Subcortical and Cortical Responses in Infants Evoked by Running Speech

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Efficient subcortical and cortical auditory processing is integral for learning spoken language. The goal of this project is to use a new tool we have developed to investigate responses to natural speech across the auditory hierarchy. Effective coding of the dynamic speech signal during development partially drives cortical development, and auditory neural deficits contribute to lifelong listening disabilities. While there are methods for measuring responses from the brainstem by the auditory brainstem response (ABR) and from the cortex (central auditory evoked potentials [CAEP]) in infants, only very short sounds can be used to evoke them which lack the complexity and richness of natural speech. Measurement of auditory responses to continuous, natural speech measured from the periphery through the brainstem and cortex opens the door for a broad spectrum of scientific inquiries including speech perception, language acquisition, and auditory processing deficits underlying various types of communication disorders. Despite the traditional stimulus limitations, the decrease in ABR and CAEP wave latencies and the increase in wave amplitudes with increasing infant age is used as a measure of maturation of brainstem myelination and cortical development for scientific and clinical purposes. Typically, only one or the other is measured, because they require different acquisition protocols. To address the limitations in non-natural stimuli and inability to measure brainstem and cortical responses simultaneously, the Maddox laboratory has pioneered a technique to measure the ABR and CAEP simultaneously to natural, running speech and shown its effectiveness in adults. In this project we will validate the technique in newborns and young infants, and define the relationship between brainstem and cortical responses over the first five months of life. In the first aim, we test the hypothesis that the novel speech ABR reflects auditory brainstem maturation. We hypothesize that the latencies of the response waves correlate with gestational age in term and preterm infants. We expect that there will be a relationship between click and speech ABR latencies, but that the developmental trajectory may be different, thereby providing new, complementary information. Based on previous literature, we also expect that trajectories will be different for term and preterm infants. In the second specific aim, we will characterize the cortical responses in infants and test the hypothesis that the size and latency of cortical responses to ongoing speech is dependent both on gestational age and wave latencies of the speech ABR. We also hypothesize that the relationships will be different for term and preterm infants. This aim is made possible by a major advantage of our speech response measure over traditional methods: the same data from which the ABR is derived also provides cortical responses. This innovative study will be the first to obtain auditory physiological measures of natural, running speech in developing infants from the brainstem to the cortex. This high-impact project will drive new studies of language acquisition and identify potential biomarkers for communication disorders due to underlying auditory processing deficiencies.