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theses

Organisms encounter numerous novel sensory signals throughout life. Thus, sensory representations in the adult brain, set up during ontogeny through the interaction of early experience with innate organizational principles, must undergo dynamic changes to adapt to the complexity of the external world. This thesis investigates how passive exposure to novel sounds modifies neural representations to facilitate recognition and discrimination, using the zebra finch model organism. Zebra finches use complex, learned acoustic signals for social communication with many parallels to human speech. Furthermore, the neural responses in an auditory structure in the zebra finch brain, Caudal Medial Nidopallium (NCM), undergo a long-term form of adaptation with repeated stimulus presentation, providing an excellent substrate to probe the neural underpinnings of adaptive sensory representations. In Experiment 1, electrophysiological activity in NCM was recorded under passive listening conditions as novel natural vocalizations were familiarized through playback. Neural decoding of stimuli using the temporal profiles of both multi-unit and single-unit neural responses improved dramatically during the first few stimulus presentations. During subsequent encounters, these signals were successfully recognized after hearing fewer initial acoustic features. Remarkably, the accuracy of neural decoding was higher when different stimuli were heard in separate blocks compared to when they were presented randomly in a shuffled sequence. Experiment 2 supported and extended these findings by showing that the rapid gains in neural decoding of natural vocalizations with passive familiarization were long-lasting, maintained for 20 hours after the initial encounter. Experiment 3 investigated how the degree of acoustic similarity between sounds related to these rapid dynamic changes in stimulus representations, using synthesized vocalizations that vary parametrically along a single dimension. Single-unit responses demonstrated that rapid differentiation of the temporal profiles of neural responses to different signals were more pronounced for stimulus pairs that are acoustically less similar to each other, although these results were mixed for multi-unit responses. Finally, in Experiment 4, the effects of passive familiarization on subsequent behavioral discrimination of two acoustically similar synthesized vocalizations were investigated. Surprisingly, this experiment did not indicate an effect of pre-exposure on behavioral responses. Taken together, the experiments in this thesis provide valuable insights into the mechanisms by which the nervous system dynamically modulates sensory representations to improve discrimination of novel complex signals over short and long time-scales. Similar mechanisms may also be engaged during processing of human speech signals, and thus may have potential translational relevance to elucidate the neural underpinnings of speech perception and comprehension difficulties.

ETD_8579

Ph.D.

Includes bibliographical references

by Efe Soyman

doi:10.7282/T3T72MP4

ETD doctoral

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