Ramifications for food formulation as fundamental research on food smells finds olfaction is uniquely a "dual" sense, in that the brain perceives the same smell molecule differently if it arrives through the nose, rather than the mouth.
Based on a recent study researchers at Yale University report that the smell of chocolate could activate different brain regions according to whether the odour was sniffed or tasted. In a joint research effort led by Dana M Small at Yale University and Thomas Hummel of the University of Dresden Medical School, the researchers launched their exploration into the brain's possible dual response to odours.
They were driven by the phenomenon that sensing an odour "orthonasally" through the nose triggers the perception that it is coming from the outside world, while sensing it through the mouth - or "retronasally" - causes the perception that it arises from the mouth. Its similar to saying that we like the 'taste' of a wine, because of its fruity or spicy notes. However, gustation refers only to the sensations of sweet, sour, salty, savoury, and bitter, and thus the pleasant 'taste' to which we refer is actually a pleasant odour sensed retronasally, say researchers.
Researchers say that pinching the nose while eating or drinking which blocks airflow from the mouth through the olfactory system - stops flavour perception and releasing the nose restores the sense of flavour in the mouth.
Examining the neural cause of olfaction's duality, researchers say they devised the first experiment to directly compare the same odorants introduced through the nose and the mouth. They claim that while several studies have examined only brain responses to retronasal olfactory stimulation, none have directly compared orthonasal and retronasal stimulation in the same subjects, or considered the possibility that the effects of route of stimulation depend on the way that odours are typically sensed.
In order to conduct the experiment scientists inserted small tubes into the noses of volunteers such: one tube ended at the nostrils and the other ended further back in the nasal passage near the throat, where odours from the mouth would originate.As they introduced odours into one tube or the other, they scanned the subject's brains using functional magnetic resonance imaging, a technique in which magnetic fields and radio waves detect increased blood flow to brain areas, which reflects increased activity. They used four odorants: chocolate for the food odour, and lavender for the non-food. They also chose two odorant chemicals - butanol and farnesol - to test a theory that the olfactory system distinguishes molecules according to whether they are more water soluble (butanol) or oily (farnesol).
Their results showed that the chocolate smell activated different brain regions according to the route of administration, supporting the duality of olfaction. The lavender odour did appear to activate different regions, but to a far lesser extent and the effect of route of delivery was greatest for the chocolate odour. The two different chemicals butanol and farnesol did not elicit significantly different brain responses according to the route of delivery, indicating that the properties of the molecules do not play a role in the response.
But researchers warn that the limited study, on just one food, means future experiments are needed to determine whether other food odours produce the same differential brain activations.