Humans and laboratory animals are thought to discriminate sensory objects using elemental perceptual features computed by neural circuits in the brain. However, it is often difficult to identify the perceptual features that animals use to make specific comparisons. In olfaction, changes in the concentration of a given odor lead to discriminable changes in both its perceived quality and intensity. Humans use perceived intensity to compare quantities of different odors. Here we establish that laboratory rats also use perceived intensity to compare concentrations of different odors and reveal the perceptual organization of this elemental feature. We first trained rats to classify concentrations of single odors as high or low. When subsequently classifying concentrations of two odors presented on different trials of the same session, rats made errors consistent with using a single intensity criterion for both odors. This allowed us to investigate the relative perceived intensity of different odor pairs. Odor intensity was not only a function of concentration, but varied also with molecular weight and exposure time. These findings demonstrate the role of perceived intensity as an elemental perceptual feature of odors in rat olfaction.
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