Patterns of developmental change in phonetic perception are critical to theory development. Many previous studies document a decline in nonnative phonetic perception between 6 and 12 months of age. However, much less experimental attention has been paid to developmental change in native-language phonetic perception over the same time period. We hypothesized that language experience in the first year facilitates native-language phonetic performance between 6 and 12 months of age. We tested 6-8- and 10-12-month-old infants in the United States and Japan to examine native and nonnative patterns of developmental change using the American English /r-l/ contrast. The goals of the experiment were to: (a) determine whether facilitation characterizes native-language phonetic change between 6 and 12 months of age, (b) examine the decline previously observed for nonnative contrasts and (c) test directional asymmetries for consonants. The results show a significant increase in performance for the native-language contrast in the first year, a decline in nonnative perception over the same time period, and indicate directional asymmetries that are constant across age and culture. We argue that neural commitment to native-language phonetic properties explains the pattern of developmental change in the first year.

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Recent experiments have demonstrated that the category goodness of speech sounds strongly influences perception in both adult and infants [Kuhl, Percept. Psychophys. 50, 93-107 (1991); Kuhl et al., Science 255, 606-608 (1992)]. Stimuli judged as exceptionally good instances of phonetic categories (prototypes) make neighboring tokens in the vowel space seem more similar, exhibiting a perceptual magnet effect. Three experiments further examined the perceptual magnet effect in adults. Experiment 1 collected goodness and identification judgments for 13 variants of the vowel /i/. Experiment 2 used signal detection theory to assess the discrimination of these tokens using a bias-free measure (d'). Experiment 3 employed multidimensional scaling (MDS) to geometrically model the distortion of the perceptual space due to the magnet effect. The results demonstrated a strong relationship between category goodness and discrimination. Vowel tokens receiving high goodness ratings in experiment 1 were more difficult to discriminate in experiment 2 and were more tightly clustered in the MDS solutions of experiment 3. These findings support the existence of a perceptual magnet effect, and may help explain some aspects of first language learning in infants and second language learning in adults.

Recent work [Iverson et al. (2003) Cognition, 87, B47-57] has suggested that Japanese adults have difficulty learning English /r/ and /l/ because they are overly sensitive to acoustic cues that are not reliable for /r/-/l/ categorization (e.g., F2 frequency). This study investigated whether cue weightings are altered by auditory training, and compared the effectiveness of different training techniques. Separate groups of subjects received High Variability Phonetic Training (natural words from multiple talkers), and 3 techniques in which the natural recordings were altered via signal processing (All Enhancement, with F3 contrast maximized and closure duration lengthened; Perceptual Fading, with F3 enhancement reduced during training; and Secondary Cue Variability, with variation in F2 and durations increased during training). The results demonstrated that all of the training techniques improved /r/-/l/ identification by Japanese listeners, but there were no differences between the techniques. Training also altered the use of secondary acoustic cues; listeners became biased to identify stimuli as English /l/ when the cues made them similar to the Japanese /r/ category, and reduced their use of secondary acoustic cues for stimuli that were dissimilar to Japanese /r/. The results suggest that both category assimilation and perceptual interference affect English /r/ and /l/ acquisition.

This study investigated changes in vowel production and perception among university students from the north of England, as individuals adapt their accent from regional to educated norms. Subjects were tested in their production and perception at regular intervals over a period of 2 years: before beginning university, 3 months later, and at the end of their first and second years at university. At each testing session, subjects were recorded reading a set of experimental words and a short passage. Subjects also completed two perceptual tasks; they chose best exemplar locations for vowels embedded in either northern or southern English accented carrier sentences and identified words in noise spoken with either a northern or southern English accent. The results demonstrated that subjects at a late stage in their language development, early adulthood, changed their spoken accent after attending university. There were no reliable changes in perception over time, but there was evidence for a between-subjects link between production and perception; subjects chose similar vowels to the ones they produced, and subjects who had a more southern English accent were better at identifying southern English speech in noise.

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.

Three experiments examined the dynamic attributes of timbre by evaluating the role of onsets in similarity judgments. In separate experiments, subjects heard complete orchestral instrument tones, the onsets of those tones, and tones with the onsets removed ("remainders"). Ratings for complete tones corresponded to those for onsets, indicating that the salient acoustic attributes for complete tones are present at the onset. Ratings for complete tones also corresponded to those for remainders, indicating that the salient attributes for complete tones are present also in the absence of onsets. Subsequent acoustic analyses demonstrated that this pattern of similarity was due to the centroid frequencies and amplitude envelopes of the tones. The results indicate that the dynamic attributes of timbre are not only present at the onset, but also throughout, and that multiple acoustic attributes may contribute to the same perceptual dimensions.

Two experiments investigated whether listeners change their vowel categorization decisions to adjust to different accents of British English. Listeners from different regions of England gave goodness ratings on synthesized vowels embedded in natural carrier sentences that were spoken with either a northern or southern English accent. A computer minimization algorithm adjusted F1, F2, F3, and duration on successive trials according to listeners' goodness ratings, until the best exemplar of each vowel was found. The results demonstrated that most listeners adjusted their vowel categorization decisions based on the accent of the carrier sentence. The patterns of perceptual normalization were affected by individual differences in language background (e.g., whether the individuals grew up in the north or south of England), and were linked to the changes in production that speakers typically make due to sociolinguistic factors when living in multidialectal environments.