Processing of local features in the zebrafish optic tectum

The optic tectum is the main visual processing area in zebrafish and is involved in a variety of visually-driven behaviours. A key question is how information about the visual environment is processed and integrated in order to generate guided behaviour. The aim of this study was to explore the response properties of tectal neurons, i.e., their preference for certain features of the visual input. To do this, I developed a custom set-up for calcium imaging and simultaneous visual stimulation in older zebrafish larvae, up to the age of 21 dpf. First, this set-up was employed to measure the spatial receptive fields of tectal neurons with small moving spots. Notably, the results suggested that receptive field development is not completed by 9 dpf as previously believed; instead, receptive field refinement continues beyond this age. The results also confirmed that receptive fields in the optic tectum were relatively large in older larvae. Based on this, I formulated the hypothesis that tectal neurons might process multiple local features simultaneously. To test how the optic tectum encodes local features, I used small, moving oriented bars and combinations of bars, i.e., angles. Tectal responses to these stimuli suggested that, not only does the optic tectum encode local features, but is also tuned to horizontal-oriented local stimuli. Finally, I used a set of moving stimuli, consisting of simple features (i.e., lines and angles) and a composite feature (i.e. square) to test how information about multiple local features was integrated by tectal neurons. The results indicated that local features are spatially integrated in a sublinear fashion. The outcomes of the work presented in this thesis add to our understanding of how visual information provided by the retina is processed within the optic tectum.