My proposed research focuses on fundamental questions in hearing and the neural mechanisms employed by the brain in the processing of communication sounds. In the natural world, the auditory system extracts critical signals, such as speech, from exceedingly complex acoustic environments. This is accomplished through mechanical and neurobiological filtering processes. Some of this filtering is accomplished within the ear and auditory nerve. However, a great deal is also accomplished through complex processing in the brain, and we know comparatively little about this. A robust understanding of auditory brain function is of clear significance to a wide array of health issues, including the development of effective auditory nerve electrical stimulation paradigms and cochlear implants for the hearing impaired. I will use neurophysiological and anatomical methods to address questions about the representation of sounds in the brain. While frequency and temporal cues are both important components in communication sounds, human behavioral studies underline the salience of temporal structure in object perception (e.g. Rasch, 1978) and in source localization (e.g. Yost & Hafter, 1987). Moreover, recent studies indicate that the preservation of time-cues must be a priority in the engineering of cochlear implants (Shannon, 1990). I will concentrate on the neural representation of temporal features of sound. In many animal communication systems, temporal coding is also of preeminent importance (Myrberg et al., 1978; Michelsen et al., 1985). I have selected a vertebrate model, the mormyrid fish, on the basis of two important criteria: 1) the fish's ear is specialized for encoding temporal features of sound, lacking a sophisticated frequency analyzer as seen in the mammalian cochlea, and 2) I have shown that this fish uses temporally patterned sounds in its relatively simple acoustic communication behavior. Using extra-cellular neurophysiological recording techniques, I will examine responses to naturalistic acoustic click-trains and other temporally modulated stimuli. I will investigate the ascending auditory system, in the mesencephalon and diencephalon, with the goal of describing the neural representation of these sounds, and identifying regions where major transformations occur. Preliminary research in the mesencephalon has already revealed a number of interesting properties, including neurons which appear to be selective for particular time-intervals. Thus, the prospects for a successful study of auditory physiology and communication in this system seem good.
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