Hormonal Modulation of Auditory Processing in a Seasonally Breeding Songbird
Caras, Melissa Lynne
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Sex steroid hormones modulate vocal communication in a number of species. While much is known about the impact of these hormones on the motor systems responsible for generating vocal output, less is known about the influence of these hormones on auditory processing. I addressed this issue in a neuroethological context by studying Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). White-crowned sparrows are wild songbirds that undergo dramatic fluctuations in sex steroid levels as a function of season. I manipulated photoperiod and hormone levels in the laboratory and found that sex steroids have disparate effects along the ascending auditory pathway. Specifically, I report the following: First, breeding condition elevates auditory brainstem response thresholds and prolongs peak latencies compared to non-breeding condition, but does not affect distortion product otoacoustic emissions. These results are described in detail in chapter 2. Next, in chapter 3, I demonstrate that single cells in the avian primary forebrain, field L, are selectively responsive to hormonal state. When plasma hormone levels are elevated, cells with monotonic responses to pure tone stimuli demonstrate enhanced auditory function, and their physiological response properties strongly correlate with the concentration of circulating hormone. Finally, the findings described in chapter 4 show that a neural code based on spike timing reliability more accurately discriminates the intensity of incoming vocalization signals compared with a spike count-based strategy, and is sensitive to hormonal condition. My results are summarized and placed into a broader context in chapter 5, and future studies are proposed.