also found in distributed source analysis using Minimum Current Estimates (MCE). Furthermore, the pre-post changes exhibited significant brain-behavior correlations between speech discrimination scores and MMF amplitudes as well as between the behavioral scores and ECD measures of neural efficiency. Together, the data provide corroborating evidence that substantial neural plasticity for second-language learning in adulthood can be induced with adaptive and enriched linguistic exposure. Like the MMF, the ECD cluster and duration measures are sensitive neural markers of phonetic learning.
Bibliographical noteFunding Information:
Funding was provided by NTT Communication Science Laboratories (Nippon Telegraph and Telephone Corporation), the University of Washington's NSF Science of Learning Center (LIFE), and the National Institute of Health, and the University of Washington's Institute for Learning and Brain Sciences. Manuscript preparation was supported in part by a visiting scholarship from Tokyo Denki University, a University of Minnesota Faculty Summer Research Fellowship, and the Grant-in-Aid of Research, Artistry and Scholarship Program administered by the Office of the Dean of the Graduate School. The authors would like to thank Dan Gammon and David Akbari for their assistance in data analysis.
- Language acquisition
- Neural efficiency
- Neural sensitivity