What’s your favorite food? And what foods make you turn up your nose in disgust? These are probably easy questions to answer, but have you ever stopped to wonder how you know what flavors you love and what flavors you hate?
You’ve probably seen that diagram of a tongue split into zones for five different tastes, and you may also know that scientists recently discovered a sixth one. Tastebuds detect different chemicals within our food that we recognize, like bitterness or sweetness, but that’s really just the start. The sensory information picked up by the tongue has to travel to the brain, and that’s where the story of flavor can really begin.
The gustatory cortex
As important as taste is to our human experience – a fact that was thrown into sharp relief when people started losing their sense of taste after a COVID-19 infection – neuroscientists know comparatively little about it when compared with our other senses.
It took many years of research for scientists to begin to understand the intricacies of how the brain perceives taste. In 2011, neurobiologist Charles S. Zuker and a US-based team published the first “gustotopic map” of the mammalian brain, using sophisticated imaging techniques to show how different parts of the cortex respond to specific tastes.
Taking this work further, Zuker co-led a study published in 2015 that manipulated the bitter and sweet areas of the gustatory cortex in mice. When the mice were given plain water whilst their bitter areas were being stimulated, they behaved as though the water tasted unpleasant. The opposite was observed when it was the sweet area of the cortex being stimulated.
They didn’t even need to be physically eating or drinking for the effect to take hold. When the mice were simply offered the choice of two chambers in their enclosure, they quickly learned to avoid one of them when their bitter taste centers were stimulated each time they went inside.
“Taste, the way you and I think of it, is ultimately in the brain,” Zuker said in a statement at the time. “Dedicated taste receptors in the tongue detect sweet or bitter and so on, but it’s the brain that affords meaning to these chemicals.”
“By using some new techniques that analyze fine-grained activity patterns, we found a specific portion of the insular cortex – an older cortex in the brain hidden behind the neocortex – represents distinct tastes,” explained senior author Adam Anderson in a statement.
The insula that Anderson refers to is an unassuming structure buried deep within the fissure of the lateral sulcus.
The work by Anderson and the team found that the map of taste perception in the human brain was likely more complicated than the one that had already been characterized in animals, but that there was one part of the insula where all of the different taste areas could be found: a gustatory cortex.
Three of the cranial nerves, the major nerves in the face and head, link the tongue to the brainstem. Information received from the taste receptors on the tongue is relayed via this pathway, eventually reaching the thalamus, which then forwards the information on to the gustatory cortex.
Like so many brain structures, you actually have two gustatory cortices, one in each hemisphere. Tastes sensed by receptors on the left side of the tongue are primarily processed in the left hemisphere, and vice versa.
The study from Anderson and colleagues also began to unravel how our likes and dislikes are represented in the gustatory cortex, something that he said had been challenging in the past: “One of the difficulties in prior work on the connection between the brain and taste specifically is that tastes come with strong associated hedonic responses, like sweet tastes good and bitter bad. So we have not known if these taste regions are really dedicated to taste, but rather hedonics or palatability of taste.”
Taste vs. flavor
We’re learning more and more about what happens in the brain to allow us to taste food. But flavor is about much more than just the combinations of chemicals sensed by our tongues. A good example of this is airplane food – never the most appetizing, mainly because consuming food thousands of meters in the air is profoundly unnatural for a human being. Everything from the dry, recycled air to the incessant noise of the engines combines to dampen your ability to enjoy your dinner in the sky.
Greater understanding of the multisensory nature of flavor has led to the birth of a new branch of science called neurogastronomy, a term that was only coined in 2006. According to its principles, flavor is not a physical entity that’s held within an item of food, but something the brain creates by combining information from all of our senses. It’s similar to how pain is only felt because our brains create the sensation in response to a stimulus that could be harmful to us.
To take one example, our sense of smell is absolutely key to our ability to perceive flavor, as you’ll no doubt be aware if you are a human with a nose who has ever had a cold.
“If I ask what flavor is, most people will say ‘taste’ – the flavor of food and the pyramid of food that we’ve created in the Western world is very much based on taste, not the smell component,” Fahmeed Hyder, a professor of biomedical engineering and radiology and biomedical imaging, told Yale Engineering Magazine.
“But a big part of flavor is actually the other part of the chemosensation – the smell components. Smell happens pretty much – not just in humans but also animals – as we chew our food. When we chew the food, molecules are released and become airborne.”
The brain receives sensory information about odors in a structure called the olfactory bulb, located right at the front between the eyes. There are two paths that odors can take to reach the olfactory bulb, and scientists are still working out how each of these contributes to taste perception.
The orthonasal route is what we traditionally think of as smelling: odors entering the nose when we inhale. There’s also the retronasal route, which is when compounds enter the nasal cavity through the back door, as it were, via the mouth when we chew food.
Hyder led a study investigating this mechanism using functional magnetic resonance imaging (fMRI) in rats, finding that there are differences in how the brain responds to stimuli received via each route.
As with taste perception via the tongue, we’re still at quite an early stage in our understanding of how the brain combines different chemical signals into what we experience as flavors. For too long, both of these senses have been neglected, but Hyder explained how this research could have much wider implications.
“It’s been shown that a lot of diseases – especially among those with onset later in life – affect smell much more than taste. That fact hasn’t been appreciated much in the treatment of disease, mainly because smell hadn’t been considered an important sensation in practiced medicine.”
“But much like how we see and hear,” he added, “taste and smell are all critical aspects to being human.”
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