Watch any dog sniffing its way down the street and it’s obvious they’re expert at localising smells. They seem to know which direction a smell is coming from, much as we do with sounds and, of course, sights. We can certainly tell where a smell is by moving nearer or further from it. But once a smell reaches our nostrils, it’s not at all clear whether or not we can then say immediately which direction it came from.
One way to do that would be to compare the relative intensity of smells hitting each of our nostrils. After all, to judge spatial distances, we compare the discrepancy between visual information hitting our two eyes, and to source sounds, we use the volume discrepancy between our ears.
Jess Porter and colleagues at the University of Berkeley designed a special mask that could deliver smells to one nostril at a time. They tested 16 participants on a range of odours and found they could say which side the smell was on with 75 per cent accuracy – far better than chance performance.
They also took advantage of their clever apparatus to find out for the first time which brain areas were active when participants were judging the location of a smell, as opposed to identifying what the smell was. They found an area in primary olfactory context, in which greater activity was correlated with better accuracy on the localisation task. They also found an area in the superior temporal gyrus that was activated more in the localisation than the identification of odours. The same region has also been implicated in auditory and visual localisation, suggesting it may be involved in using multisensory information to localise stimuli in space. Brain areas activated more by odour identification than localisation included the postcentral gyrus, and an area spanning several occipital gyri (gyri are the bulges in the cerebral cortex, sucli are the ‘valleys’).
“We conclude that humans can spatially localise an odorant to the left or right, this ability may be subserved by nostril specific receptive fields within the primary olfactory cortex, as well as mechanisms of cross-modal integration in the superior temporal gyrus”, the authors said.
Writing in the same journal issue, Jay Gottfried of Northwestern University, said “these tantalising results are sure to arouse a vigorous response in the olfactory psychophysics community, and it will be exciting to see how the story of odour source localisation unfolds”.
Porter, J., Anand, T., Johnson, B., Khan, R.M. & Sobel, N. (2005). Brain mechanisms for extracting spatial information from smell. Neuron, 47, 581-592.