Brain aging and speech perception : effects of background noise and talker variability

Speech perception can be challenging, especially for older adults. Despite the importance of speech perception in social interactions, the mechanisms underlying these difficulties remain unclear and treatment options are scarce. While several studies have suggested that decline within cortical auditory regions may be a hallmark of these difficulties, a growing number of studies have reported decline in regions beyond the auditory processing network, including regions involved in speech processing and executive control, suggesting a potentially diffuse underlying neural disruption, though no consensus exists regarding underlying dysfunctions. To address this issue, we conducted two experiments in which we investigated age differences in speech perception when background noise and talker variability are manipulated, two factors known to be detrimental to speech perception. In Experiment 1, we examined the relationship between speech perception, hearing and auditory attention in 88 healthy participants aged 19 to 87 years. In Experiment 2, we examined cortical thickness and BOLD signal using magnetic resonance imaging (MRI) and related these measures to speech perception performance using a simple mediation approach in 32 participants from Experiment 1. Our results show that, even after accounting for hearing thresholds and two measures of auditory attention, speech perception significantly declined with age. Age-related decline in speech perception in noise was associated with thinner cortex in auditory and speech processing regions (including the superior temporal cortex, ventral premotor cortex and inferior frontal gyrus) as well as in regions involved in executive control (including the dorsal anterior insula, the anterior cingulate cortex and medial frontal cortex). Further, our results show that speech perception performance was associated with reduced brain response in the right superior temporal cortex in older compared to younger adults, and to an increase in response to noise in older adults in the left anterior temporal cortex. Talker variability was not associated with different activation patterns in older compared to younger adults. Together, these results support the notion of a diffuse rather than a focal dysfunction underlying speech perception in noise difficulties in older adults

Environment- and listener-oriented speaking style adaptations across the lifespan

textThis dissertation examines how age affects the ability to produce intelligibility- enhancing speaking style adaptations in response to environment-related difficulties (noise-adapted speech) and in response to listeners’ perceptual difficulties (clear speech). Materials consisted of conversational and clear speech sentences produced in quiet and in response to noise by children (11-13 years), young adults (18-29 years), and older adults (60-84 years). Acoustic measures of global, segmental, and voice characteristics were obtained. Young adult listeners participated in word-recognition-in-noise and perceived age tasks. The study also examined relative talker intelligibility as well as the relationship between the acoustic measurements and intelligibility results. Several age-related differences in speaking style adaptation strategies were found. Children increased mean F0 and F1 more than adults in response to noise, and exhibited greater changes to voice quality when producing clear speech (increased HNR, decreased shimmer). Older adults lengthened pause duration more in clear speech compared to younger talkers. Word recognition in noise results revealed no age-related differences in the intelligibility of conversational speech. Noise-adapted and clear speech modifications increased intelligibility for all talker groups. However, the acoustic changes implemented by children when producing noise-adapted and clear speech were less efficient in enhancing intelligibility compared to the young adult talkers. Children were also less intelligible than older adults for speech produced in quiet. Results confirmed that the talkers formed 3 perceptually-distinct age groups. Correlation analyses revealed that relative talker intelligibility was consistent for conversational and clear speech in quiet. However, relative talker intelligibility was found to be more variable with the inclusion of additional speaking style adaptations. 1-3 kHz energy, speaking rate, vowel and pause durations all emerged as significant acoustic-phonetic predictors of intelligibility. This is the first study to investigate how clear speech and noise-adapted speech benefits interact with each other across multiple talker groups. The findings enhance our understanding of intelligibility variation across the lifespan and have implications for a number of applied realms, from audiologic rehabilitation to speech synthesis.Linguistic

Linguistic processing of accented speech across the lifespan.

In most of the world, people have regular exposure to multiple accents. Therefore, learning to quickly process accented speech is a prerequisite to successful communication. In this paper, we examine work on the perception of accented speech across the lifespan, from early infancy to late adulthood. Unfamiliar accents initially impair linguistic processing by infants, children, younger adults, and older adults, but listeners of all ages come to adapt to accented speech. Emergent research also goes beyond these perceptual abilities, by assessing links with production and the relative contributions of linguistic knowledge and general cognitive skills. We conclude by underlining points of convergence across ages, and the gaps left to face in future work

The Time Course of Variability Effects in the Perception of Spoken Language: Changes Across the Lifespan

Although spoken language is communicated via a rapidly varying signal, human listeners recognize spoken words both quickly and accurately. Nonetheless, variability in speech does have implications for both the processes and representations involved in spoken language perception. Moreover, variability effects have been observed across the lifespan, ranging from infants to older adults. Many factors could potentially modulate the degree to which variability affects spoken language perception. In particular, recent findings demonstrate that variability effects follow a time course, manifesting themselves at predictable points during perceptual processing. However, time course investigations are currently limited to young adults. Therefore, the current paper explores how the time course of variability effects might differ throughout the lifespan, based on predictions derived from an adaptive resonance framework

Variable Semantic Input and Novel First-Language Vocabulary Learning

Vocabulary learning involves mapping a word form to a semantic meaning. An individual asked to learn the Spanish word for “apple,” for example, must map a new word form (manzana) onto the appropriate semantic representation. Previous studies have found that acoustic variability of word forms can improve second language vocabulary acquisition (Barcroft & Sommers, 2005; Sommers & Barcroft, 2007). The current experiments investigated whether variable semantic input could have a similar beneficial effect on first language vocabulary learning. Participants learned low-frequency English vocabulary words and their definitions. Half of the words were shown with the same verbatim definition at each of the six exposures, while the other half appeared with a differently worded, but synonymous, definition at each of the six exposures. After the learning phase, two cued recall tests were administered. In the first test, each word form was supplied and participants were instructed to write its definition. In the second test, participants were given a novel definition of each word and were asked to provide the correct word form for the definition. Younger adults in Experiment 1 were more accurate for words studied in the variable condition, whereas the accuracy scores of older adults in Experiment 2 did not significantly differ by condition. These results are discussed within the TOPRA model framework (Barcroft, 2002) and the associative deficit hypothesis

The factors affectng the psychometric function for speech intelligibility

Older listeners often report difficulties understanding speech in noisy environments. Increasing the level of the speech relative to the background - e.g. by way of a hearing aid - usually leads to an increase in intelligibility. The amount of perceptual benefit that can be gained from a given improvement in signal-to-noise ratio (SNR), however, is not fixed: it instead depends entirely on the slope of the psychometric function. The shallower the slope, the less benefit the listener will receive. The aim of the research presented in this thesis was to better understand the factors which lead to shallow slopes. A systematic survey of published psychometric functions considered the factors which affect slope. Speech maskers, modulated-noise maskers, and target/masker confusability were all found to contribute to shallow slopes. Experiment 1 examined the role of target/masker confusion by manipulating masker intelligibility. Intelligible maskers were found to give shallower slopes than unintelligible ones but subsequent acoustic analysis demonstrated that modulation differences between the maskers were responsible for this effect. This was supported by the fact that the effect was seen at low SNRs. Experiment 2 confirmed that the effects of modulation and target/masker confusion occur at different SNRs. Experiments 3 and 4 demonstrated that directing attention to the target speech could "undo" the effects of target/masker confusion. In Experiments 5 and 6 a new method was developed to study whether slope effects are relevant to "real-world" situations. The results suggested that using continuous speech targets gave shallower slopes than standard speech-in-noise tests. There was little evidence found to suggest that shallow slopes are exacerbated for older or hearing-impaired listeners. It is concluded that in the complex demands of everyday listening environments the perceptual benefit received from a given gain in SNR may be considerably less than would be predicted by standard speech-in-noise paradigms.Older listeners often report difficulties understanding speech in noisy environments. Increasing the level of the speech relative to the background - e.g. by way of a hearing aid - usually leads to an increase in intelligibility. The amount of perceptual benefit that can be gained from a given improvement in signal-to-noise ratio (SNR), however, is not fixed: it instead depends entirely on the slope of the psychometric function. The shallower the slope, the less benefit the listener will receive. The aim of the research presented in this thesis was to better understand the factors which lead to shallow slopes. A systematic survey of published psychometric functions considered the factors which affect slope. Speech maskers, modulated-noise maskers, and target/masker confusability were all found to contribute to shallow slopes. Experiment 1 examined the role of target/masker confusion by manipulating masker intelligibility. Intelligible maskers were found to give shallower slopes than unintelligible ones but subsequent acoustic analysis demonstrated that modulation differences between the maskers were responsible for this effect. This was supported by the fact that the effect was seen at low SNRs. Experiment 2 confirmed that the effects of modulation and target/masker confusion occur at different SNRs. Experiments 3 and 4 demonstrated that directing attention to the target speech could "undo" the effects of target/masker confusion. In Experiments 5 and 6 a new method was developed to study whether slope effects are relevant to "real-world" situations. The results suggested that using continuous speech targets gave shallower slopes than standard speech-in-noise tests. There was little evidence found to suggest that shallow slopes are exacerbated for older or hearing-impaired listeners. It is concluded that in the complex demands of everyday listening environments the perceptual benefit received from a given gain in SNR may be considerably less than would be predicted by standard speech-in-noise paradigms

Listening Effort Outcome Measures in Adult Populations

Listening effort is being considered clinically as an important indicator of patient success with amplification. Listening effort refers to “the mental exertion required to attend to, and understand, an auditory message”. (McGarrigle et al., 2014) Cognitive ability, age, and degree of hearing impairment all must considered when measuring the effort one is exerting on a specific task. Listening effort can be measured using subjective tools, electrophysiologic measures, or with a dual-task paradigm. Subjective tools include self-reports and questionnaires. Electrophysiologic measures can consist of measuring pupil dilation, heart rate variability, and skin conductance. A dual-task paradigm is set-up with two tasks performed by a person simultaneously. The amount of decline on the secondary task compared to when that task is completed in isolation, indicates the amount of effort that was exerted on the task. Each method has its strengths as well as its limitations. This paper discusses the current research on the various methods to measuring listening effort and provides clinical applications for these outcome measures

EFFECTS OF AGING ON VOICE-PITCH PROCESSING: THE ROLE OF SPECTRAL AND TEMPORAL CUES

Declines in auditory temporal processing are a common consequence of natural aging. Interactions between aging and spectro-temporal pitch processing have yet to be thoroughly investigated in humans, though recent neurophysiologic and electrophysiologic data lend support to the notion that periodicity coding using only unresolved harmonics (i.e., those available via the temporal envelope) is negatively affected as a consequence of age. Individuals with cochlear implants (CIs) must rely on the temporal envelope of speech to glean information about voice pitch [coded through the fundamental frequency (f0)], as spectral f0 cues are not available. While cochlear implants have been shown to be efficacious in older adults, it is hypothesized that they would experience difficulty perceiving spectrally-degraded voice-pitch information. The current experiments were aimed at quantifying the ability of younger and older listeners to utilize spectro-temporal cues to obtain voice pitch information when performing simple and complex auditory tasks. Experiment 1 measured the ability of younger and older NH listeners to perceive a difference in the frequency of amplitude modulated broad-band noise, thereby exploiting only temporal envelope cues to perform the task. Experiment 2 measured age-related differences in f0 difference limens as the degree of spectral degradation was manipulated to approximate CI processing. Results from Experiments 1 and 2 demonstrated that spectro-temporal processing of f0 information in non-speech stimuli is affected in older adults. Experiment 3 showed that age-related performances observed in Experiments 1 and 2 translated to voice gender identification using a natural speech stimulus. Experiment 4 attempted to estimate how younger and older NH listeners are able to utilize differences in voice pitch information in everyday listening environments (i.e., speech in noise) and how such abilities are affected by spectral degradation. Comprehensive results provide further insight on pitch coding in both normal and impaired auditory systems, and demonstrate that spectro-temporal pitch processing is dependent upon the age of the listener. Results could have important implications for elderly cochlear implant recipients

Phonetic vowel training for child second language learners: the role of input variability and training task

Acquiring a second language speech contrast that does not exist in the native language is often difficult. High variability phonetic training (HVPT) is a wellestablished method used to train learners on specific non-native phoneme contrasts: it critically uses high variability (HV) input after earlier attempts using low variability (LV) input had proved unsuccessful. HVPT has since been successfully applied in many different adult studies. However, there is no consensus on the effect of input variation on children’s learning of non-native phoneme contrasts. This thesis aims to further investigate the effect of input variability on phonetic training for children, and examining whether they show the same HV benefit which has been argued to hold for adults. In the first set of studies, native English speaking adults and children were taught Dutch vowels in a single computerised training session, during which they received either HV or LV input. Additionally, the traditional HVPT paradigm was adapted to see if mapping vowels to orthography-like symbols representing phoneme categories was more or less effective than a vocabulary training method without such representations. Learning was stronger with training most akin to vocabulary learning, particularly for children, suggesting a benefit for a more meaningful learning context. Crucially, there was no evidence of a HV benefit for either children or adults. The second study was a two-week training study in which Dutch children of two age groups were trained on Standard Southern British English vowel contrasts. Since picture-based training had proved beneficial, this study combined both orthography and pictures in training. Potential effects of HV or LV input in training were investigated using a pre/post-test design. Older children outperformed younger children throughout, and again no evidence for a variability benefit was found. This indicates children might not benefit from high input variability