The locus coeruleus-norepinephrine (LC-NE) system has been proposed to have a critical role in regulating cognitive behavior, including arousal, perception, and decision making. However, we have limited understanding of how LC-NE modulates even simple aspects of sensory processing during perceptual behavior. Therefore, determining the fundamental neurobiology of LC-NE during quantitative perceptual behavior is a crucial first step toward understanding its role in modulating higher cognitive functions and neurological disorders. Uniquely, we have overcome several key technical limitations which have impeded fundamental mechanistic understanding of LC-NE functions. These limitations include the inability to record from chemically-defined LC- NE neurons during behavior, and the difficulty of quantifying their modulatory effects on target neurons. This proposal will test the central hypothesis that LC-NE differentially modulates cortical excitatory and inhibitory responses to facilitate sensory perception. We will determine: How does LC-NE influence behavioral outcomes? How does LC-NE modulate neuronal activity in the primary somatosensory cortex? The proposed research is innovative, in our opinion, because it allows recording and perturbing chemically-defined LC-NE neurons while simultaneously monitoring their target neurons in the cortex, during a quantitative perceptual task. This approach represents a substantive departure from the status quo. Combining dual-electrophysiological recordings, optogenetic gain-/loss-of-function, and well-controlled behavior, this novel approach allows investigating previously inaccessible mechanisms of LC-NE modulation in awake behaving mice. The research is significant because it is expected to fundamentally advance our understanding of LC-NE functions by establishing mechanistic links between LC-NE modulation of cortical sensory response and its modulation of perceptual decisions. The new techniques can be adapted to other neuromodulatory systems, and are expected to contribute to a broader understanding of brain functions. The results and new methods have the potential to help better understand LC-NE functions in higher cognitive processes, and to facilitate the development of therapeutic methods to treat neurological disorders.