Why would Musical Training Benefit the Neural Encoding of Speech? The OPERA Hypothesis

Mounting evidence suggests that musical training benefits the neural encoding of speech. This paper offers a hypothesis specifying why such benefits occur. The “OPERA” hypothesis proposes that such benefits are driven by adaptive plasticity in speech-processing networks, and that this plasticity occurs when five conditions are met. These are: (1) Overlap: there is anatomical overlap in the brain networks that process an acoustic feature used in both music and speech (e.g., waveform periodicity, amplitude envelope), (2) Precision: music places higher demands on these shared networks than does speech, in terms of the precision of processing, (3) Emotion: the musical activities that engage this network elicit strong positive emotion, (4) Repetition: the musical activities that engage this network are frequently repeated, and (5) Attention: the musical activities that engage this network are associated with focused attention. According to the OPERA hypothesis, when these conditions are met neural plasticity drives the networks in question to function with higher precision than needed for ordinary speech communication. Yet since speech shares these networks with music, speech processing benefits. The OPERA hypothesis is used to account for the observed superior subcortical encoding of speech in musically trained individuals, and to suggest mechanisms by which musical training might improve linguistic reading abilities

Individual differences in supra-threshold auditory perception - mechanisms and objective correlates

Thesis (Ph.D.)--Boston UniversityTo extract content and meaning from a single source of sound in a quiet background, the auditory system can use a small subset of a very redundant set of spectral and temporal features. In stark contrast, communication in a complex, crowded scene places enormous demands on the auditory system. Spectrotemporal overlap between sounds reduces modulations in the signals at the ears and causes masking, with problems exacerbated by reverberation. Consistent with this idea, many patients seeking audiological treatment seek help precisely because they notice difficulties in environments requiring auditory selective attention. In the laboratory, even listeners with normal hearing thresholds exhibit vast differences in the ability to selectively attend to a target. Understanding the mechanisms causing these supra-threshold differences, the focus of this thesis, may enable research that leads to advances in treating communication disorders that affect an estimated one in five Americans. Converging evidence from human and animal studies points to one potential source of these individual differences: differences in the fidelity with which supra-threshold sound is encoded in the early portions of the auditory pathway. Electrophysiological measures of sound encoding by the auditory brainstem in humans and animals support the idea that the temporal precision of the early auditory neural representation can be poor even when hearing thresholds are normal. Concomitantly, animal studies show that noise exposure and early aging can cause a loss (cochlear neuropathy) of a large percentage of the afferent population of auditory nerve fibers innervating the cochlear hair cells without any significant change in measured audiograms. Using behavioral, otoacoustic and electrophysiological measures in conjunction with computational models of sound processing by the auditory periphery and brainstem, a detailed examination of temporal coding of supra-threshold sound is carried out, focusing on characterizing and understanding individual differences in listeners with normal hearing thresholds and normal cochlear mechanical function. Results support the hypothesis that cochlear neuropathy may reduce encoding precision of supra-threshold sound, and that this manifests as deficits both behaviorally and in subcortical electrophysiological measures in humans. Based on these results, electrophysiological measures are developed that may yield sensitive, fast, objective measures of supra-threshold coding deficits that arise as a result of cochlear neuropathy

Auditory-motor entrainment and phonological skills: precise auditory timing hypothesis (PATH)

Phonological skills are enhanced by music training, but the mechanisms enabling this cross-domain enhancement remain unknown. To explain this cross-domain transfer, we propose a precise auditory timing hypothesis (PATH) whereby entrainment practice is the core mechanism underlying enhanced phonological abilities in musicians. Both rhythmic synchronization and language skills such as consonant discrimination, detection of word and phrase boundaries, and conversational turn-taking rely on the perception of extremely fine-grained timing details in sound. Auditory-motor timing is an acoustic feature which meets all five of the pre-conditions necessary for cross-domain enhancement to occur (Patel, 2011, 2012, 2014). There is overlap between the neural networks that process timing in the context of both music and language. Entrainment to music demands more precise timing sensitivity than does language processing. Moreover, auditory-motor timing integration captures the emotion of the trainee, is repeatedly practiced, and demands focused attention. The PATH predicts that musical training emphasizing entrainment will be particularly effective in enhancing phonological skills

Neural Mechanisms Underlying Hierarchical Speech-in-Noise Processing

One of the most commonly reported complaints related to hearing is difficulty understanding speech-in-noise (SIN). Numerous individuals struggle to effectively communicate in adverse listening conditions, even those with normal hearing. These difficulties are exacerbated due to age and hearing-related deficits such as hearing loss and auditory processing disorders. Despite the high prevalence of SIN deficits in individuals across the lifespan, the neural mechanisms underlying successful speech comprehension in noise are not well understood. Communication in noise is an incredibly complex process that requires efficient processing throughout the entire auditory pathway as well as contributions from higher-order cognitive processes including working memory, inhibition, and attention. In a series of studies using electrophysiologic (EEG) and behavioral measures, this dissertation evaluated the neural correlates of SIN perception across subcortical and cortical levels of the auditory system to identify how top-down and bottom-up influences aid SIN understanding. The first study examined the effects of hearing loss on SIN processing in older adults at the cortical level using frequency-specific neural oscillations (i.e., brain rhythms) and functional connectivity (i.e., directed neural transmission). We found that low-frequency alpha and beta oscillations within and between prefrontal and auditory cortices reflect the ability to flexibly allocate neural resources and recruit top-down predictions to compensate for hearing-related declines and facilitate efficient SIN perception. The second study, in younger adults, investigated the role of attention in SIN processing and how it interacts with early sensory encoding. Hierarchical processing in brainstem and cortex was assessed by simultaneously recording frequency-following responses (FFRs) and event-related potentials (ERPs) at the source level. We found that attention modulates SIN processing at both subcortical and cortical levels and strengthens bidirectional neural signaling within the central auditory pathway. A relative disengagement of corticofugal transmission was observed in noise but only for passive listening suggesting attention aids SIN perception by maintaining top-down reinforcement of acoustic feature encoding within the primary auditory pathways. Taken together, these results indicate that the neural networks engaged during SIN perception depend on a complex interplay between bottom-up and top-down factors including signal clarity, listeners hearing status, and attentional deployment

Effects of Aging and Spectral Shaping on the Sub-cortical (Brainstem) Differentiation of Contrastive Stop Consonants

Purpose: The objectives of this dissertation are to: (1) evaluate the influence of aging on the sub-cortical (brainstem) differentiation of voiced stop consonants (i.e. /b-d-g/); (2) determine whether potential aging deficits at the brainstem level influence behavioral identification of the /b-d-g/ stimuli, (3) investigate whether spectral shaping diminishes any aging impairments at the brainstem level; and (4) if so, whether minimizing these deficits improves the behavioral identification of the speech stimuli. Subjects: Behavioral and electrophysiological responses were collected from 11 older adults (\u3e 50 years old) with near-normal to normal hearing and were compared to those of 16 normal-hearing younger adults (control group). Stimuli and Methods: Speech- evoked auditory brainstem responses (Speech-ABRs) were recorded for three 100-ms long /b-d-g/ consonant-vowel exemplars in unshaped and shaped conditions, for a total of six stimuli. Frequency-dependent spectral-shaping enhanced the second formant (F2) transition relative to the rest of the stimulus, such that it reduced gain for low frequencies; and increased gain for mid and high frequencies, the frequency region of the F2 transition in the /b-d-g/ syllables. Behavioral identification of 15-step perceptual unshaped and shaped /b-d-g/ continua was assessed by generating psychometric functions in order to quantify stimuli perception. Speech ABR peak amplitudes and latencies and stop consonant differentiation scores were measured for 6 stimuli (3 unshaped stimuli and 3 shaped stimuli). Summary of Findings: Older adults exhibited more robust categorical perception, and subtle sub-cortical deficits when compared to younger adults. Individual data showed fewer expected latency patterns for the /b-d-g/ speech-ABRs in older adults as opposed to younger adults, especially for major peaks. Spectral shaping improved the stop consonant differentiation score for major peaks in older adults, such that it moved older adults in the direction of the younger adults’ responses. Conclusion: Sub-cortical impairments at least those measured in this study do not seem to influence the behavioral differentiation of stop consonants in older adults. On the other hand, cue enhancement by spectral shaping seems to overcome some of the deficits noted at the electrophysiological level. However, due to a possible ceiling effect, improvements to the originally robust perception of older adults, at the behavioral level were not found. Significance: Aging seems to reduce the sub-cortical responsiveness to dynamic spectral cues without distorting the spectral coding as evident by the “reparable” age-related changes seen at the electrophysiological level. Cue enhancement appears to increase the neural responsiveness of aged but intact neurons, yielding a better sub-cortical differentiation of stop consonants

Hearing in dementia: defining deficits and assessing impact

The association between hearing impairment and dementia has emerged as a major public health challenge, with significant opportunities for earlier diagnosis, treatment and prevention. However, the nature of this association has not been defined. We hear with our brains, particularly within the complex soundscapes of everyday life: neurodegenerative pathologies target the auditory brain and are therefore predicted to damage hearing function early and profoundly. Here I present evidence for this proposition, based on structural and functional features of auditory brain organisation that confer vulnerability to neurodegeneration, the extensive, reciprocal interplay between ‘peripheral’ and ‘central’ hearing dysfunction, and recently characterised auditory signatures of canonical neurodegenerative dementias (Alzheimer’s disease and frontotemporal dementia). In chapter 3, I examine pure tone audiometric thresholds in AD and FTD syndromes and explore the functional interplay between the auditory brain and auditory periphery by assessing the contribution of auditory cognitive factors on pure tone detection. In chapter 4, I develop this further by examining the processing of degraded speech signals, leveraging the increased importance of top-down integrative and predictive mechanisms on resolving impoverished bottom-up sensory encoding. In chapter 5, I use a more discrete test of phonological processing to focus in on a specific brain region that is an early target in logopenic aphasia, to explore the potential of auditory cognitive tests as disease specific functional biomarkers. Finally, in chapter 6, I use auditory symptom questionnaires to capture real-world hearing in daily life amongst patients with dementia as well as their carers and measure how this correlates with audiometric performance and degraded speech processing. I call for a clinical assessment of real-world hearing in these diseases that moves beyond pure tone perception to the development of novel auditory ‘cognitive stress tests’ and proximity markers for the early diagnosis of dementia and management strategies that harness retained auditory plasticity

The cerebellum and motor dysfunction in neuropsychiatric disorders

The cerebellum is densely interconnected with sensory-motor areas of the cerebral cortex, and in man, the great expansion of the association areas of cerebral cortex is also paralleled by an expansion of the lateral cerebellar hemispheres. It is therefore likely that these circuits contribute to non-motor cognitive functions, but this is still a controversial issue. One approach is to examine evidence from neuropsychiatric disorders of cerebellar involvement. In this review, we narrow this search to test whether there is evidence of motor dysfunction associated with neuropsychiatric disorders consistent with disruption of cerebellar motor function. While we do find such evidence, especially in autism, schizophrenia and dyslexia, we caution that the restricted set of motor symptoms does not suggest global cerebellar dysfunction. Moreover, these symptoms may also reflect involvement of other, extra-cerebellar circuits and detailed examination of specific sub groups of individuals within each disorder may help to relate such motor symptoms to cerebellar morphology

Individual differences in rhythmic skills: links with neural consistency and linguistic ability

Durational patterns provide cues to linguistic structure, and so variations in rhythm skills may have consequences for language development. Understanding individual differences in rhythm skills, therefore, could help explain variability in language ability across the population. We investigated the neural foundations of rhythmic proficiency and its relation to language skills in young adults. We hypothesized that rhythmic abilities can be characterized by at least two constructs, which are tied to independent language abilities and neural profiles. Specifically, we hypothesized that rhythm skills that require integration of information across time rely upon the consistency of slow, low-frequency auditory processing, which we measured using the evoked cortical response. On the other hand, we hypothesized that rhythm ic skills that require fine temporal precision rely upon the consistency of fast, higher-frequency auditory processing, which we measured using the frequency following response. Performance on rhythm tests aligned with two constructs: rhythm sequencing and synchronization. Rhythm sequencing and synchronization were linked to the consistency of slow cortical and fast frequency-following responses, respectively. Furthermore, while rhythm sequencing ability was linked to verbal memory, reading, and nonverbal auditory temporal processing, synchronization ability was linked only tononverbal auditory temporal processing. Thus, rhythm perception at different time scales reflects distinct abilities, which rely on distinct auditory neural resources. In young adults slow rhythmic processing makes the more extensive contribution to language skill