Speech is by far the most important class of acoustic stimuli for human listeners. While the coding of speech in the auditory nerve is relatively well understood, and some information is available for cochlear-nucleus neurons, very little is known about the responses of more central neurons to speech stimuli. We propose to study the responses of single neurons in the inferior colliculus (IC) of anesthetized cats to a wide variety of speech and speech-like sounds encompassing the acoustic characteristics used in English and other languages. One series of experiments will examine how the formant frequencies and fundamental frequency of vowels are coded in the discharge patterns of IC neurons, using synthetic stimuli in which these parameters ar systematically varied over a range appropriate for both male and female voices. These responses will be compared with those for sine-wave speech and amplitude-modulated tones in order to determine whether responses to vowels can be predicted from responses to simpler stimuli. Another series of experiments will study the responses to consonants differing in voicing, place, and manner of articulation. In particular, we will examine how the responses to consonants are affected by the preceding context. These responses will be compared with those to pairs of tones presented in succession in order to determine whether mechanisms such as adaptation, facilitation and long-lasting inhibition enhance the response to particular speech features. This research addresses fundamental issues in speech perception such as the neural basis for categorical perception, and the perceptual invariance of speech sounds in the face of context-dependence in their acoustic characteristics. By providing direct knowledge of the neural coding of speech in the brainstem, this research may also help design speech processors for auditory (cochlear and brainstem) implants that would be more effective by providing information appropriately coded for specific parts of the central auditory system. In combination with research on spatial hearing, this work may also lead to a better understanding of why hearing-impaired and elderly listeners have much greater difficulties understanding speech among competing sounds than do normal listeners, and may help develop new kinds of hearing aids that would perform better in noisy environments.
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