Crosslanguage studies of the perception of lexical tones have suggested that attention-modulated pitch processing at the cortical level may be influenced by language experience. We recently demonstrated that preattentive stages of pitch processing at the human brainstem level may also be influenced by language experience as reflected in the scalp-recorded human frequency following response. Our long term objective is to advance our knowledge of how pitch mechanisms in the auditory brainstem reorganize with experience to enhance encoding of behaviorally relevant sounds and to determine their role in the hierarchical processing of the temporal structure of sound. By adopting a crosslanguage approach (Mandarin, Thai, English) in tandem with a variety of speech and nonspeech stimuli varying in their lexical status and/or pitch contour, we are able to directly address questions of domain- as well as tone-specificity of the neural mechanisms mediating this pitch representation in the human brainstem.
The specific aims are to assess: (1) language-dependent specificity of pitch representation for native f0 trajectories in a speech context; (2) language-dependent specificity of pitch representation for native f0 trajectories in a nonspeech context; (3) sensitivity of pitch representation to systematic temporal degradation in native f0 trajectories in a nonspeech context; (4) contour specificity of the pitch representation of native and nonnative contour f0 trajectories in a nonspeech context; (5) role of temporal- or spectral-based mechanisms underlying pitch extraction; and (6) laterality of experience- dependent brainstem reorganization for pitch representation. We hypothesize that temporal neural mechanisms mediating pitch representation are sensitive to a particular language and tonal dimensions; that this pitch representation in the native listeners is less susceptible to temporal degradation of pitch relevant information in the stimulus even when presented in a nonspeech context; that an ear advantage may even be evident at the brainstem since the inferior colliculus and auditory cortex share the same dominant contralateral pathway. Our findings promise to provide fresh insights as to how pitch processing at the brainstem level emerges from differential demands on auditory and linguistic processes that are shared by speech and nonspeech stimuli alike. Of relevance to the public health, a better understanding of how pitch encoding is influenced by language experience at the brainstem level will allow us to assess differentially the integrity of pitch representation in our increasingly multilingual population; to monitor non-invasively changes in pitch representation after retraining of language/hearing-impaired listeners from different language backgrounds; and to develop optimal, language-sensitive signal processing strategies for hearing prosthetic devices used by hearing-impaired listeners. Thus, auditory electrophysiology at the brainstem level can provide us with fresh insights into patients' difficulties in hearing and understanding speech and assist in the development of optimal signal processing strategies to remedy degraded neural representation. Our ability to favorably influence the recovery from hearing-related impairments will depend upon a better understanding of the neurobiological basis of language-dependent pitch processing in the human brainstem. The scalp recorded frequency following response (FFR) provides a noninvasive neural index of pitch representation at the brainstem level. If the aims of this project are achieved, FFRs can be incorporated into clinical practice to assess the integrity of pitch representation in a target population, to index changes in pitch representation before and after training, and to evaluate signal processing algorithms designed to recover degraded neural representation of pitch in conventional hearing aids and/or cochlear implants. ? ? ?
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