SNIFFING AND SMELLING - SEPARATE SUBSYSTEMS IN THE HUMAN OLFACTORY CORTEX

The sensation and perception of smell (olfaction) are largely dependen t on snifting, which is an active stage of stimulus transport and ther efore an integral component of mammalian olfaction(1,2). Electrophysio logical data obtained from study of the hedgehog, rat, rabbit, dog and monkey indicate that sniffing (whether or not an odorant is present) induces an oscillation of activity in the olfactory bulb, driving the piriform cortex in the temporal lobe, in other words, the piriform is driven by the olfactory bulb at the frequency of sniffing(1-4). Here w e use functional magnetic resonance imaging (fMRI) that is dependent o n the level of oxygen in the blood to determine whether sniffing can i nduce activation in the piriform of humans, and whether this activatio n can be differentiated from activation induced by an odorant, We find that sniffing, whether odorant is present or absent, induces activati on primarily in the piriform cortex of the temporal lobe and in the me dial and posterior orbito-frontal gyri of the frontal lobe. The source of the sniff-induced activation is the somatosensory stimulation that is induced by air flow through the nostrils. In contrast, a smell, re gardless of snifting, induces activation mainly in the lateral and ant erior orbito-frontal gyri of the frontal lobe. The dissociation betwee n regions activated by olfactory exploration (sniffing) and regions ac tivated by olfactory content (smell) shows a distinction in brain orga nization in terms of human olfaction.