Author: Feng, Yan
Title: Categorical speech perception across the lifespan
Advisors: Peng, Gang (CBS)
Degree: Ph.D.
Year: 2022
Subject: Speech perception
Speech perception in children
Speech perception in older people
Hong Kong Polytechnic University -- Dissertations
Department: Department of Chinese and Bilingual Studies
Pages: xiv, 165 pages : color illustrations
Language: English
Abstract: Speech perception is an integral part of the speech chain. Since the speech information is delivered by continuous acoustic signals, how listeners perceive these continuous signals as discrete phonemes, so-called categorical speech perception, attracts researchers' attention. With the brain growth and shrinkage across the lifespan, how the categorical speech perception developing and degenerating remains unclear. What neural mechanisms older adults adapt to actively compensate for the brain shrinkage during categorical speech perception are also unknown. Especially, there is an urgent need to figure out these questions in Mandarin-speaking older adults to cope with the challenges of the increased aging population in China. This dissertation aims to shed light on these questions and bridge the research gap. To investigate the maturation of categorical speech perception during childhood, we conduct a cross-sectional study in four/six/10-year-old children, 14-year-old adolescents, and young adults. The maturation processes of different Mandarin phonemes are asynchronous. The identification and discrimination abilities also show different developmental trajectories. Children aged four years already could identify Mandarin vowels (/u/-/i/) as well as the adults could. Children aged six years have achieved an adult-like ability to discriminate vowels and to identify and discriminate tones (Tone 1-Tone 2). Children aged 10 years could identify aspiration of stops (/p/-/ph/) maturely. Adolescents 14 years of age could discriminate aspiration of stops as well as adults could, and they could also maturely identify and discriminate the transition of stops (/p/-/t/). The observation that tone and vowel perception are easier to achieve supports the phonological saliency hypothesis. To explore the degeneration of categorical speech perception during late adulthood, we directly compare older adults below 65 years old and those above 75 years old with young adults. Older adults below 65 years of age could perceive all Mandarin phonemes categorically as well as the young adults could, revealing that they still maintain the normal categorical speech perception. The perception of vowels also remains intact in the older adults above 75 years old. However, their perception of consonant transition has degraded, as have their abilities to discriminate tones and aspiration of consonants. The perception of tones and consonants degenerate earlier than vowel perception does. Furthermore, hearing loss may partially explain their decreased perceptual abilities.
In addition to hearing loss, we also examine the effect of cognitive decline on categorical speech perception. We find that older adults with normal cognitive ability could maintain a normal ability to perceive Mandarin tones, vowels, and aspiration of consonants categorically. They could also identify the transition of consonants as well as young adults could, whereas their transition discrimination has degenerated. Those with mild cognitive impairment (MCI) maintain intact identification of tones, vowels, and aspiration of consonants, but their ability to identify the transition of consonants has degraded. They also show a decreased ability to discriminate all phonemes. Thus, cognitive impairment poses a threat to phoneme discrimination in older adults. Moreover, the perception of consonant transition degrades earlier than other phonemes do in the older adults. When we look at categorical speech perception across the human lifespan, our findings indicate that categorical speech perception follows an inverted-U trajectory. Categorical speech perception develops during childhood, enters a stable stage during early adulthood, and then degenerates during the sunset years of life. In addition, the perceptual development and later degeneration of different Mandarin phonemes are asynchronous. During childhood, the categorical perception of Mandarin vowels is acquired earlier than that of consonant transition. However, during the sunset of life, the categorical perception of consonant transition degrades earlier than that of vowels does. Thus, in support of the order of language acquisition and loss proposed by Jakobson (1968), we report that the phonemes acquired later may be lost earlier during one's lifespan. Nonetheless, aging is not 'development in reverse'. To further uncover the neural mechanisms underlying the degeneration of categorical speech perception, we investigate event-related potential components in older adults with normal cognitive ability and those with MCI. With young adults as a control group, we observe increased cortical excitability in older adults during early phonetic and phonological processing. They show larger P2 amplitude across different phonemes. In addition, they show a posterior-anterior shift of scalp distribution of P2 waves. In support of the PASA model, we believe that the age-related neural reorganisation could be regarded as an active adaptive compensation in older adults. Furthermore, healthy older adults also present some unique characteristics that are absent in those with MCI. During early auditory processing, the healthy older adults show a more negative N1 amplitude when perceiving the aspiration of consonants, and during relatively late phonological processing of transition of consonants, they exhibit a larger P300 amplitude as a compensatory mechanism that appears to help them achieve normal between-category discrimination. More work is needed to specify how and to what extent internal adaptive neural changes and external training regimens could promote successful aging.
Rights: All rights reserved
Access: open access

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