Odor quality and odorant structure are functionally dissociable in human piriform cortex
In the present study, we used functional magnetic resonance imaging (fMRI) techniques to determine whether human piriform cortex encodes information about perceptual or structural determinants of smell (Gottfried et al., Neuron 2006). As used here, the term "quality" denotes odor-object identity, i.e., the perceptual character of a smell emanating from an odorous object (in contrast to other odor qualities such as intensity or valence). We scanned healthy subjects during an olfactory version of fMRI cross-adaptation in which odor quality and odorant structure were independently manipulated. On each trial, subjects made two successive sniffs to pairs of odorants that varied either in perceptual quality ("lemon-like" or "vegetable-like") or molecular functional group (alcohol or aldehyde). We hypothesized that if piriform cortex (PC) represents odor quality, independent of functional group, then sequential presentation of qualitatively similar odorant pairs should elicit cross-adapting (decreased) neural activity, by comparison to qualitatively dissimilar pairs. In contrast, structural (group) features of odorants should have no impact on neural cross-adaptation within such a region.
Our results indicate a double dissociation in PC, whereby posterior regions encoded perceptual quality (but not molecular functional group) and anterior regions encoded functional group (but not quality), in accordance with animal models suggesting that PC is anatomically organized along an anterior-posterior axis. A second complementary study based on an independent group of subjects and a different set of odorants (Gottfried et al., 2006; Experiment 2) confirmed these same findings. We speculate that the presence of structure-based codes in anterior PC would help maintain the fidelity of sensory information arising from the OB. In turn, the presence of quality-based codes in posterior PC, independent of any simple structural configuration, implies that synthetic mechanisms are critical for the neural organization of odor objects, much in the same way that visual objects are assembled in visual association cortex.