Categorization allows organisms to efficiently extract relevant information from a diverse environment. Because of the multidimensional nature of odor space, this ability is particularly important for the olfactory system. However, categorization relies on experience, and the processes by which the human brain forms categorical representations about new odor percepts are currently unclear. Here we used olfactory psychophysics and multivariate fMRI techniques, in the context of a paired-associates learning task, to examine the emergence of novel odor category representations in the human brain. We found that learning between novel odors and visual category information induces a perceptual reorganization of those odors, in parallel with the emergence of odor category-specific ensemble patterns in perirhinal, orbitofrontal, piriform, and insular cortices. Critically, the learning-induced pattern effects in orbitofrontal and perirhinal cortex predicted the magnitude of categorical learning and perceptual plasticity. The formation of de novo category-specific representations in olfactory and limbic brain regions suggests that such ensemble patterns subserve the development of perceptual classes of information, and imply that these patterns are instrumental to the brain's capacity for odor categorization.

Learning associations between sensory stimuli and outcomes, and generalizing these associations to novel stimuli, are a fundamental feature of adaptive behavior. Given a noisy olfactory world, stimulus generalization holds unique relevance for the olfactory system. Recent studies suggest that aversive outcomes induce wider generalization curves by modulating discrimination thresholds, but evidence for similar processes in olfaction does not exist. Here, we use a novel olfactory discrimination learning paradigm to address the question of how outcome valence impacts associative learning and generalization in humans. Subjects underwent discrimination learning, where they learned to associate odor mixtures with either aversive (shock) or neutral (air puff) outcomes. We find better olfactory learning for odors associated with aversive compared to neutral outcomes. We further show that generalization gradients are also modulated by outcome valence, with the shock group exhibiting a steeper gradient. Computational modeling revealed that differences in generalization are driven by a narrower excitatory gradient in the shock group, indicating more discriminatory responses. These findings provide novel evidence that olfactory learning and generalization are strongly affected by the valence of outcomes. This adaptive mechanism allows for behavioral flexibility in novel situations with related stimuli and with outcomes of different valences. Because odor stimuli differ considerably from one encounter to the next, adaptive generalization may be especially important in the olfactory system.