Letters to Nature

Nature 392, 282-286 (19 March 1998) | doi:10.1038/32654; Received 25 August 1997; Accepted 9 December 1997

Sniffing and smelling: separate subsystems in the human olfactory cortex

N. Sobel1, V. Prabhakaran1, J. E. Desmond2, G. H. Glover3, R. L. Goode4, E. V. Sullivan1,5 & J. D. E. Gabrieli1,2

  1. Program in Neuroscience, Standford University, Standford, California 94305, USA
  2. Departments of Psychology, Standford University, Standford, California 94305, USA
  3. Departments of Radiology, Standford University, Standford, California 94305, USA
  4. ENT Surgery, Palo Alto VA Hospital , Palo Alto, California 94305, USA
  5. Psychiatry & Behavioral Sciences, Standford University, California 94305, USA

Correspondence to: N. Sobel1 Correspondence and requests for materials should be addressed to N.S. (e-mail: Email: nsobel@leland.stanford.edu).

The sensation and perception of smell (olfaction) are largely dependent on sniffing, which is an active stage of stimulus transport and therefore an integral component of mammalian olfaction1,2. Electrophysiological 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 sniffing3, 4, 5, 6. Here we use functional magnetic resonance imaging (fMRI) that is dependent on the level of oxygen in the blood to determine whether sniffing can induce activation in the piriform of humans, and whether this activation can be differentiated from activation induced by an odorant. We find that sniffing, whether odorant is present or absent, induces activation primarily in the piriform cortex of the temporal lobe and in the medial 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, regardless of sniffing, induces activation mainly in the lateral and anterior orbito-frontal gyri of the frontal lobe. The dissociation between regions activated by olfactory exploration (sniffing) and regions activated by olfactory content (smell) shows a distinction in brain organization in terms of human olfaction.