The senses of taste and smell are closely intertwined, so much so that simply inhaling the aroma of favorite foods can trigger the memory of their flavor without any deliberate thought.
Recent research featuring brain imaging and machine learning techniques has, for the first time, identified the specific brain area involved in this sensory interrelation in humans. This key region is known as the insula, as detailed by researchers in a study published on September 12 in Nature Communications.
According to Ivan de Araujo, a neuroscientist at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, these findings help clarify why individuals develop cravings for certain foods or feel repulsed by others.
As mouthfuls of food are consumed, smell and taste begin reinforcing each other, explains Putu Agus Khorisantono, a neuroscientist at Karolinska Institutet in Stockholm. Certain food compounds engage our sweet, salty, sour, bitter, or umami taste receptors. Other compounds can penetrate through the roof of the mouth, triggering olfactory receptors located at the back of the nasal cavity. These “retronasal odors” allow us to differentiate between foods like mangoes and peaches, which may both have primarily sour tastes but are distinguished by their unique aromas, Khorisantono elaborates.
While flavor perception relies on the integration of these sensory signals, understanding where this integration occurs in the brain has posed a challenge for scientists.
In the study, Khorisantono and collaborators engaged 25 participants with specially designed beverage droplets that targeted only their taste or retronasal smell receptors during multiple brain activity scans. These individuals had previously associated specific combinations of aromas and tastes with particular flavor profiles.
The research team identified potential brain regions by manipulating results from the scans and employing these to train machine learning algorithms. Initially, the algorithms distinguished between brain responses to sweet and savory flavors. Subsequently, Khorisantono evaluated the algorithms’ proficiency in predicting what participants had smelled based on their brain imaging.
A standout algorithm was trained on scans from the insula, a brain region once believed to respond solely to taste sensations. Khorisantono suggests that its involvement indicates that the insula processes both aroma and taste in comparable ways, as indicated by similar patterns observed in scans when participants experienced either the aroma or taste of the same flavor. Other brain regions may derive flavor information from the insula, which stands as an initial processing hub for flavor perception, according to Khorisantono.
The insula associates closely with internal bodily functions, like digestion, and subtly shapes our eating habits and body perceptions. However, studying this region presents difficulties due to its deep location within the brain. Research in rodent models has shown that the insula is involved in smell processing, with certain neurons responding distinctively to specific smell-taste combinations, suggesting its role in flavor interpretation.
Investigating its functions in humans has only become feasible due to the advanced computational and statistical methods employed in this study, notes de Araujo.
Khorisantono’s focus has been solely on retronasal smells, but he’s also curious about whether aromas that elicit food cravings—such as those encountered while passing a cheese counter or restaurant—impact the insula. His ongoing research will explore this possibility.
