Progression of variable repeats of introductory notes to the stable zebra finch song.

Abstract

Understanding how the brain initiates movements is an active field of research in neuroscience and is important both from the perspective of understanding how the brain works and for treating movement disorders, where movements fail to initiate. While most existing studies have looked at initiation of simple movements using instruction based paradigms, what happens before more natural movements remains poorly understood. In my thesis I addressed this by looking at a natural behavior, the song of the zebra finch. The zebra finch is an Australian songbird and it's song, consisting of a stereotyped sequence of sounds separated by silent gaps, is a well-studied example of a natural movement. Zebra finches begin their songs by repeating short variable notes called introductory notes (INs) before switching to their stereotyped song sequence. During the transition to song, intervals between successive introductory notes speed up and change in their temporal and sound properties towards a consistent state, suggesting a preparatory role for song, possibly similar to preparatory neural activity before the initiation of simple movements. To further understand what INs represent, I test various hypotheses to understand the behavioral and neural mechanisms of how INs progress to the song. I show that INs progress to song independent of real-time sensory feedback from the periphery, suggesting these notes are not produced to calibrate the vocal apparatus. Further, extensive behavioral analysis on the relationship between properties of INs and the upcoming song showed that the acoustic properties of INs are correlated with the acoustic properties of song syllables suggesting shared control. The intervals between INs and their increase in speed is unique to INs and not a property of other song syllables or other song syllable repeats. These results suggest that the intervals between successive INs and their speeding up before song reflect preparation in the songbird brain before song initiation, but changes in acoustic properties of INs reflect changes associated with repetition of a vocalization. To further understand how the brain encodes progression I analyzed the singing-related activity of neurons in two nuclei of the song motor pathway. These analyses revealed differences in neural activity for the first, middle and last INs before song, only in premotor nucleus HVC and not in downstream motor nucleus RA. These results suggest different combination of neurons in premotor area HVC being active at first, middle and last repeat of INs. This suggests a possible mechanism for IN progression where the brain keeps track of past and future syllables to transition from repeating INs to song. These mechanisms hold parallels to variable sequencing of songs in other songbird species and implicate a conserved function of HVC for encoding stereotyped and variable sequences. Overall, these results demonstrate interesting similarities and differences with initiation of simple, instruction-based movements and highlight potential for using songbirds to understand how the brain initiates natural movements.