Object size classification and sensorimotor decision making in the larval zebrafish.

An organism's survival depends on its success in evading threats and acquiring resources. These basic needs require that sensory information be processed to maximize behavioral outputs in a given environment. The appropriate classification of visual objects can be used to drive context specific behavior (e.g. avoiding potential threats or pursuing potential food sources). In this work using the larval zebrafish as a model, I explore two central questions: How is visual information classified? Where along the visuomotor pathway is "positive" or "negative" value assigned to perceived objects? This work succeeds in linking visual perception (evaluation of object size) to action selection (deciding between approach and avoidance). Chapter 2 describes the development of behavioral paradigms to evaluate object size classification. A free swimming assay for the larval zebrafish was developed that elicited both approach and avoidance through the presentation of moving dots of different sizes. This assay was used to identify neural components mediating both approach and avoidance behaviors. Chapters 3 and 4 investigate the roles of specific neuronal populations that contribute to the implementation of size-mediated behaviors. A significant advance is the identification of a population of glutamatergic interneurons residing in the optic tectum which are components of a neural pathway for approach towards small objects. Ablation of these neurons results in a shift from approach to avoidance when small moving dots are presented. Conversely activating these interneurons enhances approaches to small dots. These results suggests that neural circuits in the optic tectum are actively involved in object classification which biases behavioral output towards approach or avoidance