Ramifications for food formulation as fundamental research on chocolate 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.
Researchers at Yale University in the US report the smell of chocolate activated different brain regions according to whether the odour was sniffed or tasted.
How consumers sense food is crucial knowledge for a food industry constantly organising the building blocks of new food formulations.
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.
"The illusion that retronasally perceived odours are localised to the mouth is so powerful that people routinely mistake retronasal olfaction for taste," they write.
For example, we may say 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, they add.
The role of olfaction in taste is powerful. The scientists point out that pinching the nose while eating or drinking - which blocks airflow from the mouth through the olfactory system - stops flavour perception. Releasing the nose restores the sense of flavour in the mouth.
"The fact that the olfactory referral illusion is maintained even though the subject is now aware that the experience is related to an event in the nose demonstrates that olfactory referral is robust and cognitively impenetrable," continue the researchers.
Examining the neural cause of olfaction's duality, the researchers devised the first experiments to directly compare the same odorants introduced through the nose and the mouth.
They claim that while several studies have examined 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.
"For example, food odours are normally experienced both orthonasally and retronasally, whereas nonfood odors are perceived only orthonasally. Therefore, it is possible that the route of stimulation may have different effects for food versus nonfood odors," writes Small.
The 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).
The researchers found 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.
"The effect of route of delivery was greatest for the chocolate odour," report the researchers.
This raises the possibility that odorant administration interacts with experience to engage unique brain regions, they continue.
"Olfactory referral induced by retronasal stimulation creates a differential reward context for food, but not for non-food odours, by signaling availability versus receipt of food," they conclude.
But the 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.
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.
Full findings for the study are published in the August 18, 2005, issue of Neuron.
Previous research into taste has revealed that the human tongue has about 10,000 taste buds with five taste sensations: sweet, bitter, and umami, which work with a signal through a G-protein coupled receptor; salty and sour which work with ion channels.
Contrary to popular understanding, taste is not experienced on different parts of the tongue. Though there are small differences in sensation, which can be measured with highly specific instruments, all taste buds, essentially clusters of 50 to 100 cells, can respond to all types of taste.
Taste buds (or lingual papillae) are small structures on the upper surface of the tongue that provide information about the taste of food being eaten.