How exactly do our brains sort between the five taste groups: sweet, sour, salty, bitter and umami? We've now got a much better idea, thanks to research that has pinned down where in the brain this taste processing happens.
It's an area of the brain scientists have previously suspected could be responsible for sorting tastes, and which has been linked to taste in rodents, but this new study is much more precise in figuring out the role it plays in decoding what our tongues are telling us.
"We have known that tastes activate the human brain for some time, but not where primary taste types such as sweet, sour, salty, and bitter are distinguished," says one of the team, Adam Anderson from Cornell University in New York.
"By using some new techniques that analyse 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."
Anderson and his team used detailed fMRI scans of 20 adults as well as a new statistical model to dig deeper than previous studies into the link between the insular cortex and taste. This helped separate the taste response from other related responses – like the disgust we might feel when eating something sour or bitter.
Part of the problem in pinning down the taste-testing parts of the brain is that multiple regions of neurons get busy whenever we're eating something. However, this study helps to cut through some of that noise.
In particular, it seems that different tastes don't necessarily affect different parts of the insular cortex, but rather prompt different patterns of activity. Those patterns help the brain determine what it's tasting.
For example, one particular section of the insular cortex was found to light up – in terms of neural activity – whenever something sweet was tasted. It's a literal sweet spot, in other words, but it also showed that different brains have different wiring.
"While we identified a potential sweet spot, its precise location differed across people and this same spot responded to other tastes, but with distinct patterns of activity," says Anderson.
"To know what people are tasting, we have to take into account not only where in the insula is stimulated, but also how."
The work follows on from previous research showing just how big a role the brain plays in perceiving taste. It used to be thought that receptors on the tongue did most of the taste testing, but now it seems the brain is largely in charge of the process.
That prior study showed how switching certain brain cells on and off in mice was enough to prevent them from distinguishing between sweet and bitter. The conclusion is that while the tongue does identify certain chemicals, it's the brain that interprets them.
The new research adds even more insight into what's going on in the brain in humans when we need to work out what we're tasting – and shows just how important a job the insular cortex is doing.
"The insular cortex represents experiences from inside our bodies," says Anderson. "So taste is a bit like perceiving our own bodies, which is very different from other external senses such as sight, touch, hearing or smell."
The research has been published in Nature Communications.