Project 2 - Quantitative behavioral analysis and imaging - Abstract Behavior is the foundation for systems neuroscience. Its quantitative description and algorithmic analyses need to be the first and essential step on the journey to generating a realistic circuit model of any brain. Whether fly, fish, rat or human, for an animal to exhibit purposeful behavior, it must be endowed with the computational ability to map patterns of sensory input to patterns of motor output. The quantifiable rules of sensory-motor mapping define the algorithms of behavioral strategy. These computational rules determine what the underlying neural circuits are actually doing. The critical test of a nervous system is whether it can provide computational solutions for behavioral challenges that the animal faces in natural environments. As such, it is the behavioral algorithms that dictate the questions and framework for any model of neural implementation and it is therefore necessary to turn behavior into a conceptual framework that can fully integrate genetic, anatomical, and physiological data. In the first part of Project 2, we describe a fleet of behavioral set-ups and assays that allow the quantitative description of fish behavior at various levels of detail and throughput. To perform a neural implementation of these algorithms and generate a realistic whole-brain model for larval zebrafish we need, in addition to knowledge about the anatomical structure of the circuits, detailed information about neural activity patterns during behavior. To that end, we have designed a variety of assays that are compatible with brainwide imaging of neural activity in freely swimming and tethered larval zebrafish. These assays in conjunction with Ca2 + imaging are described in the second part of Project 2. Furthermore, tests for causality and the rigorous validation of circuit models require targeted perturbation experiments, where identified neuronal cell types can be silenced or activated in a targeted way. The tethered behavioral assays described in this project provide the ideal setting to perform such perturbations in the context of a controlled behavioral paradigm. We conclude, in the third part of Project 2, by describing a specific application of these assays that shed light on the role that various modulatory neurotransmitters, such as oxytocin and serotonin, play in regulating internal states involving hunger, stress or loneliness.