SUMMARY OF THE FUNDED PROJECT To survive, organisms must both accurately represent stimuli in the outside world and use that representation to generate beneficial behavioral actions. Historically, these two processes – the mapping from stimuli to neural responses, and the mapping from neural activity to behavior – have largely been treated separately. Of the two, the former has received the most attention. Often referred to as the “neural coding problem,” its goal is to determine which features of neural activity carry information about external stimuli. This approach has led to many empirical and theoretical proposals about the spatial and temporal features of neural population activity, or “neural codes,” that represent sensory information. However, there is still no consensus about the neural code for most sensory stimuli in most areas of the nervous system. The lack of consensus arises in part because, while it is established that certain features of neural population responses carry information about specific stimuli, it is unclear whether the brain uses (“reads”) the information in these features to form sensory perceptions. We have developed a theoretical framework, based on the intersection of coding and readout, to approach this problem. Experimentally informing this framework requires manipulating patterns of neuronal activity based on, and at the same spatiotemporal scale as, their natural firing patterns during sensory perception. This work must be done in behaving animals because it is essential to know which neural codes guide behavioral decisions. In the first phase of this project (funded by the BRAIN Initiative), we developed the technology necessary for realizing this goal. In the present proposal, we will further explore our patterned neuronal stimulation technology developed under the parent U19 to answer outstanding questions about neural coding and readout in the olfactory system that remain unaddressed to date under the parent award. We will pioneer the capacity to determine what neural dynamics within a population of cells are encoding behaviorally relevant information, and to determine the functional connectivity between these cells constituting this population. We will also develop tools and analysis methods to make targeted perturbations with holographic photostimulation to probe these dynamics to determine how they guide behavior. Finally, we will study neural coding principles across changes in behavioral state and during learning to determine how internal context and experience shape coding and readout. The contributions of the proposed work will be three-fold. First, we will both use and develop tools from the parent U19 to test theories of how neural populations encode and decode information throughout the brain. Second, we will reveal fundamental principles of spatiotemporal neural coding and readout in the olfactory systems of behaving animals using our data under the parent award to as a jumping off point. And t...