Catherine T. Best

Examination of Perceptual Reorganization for Nonnative Speech Contrasts: Zulu Click Discrimination by English­ Speaking Adults and Infants

The language environment modifies the speech perception abilities found in early development. In particular, adults have difficulty perceiving many nonnative contrasts that young infants discriminate. The underlying perceptual reorganization apparently occurs by 10-12 months. According to one view, it depends on experiential effects on psychoacoustic mechanisms. Alternatively, phonological development has been held responsible, with perception influenced by whether the nonnative sounds occur allophonically in the native language. We hypothesized that a phonemic process appears around 10-12 months that assimilates speech sounds to native categories whenever possible; otherwise, they are perceived in auditory or phonetic (articulatory) terms. We tested this with English-speaking listeners by using Zulu click contrasts. Adults discriminated the click contrasts; performance on the most difficult (80% correct) was not diminished even when the most obvious acoustic difference was eliminated. Infants showed good discrimination of the acoustically modified contrast even by 12-14 months. Together with earlier reports of developmental change in perception of nonnative contrasts, these findings support a phonological explanation of language-specific reorganization in speech perception.

Perceptual equivalence of acoustic cues in speech and nonspeech perception

Trading relations show that diverse acoustic consequences of minimal contrasts in speech are equivalent in perception of phonetic categories. This perceptual equivalence received stronger support from a recent finding that discrimination was differentially affected by the phonetic cooperation or conflict between two cues for the /slIt/-/splIt/contrast. Experiment 1 extended the trading relations and perceptual equivalence findings to the /sei/-/stei/contrast. With a more sensitive discrimination test, Experiment 2 found that cue equivalence is a characteristic of perceptual sensitivity to phonetic information. Using “sine-wave analogues” of the /sei/-/stei/stimuli, Experiment 3 showed that perceptual integration of the cues was phonetic, not psychoacoustic, in origin. Only subjects who perceived the sine-wave stimuli as “say” and “stay” showed a trading relation and perceptual equivalence; subjects who perceived them as nonspeech failed to integrate the two dimensions perceptually. Moreover, the pattern of differences between obtained and predicted discrimination was quite similar across the first two experiments and the “say”-“stay” group of Experiment 3, and suggested that phonetic perception was responsible even for better-than-predicted performance by these groups. Trading relations between speech cues, and the perceptual equivalence that underlies them, thus appear to derive specifically from perception of phonetic information.

Identification and discrimination of Mandarin Chinese tones by Mandarin Chinese vs. French listeners

Previous work has not yielded clear conclusions about the categorical nature of perception of tone contrasts by native listeners of tone languages. We reopen this issue in a cross-linguistic study comparing Taiwan Mandarin and French listeners. We tested these listeners on three tone continua derived from natural Mandarin utterances within carrier sentences, created via a state-of-the-art pitch-scaling technique in which within-continuum interpolation was applied to both f0 and intensity contours. Classic assessments of categorization and discrimination of each tone continuum were conducted with both groups of listeners. In Experiment 1, Taiwanese listeners identified the tone of target syllables within carrier sentence context and discriminated tones of single syllables. In Experiment 2, both French and Taiwanese listeners completed an AXB identification task on single syllables. Finally, French listeners were run on an AXB discrimination task in Experiment 3. Results indicated that Taiwanese listeners’ perception of tones is quasi-categorical whereas French listeners’ is psychophysically based. French listeners nevertheless show substantial sensitivity to tone contour differences, though to a lesser extent than Taiwanese listeners. Thus, the findings suggest that despite the lack of lexical tone contrasts in the French language, French listeners are not absolutely “deaf” to tonal variations. They simply fail to perceive tones along the lines of a well-defined and finite set of linguistic categories.

Divergent developmental patterns for infants' perception of two nonnative consonant contrasts

Abstract Young infants discriminate nonnative and native consonant contrast, yet 10–12-month-olds discriminate most nonnative contrasts poorly, like adults. However, English-speaking adults and 6–14 month-old infants discriminate Zulu clicks, consistent with a model predicting that listeners who have a native phonology assimilate nonnative consonants to native categories when possible but hear non-assimilable (NA) consonants as nonspeech sounds (Best, McRoberts, & Sithole, 1988). Non-assimilable contrasts, thus, avoid language-specific effects and are discriminated, whereas consonants assimilated equally into a single category (SC) are discriminated poorly by listeners showing language-specific influences; other possible assimilation patterns show poor to excellent discrimination. This study directly compared discrimination of NA clicks and SC ejectives by 6–8- and 10–12-month-olds with a conditioned fixation habituation procedure. Consistent with predictions, the younger group discriminated both nonnative contrats and a control English contrast, whereas the older group discriminated only the NA and English contrasts.

Cross-language Perception of Non-native Tonal Contrasts: Effects of Native Phonological and Phonetic Influences

This study examined the perception of the four Mandarin lexical tones by Mandarin-naïve Hong Kong Cantonese, Japanese, and Canadian English listener groups. Their performance on an identification task, following a brief familiarization task, was analyzed in terms of tonal sensitivities (A-prime scores on correct identifications) and tonal errors (confusions). The A-prime results revealed that the English listeners’ sensitivity to Tone 4 identifications specifically was significantly lower than that of the other two groups. The analysis of tonal errors revealed that all listener groups showed perceptual confusion of tone pairs with similar phonetic features (T1—T2, T1—T4 and T2—T3 pairs), but not of those with completely dissimilar features (T1—T3, T2—T4, and T3—T4). Language-specific errors were also observed in their performance, which may be explained within the framework of the Perceptual Assimilation Model (PAM: Best, 1995; Best & Tyler, 2007). The findings imply that linguistic experience with native tones does not necessarily facilitate non-native tone perception. Rather, the phonemic status and the phonetic features (similarities or dissimilarities) between the tonal systems of the target language and the listeners’ native languages play critical roles in the perception of non-native tones.

Development of Phonological Constancy Toddlers' Perception of Native- and Jamaican-Accented Words

Efficient word recognition depends on detecting critical phonetic differences among similar-sounding words, or sensitivity to phonological distinctiveness, an ability evident at 19 months of age but unreliable at 14 to 15 months of age. However, little is known about phonological constancy, the equally crucial ability to recognize a words identity across natural phonetic variations, such as those in cross-dialect pronunciation differences. We show that 15- and 19-month-old children recognize familiar words spoken in their native dialect, but that only the older children recognize familiar words in a dissimilar nonnative dialect, providing evidence for emergence of phonological constancy by 19 months. These results are compatible with a perceptual-attunement account of developmental change in early word recognition, but not with statistical-learning or phonological accounts. Thus, the complementary skills of phonological constancy and distinctiveness both appear at around 19 months of age, together providing the child with a fundamental insight that permits rapid vocabulary growth and later reading acquisition.

Development of infant ear asymmetries for speech and music.

Groups of 2-, 3-, and 4-month olds were tested for dichotic ear differences in memory-based phonetic and music timbre discriminations. A right-ear advantage for speech and a left-ear advantage (LEA) for music were found in the 3- and 4-month-olds. However, the 2-month-olds showed only the music LEA, with no reliable evidence of memory-based speech discrimination by either hemisphere. Thus, the responses of all groups to speech contrasts were different from those to music contrasts, but the pattern of the response dichotomy in the youngest group deviated from that found in the older infants. It is suggested that the quality or use of lefthemisphere phonetic memory may change between 2 and 3 months, and that the engagement of right-hemisphere specialized memory for musical timbre may precede that for left-hemisphere phonetic memory. Several directions for future research are suggested to determine whether infant short-term memory asymmetries for speech and music are attributable to acoustic factors, to different modes or strategies in perception, or to structural and dynamic properties of natural sound sources.

Emergence of Language-Specific Constraints in Perception of Non-Native Speech: A Window on Early Phonological Development

Adults have difficulty discriminating many non-native speech contrasts, yet young infants discriminate both native and non-native contrasts. Language-specific constraints appear by 10–12 months. Evidence presented here suggests that mature listeners’ discrimination is constrained by perceived similarities between non-native sounds and native categories, and that this native language influence may not be fully developed at 10–12 months. The findings suggest that young infants have broadly-tuned perception of phonetic details. Next, they begin to discern equivalence classes that roughly correspond to native phonemes. Perception of phonological contrasts, however, depends on recognition of their linguistic function, and thus develops later. But what sort of information in speech forms the basis for perception of equivalence classes or phonemic contrasts? I argue that distal articulatory gestures, rather than proximal auditory-acoustic cues or abstract phonetic features, are the primitives both for adults’ perceptual assimilations of non-native phones and for infants’ emerging recognition of native categories.

Cognitive processing deficits in reading disabilities: a prefrontal cortical hypothesis.

Much research investigating the neuropsychological underpinnings of reading disabilities has emphasized posterior brain regions. However, recent evidence indicates that prefrontal cortex may also play a role. This study investigated cognitive processes that are associated with prefrontal and posterior brain functions. Subjects were 12-year-old reading disabled and nondisabled boys. Discriminant analysis procedures indicated that measures of prefrontal functions distinguished between the two groups better than measures of posterior functions. The results suggest that reading disabled boys have difficulty with cognitive processes involving selective and sustained attention, inhibition of routinized responses, set maintenance, flexibility in generating and testing alternative hypotheses, and phonemically based language production.

Young infants’ perception of liquid coarticulatory influences on following stop consonants

Phonetic segments are coarticulated in speech. Accordingly, the articulatory and acoustic properties of the speech signal during the time frame traditionally identified with a given phoneme are highly context-sensitive. For example, due to carryover coarticulation, the front tongue-tip position for HI results in more fronted tongue-body contact for a /g/ preceded by /l/ than for a /g/ preceded by /r/. Perception by mature listeners shows a complementary sensitivity—when a synthetic /da/-/ga/ continuum is preceded by either /al/ or /ar/, adults hear more /g/s following HI rather than Irl. That is, some of the fronting information in the temporal domain of the stop is perceptually attributed to /l/ (Mann, 1980). We replicated this finding and extended it to a signaldetection test of discrimination with adults, using triads of disyllables. Three equidistant items from a /da/-/ga/ continuum were used preceded by /al/ and /ar/. In the identification test, adults had identified item ga5 as “ga”, and dal as “da”, following both /al/ and /ar/, whereas they identified the crucial item d/ga3 predominantly as “ga” after /al/ but as “da” after /ar/. In the discrimination test, they discriminated d/ga3 from dal preceded by /al/ but not /ar/; compatibly, they discriminated d/ga3 readily from ga5 preceded by /ar/ but poorly preceded by /al/. We obtained similar results with 4-month-old infants. Following habituation to either ald/ga3 or ard/ga3, infants heard either the corresponding ga5 or dal disyllable. As predicted, the infants discrimi-nated d/ga3 from dal following /al/ but not /ar/; conversely, they discriminated d/ga3 from ga5 following /ar/ but not /al/. The results suggest that prelinguistic infants disentangle consonant-consonant coarticulatory influences in speech in an adult-like fashion.