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.

Distributed neural activity patterns are widely proposed to underlie object identification and categorization in the brain. In the olfactory domain, pattern-based representations of odor objects are encoded in piriform cortex. This region receives both afferent and associative inputs, though their relative contributions to odor perception are poorly understood. Here, we combined a placebo-controlled pharmacological fMRI paradigm with multivariate pattern analyses to test the role of associative connections in sustaining olfactory categorical representations. Administration of baclofen, a GABA(B) agonist known to attenuate piriform associative inputs, interfered with within-category pattern separation in piriform cortex, and the magnitude of this drug-induced change predicted perceptual alterations in fine-odor discrimination performance. Comparatively, baclofen reduced pattern separation between odor categories in orbitofrontal cortex, and impeded within-category generalization in hippocampus. Our findings suggest that odor categorization is a dynamic process concurrently engaging stimulus discrimination and generalization at different stages of olfactory information processing, and highlight the importance of associative networks in maintaining categorical boundaries.DOI: http://dx.doi.org/10.7554/eLife.13732.001

Part-set cuing inhibition refers to the counterintuitive finding that hints--specifically, part of the set of to-be-remembered information--often impair memory performance in free recall tasks. Although inhibition is the most commonly reported result, part-set cuing facilitation has been shown with serial order tasks. The present study examined the influence of part-set cuing for spatial locations using novel methods and materials. Participants viewed the construction of Snap Circuit objects and then attempted to reconstruct the objects in either the presence or absence of part-set cues. Two experiments revealed robust part-set cuing facilitation on the spatial memory tasks. Generally, these results are consistent with the predictions of the retrieval strategy disruption hypothesis (e.g., D. R. Basden & Basden, 1995) and the 2- and 3-mechanism accounts of part-set cuing (Bäuml & Aslan, 2006; Bäuml & Samenieh, 2012).

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