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Researchers have visualised what happens when certain smells or aromas are interpreted by the brain as taste, helping to explain why things like flavoured waters are perceived as sweet despite containing no added sugar.
“We saw that the taste cortex reacts to taste-associated aromas as if they were real tastes,” says Dr Putu Agus Khorisantono, a cognitive and behavioural neuroscientist at the Karolinska Institutet, Sweden.
“The finding provides a possible explanation for why we sometimes experience taste from smell alone, for example in flavoured waters. This underscores how strongly odours and tastes work together to make food pleasurable, potentially inducing craving and encouraging overeating of certain foods.”
The flavours we perceive when eating or drinking arise from a combination of taste and the smell of ‘retronasal odours’ which reach the nose via the back of the throat.
This is why pinching your nose can make it easier to consume food which would otherwise be too gross to stomach, or suffering from a blocked nose is often accompanied by a loss of taste.
The team used functional magnetic resonance imaging (fMRI) of 25 healthy adults to investigate what happens when the brain integrates these parallel taste and smell signals into the perception of an overall flavour.
Th participants were initially taught to recognise a sweet-tasting solution with a sweet aroma, and savoury-tasting solution with a savoury smell.
They were then given either a tasteless aroma or taste without a smell while having their brain imaged.
The researchers trained an algorithm to pick up on the different patterns in brain activity associated with sweet and savoury tastes. They then tested whether the same patterns were present when the participants were given only sweet or savoury aromas.
They found that savoury and sweet tastes evoked activity in the granular, dysgranular and agranular portions of the insular cortex. The insula is part of the brain structure responsible for the perception of taste – the gustatory cortex.
Sweet and savoury odours prompted activity in the piriform cortex – associated with sense of smell – as well as in the dysgranular and agranular insula.
The findings suggest that the perception of flavour is integrated in the dysgranular and agranular insula, based on input from the granular insula and the piriform cortex.
“This shows that the brain does not process taste and smell separately but rather creates a joint representation of the flavour experience in the taste cortex,” explains Dr Janina Seubert, a cognitive neuroscientist and senior researcher at Karolinska Institutet.
“This mechanism may be relevant for how our taste preferences and eating habits are formed and influenced.”
The team now plans to investigate whether the same mechanism applies to ‘orthonasal odours’, or external smells.
“We want to find out whether the activation pattern in the brain’s taste cortex changes from salty to sweet when we walk from the cheese aisle to the pastries in the supermarket,” says Khorisantono.
“If so, this could have a significant impact on the foods we choose to consume.”
The research is published in the journal Nature Communications.