Neural Responses to Auditory Rhythms in the Zebra Finch

NEURAL RESPONSES TO AUDITORY RHYTHMS IN THE ZEBRA FINCH By Jennifer A. Lampen Rhythm is important in the production of motor sequences such as speech and song. Deficits in rhythm processing have been implicated in a range of human disorders including some that affect speech and language processing, including stuttering, specific language impairment, and dyslexia. Songbirds provide a tractable model for studying the neural underpinnings of rhythm processing due to parallels with humans in neural structures and vocal learning patterns. In the experiments conducted for this dissertation, I investigated the effect of rhythmicity of song stimuli on expression of the immediate early gene ZENK in the adult zebra finch brain. I also investigated development of rhythmic discrimination in the juvenile brain, and estradiol (E2) effects on rhythm perception in adult birds. In adult zebra finches, ZENK was increased in response to arrhythmic compared to rhythmic song in the auditory association cortex homologs, caudomedial nidopallium (NCM) and caudomedial mesopallium (CMM), and the avian amygdala, nucleus taeniae (Tn). CMM also had greater ZENK labeling in females than males. These auditory areas may be detecting errors in arrhythmic song when comparing it to a stored template of how conspecific song is expected to sound. CMM may also be important for females in evaluating songs of potential mates. Increased neural activity in Tn may be related to the value of song for assessing mate choice and bonding or to perception of arrhythmic song as aversive. Before formation of the template for song that young birds memorize, expression of ZENK was increased in NCM of birds exposed to rhythmic relative to arrhythmic song. During template formation, ZENK expression was increased in CMM of birds exposed to arrhythmic relative to rhythmic song. These results suggest that the youngest birds may be predisposed to respond to a more natural stimulus, and a template may be required for arrhythmic song to elicit increased neural activity. Rhythm discrimination in CMM may be strongest at life stages, such as during template memorization, when birds are most focused on external auditory signals. Compared to data from adults, it also appears that functional development across the brain regions investigated continues to maturity. In adult zebra finches treated with a control or E2 or the aromatase inhibitor fadrozole (to increase or decrease estrogen availability), ZENK mRNA was significantly greater in the left hemisphere within NCM, CMM, and Tn. The overall pattern for left lateralization parallels the left lateralization of language processing in humans and may suggest that this hemisphere is specialized for processing conspecific vocalizations. Main effects of sex were detected in both auditory regions, with increased ZENK in males in NCM and in females in CMM. The reversed sex differences in NCM and CMM suggest that males and females differentially rely on components of the auditory forebrain for processing conspecific song. In CMM, an effect of hormone treatment also existed. While no pairwise comparison was statistically significant, the pattern suggested greater ZENK expression in control compared to both fadrozoleand E2treated birds. In NCM, an interaction between sex and hormone treatment suggested that the sex effect was restricted to control animals. The hormone effects suggest that an optimal level of estradiol may exist for processing rhythmicity of auditory stimuli. Together, these studies represent the first step in establishing the zebra finch as a model for human rhythm perception and disorders with disruptions in rhythm processing. This work suggests multiple brain regions that should be assessed in more detail for their involvement in human rhythm processing and disorders. A potential for a learned aspect of rhythm discrimination is also indicated, suggesting that rhythm training may aid those with disorders involving rhythm processing deficits. In addition, the establishment of the zebra finch as a model provides the opportunity to conduct more mechanistic studies into the basis of rhythm perception.