During early life neural codes must develop to appropriately transform sensory inputs into behavioral outputs. Here we demonstrate a direct link between the maturity of neural coding in the visual brain and developmental changes in visually-guided behavior. In zebrafish larvae we show that visuallydriven hunting behavior improves from 4 to 15 days post-fertilization, becoming faster and more accurate. During the same period population activity in the optic tectum refines, leading to improved decoding and information transmission of spatial position, particularly in the representation of the frontal visual field. Remarkably, individual differences in decoding can predict each fish's hunting success. Together these results show how the neural codes required to subserve a natural behavior emerge during development.

Every action in response to some environmental state is preceded by a cascade of activity in the brain. Information about the environment is perceived and enters the brain through sensory systems, such as the visual, auditory, or somatosensory system, is processed as it travels downstream along the particular sensory pathway and at the end may elicit for example a motor response. In order to achieve this, the brain is made up of a multitude of neurons, ranging from less than one hundred in simple organisms to several billion cells in great apes. These discrete units of neural signalling facilitate the propagation of information through electrochemical processes. As the animal develops and gains experience about the environment these circuits and the neural code, the representation of information in neural activity, undergo changes that likely allow the animal to act more efficiently. Key insights into the functioning of the brain can be gained from studying its development. And in that sense understanding the development of the neural code can give new insight into how the brain processes information. In order to do so, we turned to the larval zebrafish as a model system and in this system studied the development of neural activity in the optic tectum. The optic tectum is a brain structure that is homologous to the superior colliculus in the mammalian brain and an integral part of the visual systems in vertebrates such as fish. In recent years, the zebrafish has become a popular model system in systems neuroscience because of the interesting feature that in their larval stage zebrafish are translucent. This together with their small size allows for in vivo microscopy and using genetically encoded calcium indicators this enables non-invasive recording of neural activity from large neural populations at single cell resolution. Contributions by others to this thesisGeoffrey J. Goodhill and Lilach Avitan have provided support and guidance in the analysis and interpretation of data as well as feedback on the writing. The calcium imaging experiments including the preprocessing of the calcium imaging recordings were performed by Lilach Avitan, Zac Pujic, Biao Sun, Shuyu Zhu, Matthew Van De Poll, and Ann-Elin Myhre. The images of zebrafish shown were taken by Zac Pujic. Statement of parts of this thesis submitted to qualify for the award of another degreeNo works submitted towards another degree have been included in this thesis. Research involving human or animal subjectsAll animal procedures were approved by The University of Queensland Ethics Committee, Approval Number QBI/152/16/ARC. The approval certificate is included in Appendix A.