The olfactory bulb (OB) is the first processing station of olfactory sensory information in the brain. This subcortical area sends information to several olfactory cortical centers and receives direct back projections forming extensive cortico-subcortical loops. So far, the precise physiological function of these loops is poorly described. A prominent theory derived from computational modeling suggests that olfactory cortico-subcortical loops relay odor-evoked neuronal activity back to the OB, thereby enhancing the detection of "foreground" odors in noisy backgrounds. This ‘Figure-background separation’ is essential for many sensory processing modalities. In this tandem proposal between the Rothermel and Schwarz labs, we will study the relation of reciprocal neuronal activity between OB and ist associated cortical centers to uncover their contribution to figure-background separation. Specifically, we will functionally compare the cortical loops formed between OB and anterior olfactory nucleus (AON), as well as OB and anterior piriform cortex (APC). To this end, we will employ dual-color 2P Ca2+ imaging of cortical fibers and OB output neurons simultaneously (Objective 1) to investigate their spatio-temporal relation in different olfactory tasks. Data from these experiments will be used to refine existing OB-olfactory cortex models of figure background separation (Objective 2). In Objective 3 we will then establish an activity-dependent labeling approach for the olfactory cortex. Odor-dependent labeling of active AON and APC neurons will allow insights into their differential connectomic/projectomic composition in unprecedented detail using light sheet fluorescence expansion microscopy (LSFEM). Finally, the expression of optogenetic actuators in activity-tagged neurons will allow us to probe into the causal relationship of AON/APC activity in odor detection and figure background separation (Objective 4). Successful completion of this project will fundamentally increase our understanding of the physiological implications of OB-cortical loops for olfactory processing and odor-guided behaviors.