PROJECT SUMMARY Attention is a critical executive function. Attention adaptively shapes sensory information in the brain such that signals associated with relevant cues are enhanced. In some sensory systems, this is accomplished by top- down inputs from the medial prefrontal cortex (mPFC) to the sensory thalamus, which ultimately results in attentional modulation of sensory representations within sensory cortex. While it was once proposed that the lack of thalamic relay within the olfactory system might indicate that attention could not be directed to odors, it is now clear that odor-directed attention does occur and may involve modulation of the olfactory cortex. For example, odor-responsive neurons in the olfactory tubercle (OT), one olfactory cortex region, exhibit enhanced odor signal-to-noise during attention in a manner that may facilitate odor perception. Still, the neural mechanisms underlying attention-dependent changes in odor representations are completely unknown. Because odor information is not processed by the thalamus before arriving at the OT, studying attentional modulation of odor information among these neurons will reveal novel insights into mechanisms of top-down attention in the brain. This project will use rodents to identify circuits integral for selective attention to odors using in vivo calcium imaging and optogenetic approaches in the context of a cutting-edge attention paradigm. Given the attentional modulation of OT single unit responses, and the importance of the mPFC for attention, this project will focus on the direct projection from the mPFC into the OT. First, this project will define attention-dependent responses in specific populations of OT-projecting mPFC neurons by monitoring aggregate calcium signals in the context of selective attention to odors. Next, optogenetic perturbation of these same populations will determine the behavioral relevance of OT-projecting mPFC neurons for selective attention to odors. Ultimately, the outcomes of this project will identify top-down neural circuits that orchestrate the effects of attention on odor processing and perception.