Central Nervous System Processing of Complex Acoustic Signals

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Project Summary: In perceiving acoustic communication signals, two of the most important levels at which the central nervous system (CNS) must process sounds are (1) temporal fine structure (TFS) – rapid changes in the frequency and amplitude within the envelope of the speech waveform – and (2) sequential patterns in the structure of acoustic elements. In human language, these abilities are important both for decoding information from speech and in learning to accurately produce words and sentences. While much has been learned about how the auditory cortex in humans computes complex acoustic signals through using non-invasive techniques, it is not known how neurons of the auditory system process the acoustic communication signals at these multiple levels. Vocal learning birds with their complex, learned vocal repertoires and sequentially patterned songs provide very good models for understanding how the brain processes complex acoustic signals. Songbirds, such as zebra finches, and parrots, such as budgerigars, are especially attractive models for asking how the CNS processes complex acoustic signals as the birds can hear TFS at a level that surpasses the capability of humans and other mammals, and the neural mechanisms involved in TFS and sequence processing have been little explored. Moreover, recent experiments have suggested that changes in TFS are much more discriminable to zebra finches than changes in the sequential pattern of song syllables. This is in contrast to humans and budgerigars, for which changes to sequence are very salient. In perception, the coding of neurons in primary and secondary auditory regions for TFS and sequential patterns may help explain why species differ in processing these features. Examining these capacities at the single- and multi-unit level in vocal learning birds could help us, thereby, further understand CNS processing of acoustic communication signals and address central auditory disorders affecting human language. To determine the neural basis for auditory pattern recognition and processing of TFS, I propose the following 2 specific aims: In aim 1, I will compare the discriminability of TFS and sequential patterns in zebra finches, a songbird model, and budgerigars, a parrot model. I will pit these two auditory levels against each other in psychoacoustics testing using Schroeder waveforms, synthetic stimuli that can be manipulated so that only TFS or the sequence of elements is changed, and I will also obtain thresholds in the two species for hearing changes to sequence. In aim 2, I will compare how neurons in auditory regions of the zebra finch and budgerigar forebrains code TFS and sequence information in complex acoustic signals. I will measure single- and multi-unit extracellular selectivity to the TFS of Schroeder waveforms in the primary auditory region of zebra finches and budgerigars and I will measure dishabituation to changes in sequence in a secondary auditory region. In summary, the proposed project will improve scientific knowledge about complex auditory perception by linking behavioral data about TFS and pattern processing with neural correlates in the auditory system.