The differential effects of vowel and onset consonant lengthening on speech segmentation: Evidence from Taiwanese Southern Min.

A review of previous speech segmentation research suggests the prediction that listeners of Taiwanese Southern Min (TSM), a lexical tone language, would exploit vowel lengthening and syllable-onset consonant lengthening to locate word ends and beginnings, respectively. Yet, correlations between segment duration and tone identity in tone languages along with some TSM-specific phonological phenomena may work against such use. Two artificial language learning experiments examined TSM listeners' use of the lengthening cues. The listeners heard the words of an artificial language (e.g., /ba.nu.me/) repeated continuously and identified them in a subsequent two-alternative forced-choice test. Experiment I revealed that their segmentation benefits from and only from word-initial onset lengthening or word-final vowel lengthening, supporting the prediction. Experiment II further demonstrated that these two cues in combination synergistically support segmentation at least when compared to word-initial onset lengthening alone, consistent with previous findings regarding complementary cues. These results furnish additional evidence that vowel and onset consonant lengthening affect segmentation in different ways, possibly reflecting a functional division between vowels and consonants that is supported by some prosody-computing mechanism. Additionally, vowel lengthening seems to affect segmentation to a greater extent than onset consonant lengthening. Possible explanations for this and further issues are discussed.

Reduced neural distinctiveness of speech representations in the middle-aged brain.

Speech perception declines independent of hearing thresholds in middle-age, and the neurobiological reasons are unclear. In line with the age-related neural dedifferentiation hypothesis, we predicted that middle-aged adults show less distinct cortical representations of phonemes and acoustic-phonetic features relative to younger adults. In addition to an extensive audiological, auditory electrophysiological, and speech perceptual test battery, we measured electroencephalographic responses time-locked to phoneme instances (phoneme-related potential; PRP) in naturalistic, continuous speech and trained neural network classifiers to predict phonemes from these responses. Consistent with age-related neural dedifferentiation, phoneme predictions were less accurate, more uncertain, and involved a broader network for middle-aged adults compared with younger adults. Representational similarity analysis revealed that the featural relationship between phonemes was less robust in middle-age. Electrophysiological and behavioral measures revealed signatures of cochlear neural degeneration (CND) and speech perceptual deficits in middle-aged adults relative to younger adults. Consistent with prior work in animal models, signatures of CND were associated with greater cortical dedifferentiation, explaining nearly a third of the variance in PRP prediction accuracy together with measures of acoustic neural processing. Notably, even after controlling for CND signatures and acoustic processing abilities, age-group differences in PRP prediction accuracy remained. Overall, our results reveal "fuzzier" phonemic representations, suggesting that age-related cortical neural dedifferentiation can occur even in middle-age and may underlie speech perceptual challenges, despite a normal audiogram.

The effect of lengthening aspiration on speech segmentation.

Listeners tend to perceive longer vowels as word-final and include the aspiration of a syllable-onset consonant in metalinguistically judging the duration of the following vowel. It may be hypothesized that longer aspiration is interpreted as extra vowel duration and hence, a word-finality cue. Alternatively, if aspiration is perceived as part of the onset, longer aspiration should be interpreted as word-initial, consistent with previous findings on consonant lengthening [White, Mattys, Stefansdottir, and Jones (2015). J. Acoust. Soc. Am. 138(2), 1214-1220]. An artificial language learning experiment with Taiwanese Southern Min listeners showed that lengthening aspiration in word-initial but not word-final syllables improved speech segmentation, supporting the second but not the first hypothesis.

The Language-specific Use of Fundamental Frequency Rise in Segmentation of an Artificial Language: Evidence from Listeners of Taiwanese Southern Min.

Experience with native-language prosody encourages language-specific strategies for speech segmentation. Conflicting findings from previous research suggest that these strategies may not be abstracted away from the acoustic manifestation of prosodic features in the native speech. Using the artificial language learning paradigm, the current study explores this possibility in connection with listeners of a lexical tone language called Taiwanese Southern Min (TSM). In TSM, the only rising lexical tone occurs almost only on the final syllable of the language's tone sandhi domain and is phonetically associated with final lengthening. Based on these observations, Experiment I examined what constituted a sufficient finality cue for use by TSM listeners to support segmentation: (a) final fundamental frequency (F0) rise only; or (b) final F0 rise conjoined with final lengthening. The results showed that segmentation was inhibited by the former cue but facilitated by the latter. Experiment II showed that the facilitation cannot be attributed entirely to final lengthening, as a null effect was found when final lengthening was the sole prosodic cue to segmentation. It is thus assumed that acoustic details as fine-grained as the lengthening of the rising tone are involved in the modulation of the segmentation strategy whereby TSM listeners perceive F0 rise as signaling finality. The inhibitory effect of final F0 rise alone found in Experiment I motivated Experiment III, which revealed that initial F0 rise in the absence of lengthening cues improved TSM listeners' segmentation. It is speculated that such use of initial F0 rise might reflect a cross-linguistic segmentation solution.