Locating objects of interest in space is a vital aspect of sensation. In most mammals, olfaction plays an important role in localization, yet the neural basis of this ability remains poorly understood. As animals move toward an odor source, the odor concentration changes from sniff to sniff. These concentration changes have been shown to guide odor source localization in studies of freely moving animals. Yet studies in freely moving animals do not allow sufficient stimulus control for precise study of olfactory neuronal or behavioral responses. We have developed a system for presenting stimuli that rapidly change concentration to head- fixed mice. In this proposal, we will use this system to mimic odor concentration fluctuation during source localization and to study how the mouse olfactory bulb represents stimuli that change concentration from sniff to sniff. In preliminary experiments, we have found that a subset of neurons in the olfactory bulb is sensitive to concentration changes, modulating their activity when the odor concentration changes across sniffs. In Specific Aim 1, we will investigate the novel response properties of these neurons in electrophysiological recordings. In Specific Aim 2, we will take an optogenetic approach to test whether these concentration change sensitive neurons are a functional cell type. Finally, in Specific Aim 3, we will record in animals performing a precise psychophysical task in which they report detection of concentration changes. We will ask whether activity in concentration change sensitive neurons can predict the animal's perceptual report. By defining the neural representation of ethologically relevant odor dynamics, our work will advance a general understanding of how the brain operates with input that changes through time, which is essential to guide successful behavior.