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Olfactory perception relies on the brain's ability to extract and represent different features of an odor stimulus. For example, intensity and identity are fundamental features of the olfactory percept that must be encoded separately for accurate perception; the same odor can be encountered at different strengths. Understanding neural representations of olfactory perceptual features is fundamental to understanding the olfactory system, however the neural correlates of odor intensity and identity are poorly understood in the human brain. The field of olfaction typically uses chemistry as a proxy for perception and focuses on manipulating physical properties of a stimulus to look for neural correlates. For example, chemical concentration and molecular identity are typically used as proxies for odor intensity and identity. However, the world that animals perceive is not a complete representation of physical reality; it is a product of the particular sensory systems that each species has acquired in the course of evolution. To understand how the brain forms representations of perceptual features of odors, we must identify relationships between features of neural responses and perceptual ratings. While novel behavioral paradigms in rodents can provide good estimates of perceptual ratings of different features of odors (intensity, identity), humans can directly provide perceptual ratings of different features of an odor. Therefore, human work is necessary to fully understand olfactory perception. The missing link between neural signatures and perceptual properties of odor is a critical gap in our understanding of odor coding. Here, we will combine human intracranial recordings-including direct neural recordings from human olfactory cortex-with simultaneous delivery of precisely controlled odors of varying intensity/identity and trial-by-trial collection of psychophysical ratings to understand how odors are represented in the human brain. Accurate olfactory psychophysical ratings are a critical part of our proposal. Therefore, we will work with leading experts in human psychophysics at Monel Chemical Senses Center, Dr. Pamela Dalton and Dr. Joel Mainland. As Co-Investigators on this grant, their expertise in human psychophysics will combine with my lab's expertise in human olfactory electrophysiology to provide the field with an understanding of the mappings between olfactory perception and neural codes in the human brain. To identify a neural representation of odor identity, it is necessary to dissociate molecular structure from perceptual identity/similarity. Whether the olfactory system employs a coding scheme in which the similarity between neural features is systematically related to perceptual similarity is unknown. Experiments for this aim will acquire perceptual responses while manipulating perceptual similarity. In Experiment 1A, we will examine features of neural responses during natural behavior of freely naming odors. In Experiment 1B, we will manipulate perceptual similarity with odor metamers and collect perceptual and neural data on each trial. Preliminary data suggest that the earliest, high-frequency components of the piriform response can optimally decode perceptual identity.