The five senses are usually studied in isolation and there is no doubt that this ‘divide and conquer’ method has given us very valuable insight into the way the brain processes sensory information. However, in our daily life, we combine inputs from all sensory channels to make sure we know what’s happening around us. If there is a fire in your house, it doesn’t really matter if you saw a flame, smelt the smoke, felt the warmth, or heard the fire alarm, you’d better get out fast.
Some senses seem to be more closely associated than others: olfaction and taste combine to give the percept of flavour, whereas associating odour and sounds seem less obvious. However, a team from the flavour and fragrance company Givaudan recently reported that odours could be reliably matched to pitch[1], an idea already put forward by a perfumer more than a hundred years ago.[2] A recent study by Wesson and Wilson[3], relayed by a story in Scientific American, suggests a possible neural mechanism for olfactory-auditory interactions.
While studying the responses to odours of neurons in the olfactory tubercle (a part of the basal forebrain) of rats, they discovered by chance that these neurons were also responding to sounds, providing one more confirmation of Asimov’s statement: “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ (‘I found it!’) but rather ‘hmm….that’s funny…’ ”. So, on top of confirming that the olfactory tubercle is activated by odours (65% of the units they recorded from), they showed that 19% of them also responded to a tone. Furthermore, 29% of the units tested showed a modulation of the response (either a superadditive or suppressive effect) when both the odour and the sound were presented simultaneously.
These results are of course still far from explaining the mechanism of the associations observed by the Givaudan team, but they show that cross-modal modulations happen early in sensory processing. Further research is needed to explore the range of sounds the olfactory tubercle responds to, as well as how the observed modulations impact behaviour.
Step by step, through both physiological and behavioural studies, we are getting a better understanding of the importance of cross-modal modulations in our perception of the world. ‘Divide and conquer’ might have been a good strategy to study the senses, but now it might be time to try and put the pieces back together.
Anne-Sylvie Crisinel is currently PhD student at the Department of Experimental Psychology, Oxford University.
Peeples, L. (2010). Making Scents of Sounds: Noises May Alter How We Perceive Odors Scientific American
References
[1] Belkin, K., Martin, R., Kemp, S., & Gilbert, A. (1997). AUDITORY PITCH AS A PERCEPTUAL ANALOGUE TO ODOR QUALITY Psychological Science, 8 (4), 340-342 DOI: 10.1111/j.1467-9280.1997.tb00450.x
[2] Piesse, G. (1867). The art of perfumery, and the methods of obtaining the odors of plants. By G. W. Septimus Piesse. Second American, from the third London edition. Lindsay & Blakiston, Philadelphia, 1867 Journal of the Franklin Institute, 83 (6), 423-424 DOI: 10.1016/0016-0032(67)90381-X
[3] Wesson DW, & Wilson DA (2010). Smelling sounds: olfactory-auditory sensory convergence in the olfactory tubercle. The Journal of neuroscience : the official journal of the Society for Neuroscience, 30 (8), 3013-21 PMID: 20181598