This project set up the first steps to mapping the songbird brain in high detail and to understand how their songs are learned. The way songbirds learn to sing is a complex behavior with many parallels to the way humans learn speech and language. But the exact ways in which this happens are unclear. Given the many parallels between human speech acquisition and song learning in birds, we believe that our insights will be generalizable and translatable to humans as well, and might help us to better understand and develop treatment strategies for speech-associated neuropathologies.
The research focus was on resolving the network architecture of vocal learning and production in a small songbird, the zebra finch. This bird has a unique neural anatomy in which functionally and anatomically discrete areas called song nuclei form an interconnected network. To understand neural mechanisms involved with song memorization and the emergence of adult song, DESYNE aimed at mapping the synaptic networks of synapse connections within a brain area called HVC. The idea was to explore the connections to understand how the experience of song learning worked.
Neural circuit research in songbirds is technically very challenging, which is why a large emphasis has been put on the development of methods to enable such investigations. As a result, the project produced exciting novel technology to study the detailed structure and function of specific network components during vocal learning.
This project provides two exciting imaging approaches to study entire brains or large portions of the brain with subcellular resolution. Through chemicals processes the brain tissue can be made transparent for light (tissue clearing) and even physically expanded through a swellable hydrogel strategy called expansion microscopy. The two approaches allow us to either study the long range projections between brain areas, or in more detail the connections within one entire brain nucleus. Further, we developed a virus which allows to selectively modify the genome of certain neurons underlying song learning and production. This virus can be used, for example, to introduce fluorescent dyes into certain parts of the circuits underlying learned vocalization. A detailed light microscopic analysis of the structures in the bird's brain is possible within just three days of injection, which meets the rapid pace of song learning. Neuronal activity can also be measured in vivo using genetic sensors, like calcium indicators, which emit a fluorescent signal whenever a neuron is firing. The virus can be used with zebra finches, bengalese finches and canaries, among others, but also in dopaminergic circuits of mice, which in the past could only be genetically targeted through more dangerous viruses, such as the rabies virus. We hope that this new differentiability will provide novel and more accurate insights into the function and development of the neural circuits underlying song and language learning, a necessary step towards developing medical treatment strategies for diseases that also affect the speech center, such as autism and attention deficit syndrome.We believe our technologies will help us and many other labs to study the neural circuits in songbirds with unprecedented detail and accuracy.
