Smell and taste are the least studied of all senses. Very little is known about chemosensory information processing beyond the level of receptor neurons. Every morning we enjoy our coffee thanks to our brains ability to combine and process multiple sensory modalities. Meanwhile, we can still review a document on our desk by adjusting the weights of numerous sensory inputs that constantly bombard our brains. Yet, the smell of our coffee may remind us that pleasant weekend breakfast through associative learning and memory. In the proposed project we will explore the function and the architecture of neural circuits that are involved in olfactory and gustatory information processing, namely habenula and brainstem. Moreover we will investigate the fundamental principles underlying multimodal sensory integration and the neural basis of behavior in these highly conserved brain areas.To achieve these goals we will take an innovative approach by combining two-photon calcium imaging, optogenetics and electrophysiology with the expanding genetic toolbox of a small vertebrate, the zebrafish. This pioneering approach will enable us to design new types of experiments that were unthinkable only a few years ago. Using this unique combination of methods, we will monitor and perturb the activity of functionally distinct elements of habenular and brainstem circuits, in vivo. The habenula and brainstem are important in mediating stress/anxiety and eating habits respectively. Therefore, understanding the neural computations in these brain regions is important for comprehending the neural mechanisms underlying psychological conditions related to anxiety and eating disorders. We anticipate that our results will go beyond chemical senses and contribute new insights to the understanding of how brain circuits work and interact with the sensory world to shape neural activity and behavioral outputs of animals.