Auditory Information Processing in the Midbrain

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DESCRIPTION: (Adapted from applicant's abstract): The long term goal of this research is to identify mechanisms and pathways that give rise to the encoding and representation of information conveyed by vocal cornmuniction signals. Communication by sound employs spectrally and temporally complex signals, and their analyses in the central nervous system require integration across spectral and temporal elements in the signals. Such integration, once considered an exclusive role of auditory cortex, now appears to occur in sub-cortical auditory regions. This proposal focuses on the neuronal mechanisms and structural features of temporal and spectral integration in the auditory midbrain. Understanding where and what types of specialized processing of speech-like sounds occurs in particular pathways to auditory cortex will provide a fuller understanding of the bases of language perception disorders and potential intervention strategies.One form of time- and frequency-sensitive integration, common in the forebrains of a wide range of vertebrates, is performed by combinatorial neurons that respond best when distinct spectral elements in vocalizations are combined in specific temporal relationships. The proposed research examines how and where combinatorial responses originate in the mustached bat. In the auditory cortex of this species, combinatorial responses are abundant and well-characterized, and there is a good understanding of their potential significance in sonar and social communication behaviors. These neurons are also abundant in the inferior colliculus (IC), and may be formed there. The first two specific aims examine how brainstem auditory neurons contribute to the combinatorial responses in the inferior colliculus that analyze the bat's sonar echoes. The first aim will examine the neurophysiological properties of neurons in the cochlear nucleus that may contribute to the construction of these combinatorial responses. The second aim is to use anterograde tracing methods to determine what projections from the auditory brainstem provide the basis for frequency integration by combinatorial neurons in the inferior colliculus. The third specific aim uses physiological recording and local application of drugs to examine the mechanisms operating in the inferior colliculus that create the temporally sensitive facilitation that characterizes combinatorial responses. The fourth specific aim uses the same physiological/pharmacological techniques to study a class of combinatorial neurons that may respond to social vocalizations. This aim will test whether there is a fundamental mechanistic link between the well-studied combinatorial neurons that analyze sonar echoes and those that analyze other complex sounds, such as social vocalizations. These mechanisms may be similar to those operating at early stages in the analysis of speech sounds.