How The Brain Perceives Flavor
If you go back and listen to my interview with Professor Dan McCall, we talk about the route that flavor takes from the mouth and nose to the brain, which is where we experience it. And it’s important to note here that flavor is NOT one of the five senses—but rather, a fusion of two of them: taste and smell.
One obviously intriguing question is, how do our brains fuse the taste and smell data from these two systems into the unified experience of flavor? What does that process look like? This is something scientists have studied for ages.
In his book, Neurogastronomy, neuroscientist Gordon Shepherd succinctly describes the process like this:
“In sum: form a pointillist image, process it locally, format it globally, represent it in memory, enhance it with emotion, and perceive it consciously. Each of these steps is performed by its own [neural] microcircuit.”
Wow. That’s about as pithy of a statement about neuroscience as you’re ever going to find. So let’s break it down into steps.
Step 1: Form a Pointillist Image
What the heck does that mean?
Well, Dr. Gordon Shepherd, who is going to be our hunting guide here for the next segment of this episode, proposed a really fascinating metaphor that helps us to understand how we create things called “smell images” in the brain.
And he used the work of 19th century neo-impressionist painters to do it.
The painting technique of pointillism is basically when an artist will use a bunch of small dots or splotches on a canvas to create a larger picture. Instead of using sweeping strokes, all she or he uses are a collection of tiny dots and dabs. And if you’ve ever seen the impressionist work of famous painters like Van Gogh or Monet, this is basically their method, but on steroids.
Perhaps the most famous pointillist painter was Paul Signac, but my personal favorite is Théo van Rysselberghe of Belgium, especially when his work features the play of light on water.
And basically what Dr. Shepherd is saying is this: if you’ve ever walked into a room with large pointillist paintings on the wall, you might not, upon first entering the room, even notice that they’re made up of tiny little dots and dabs. Remember, this was the same shape-making tendency that helped our ancestors stay one step ahead of predators. We take an image that’s grainy or incomplete, and we kind of smooth out the edges to make it into the shape we think it should be, based on our brain’s visual schemas. But then, as you walk closer to these paintings, the dots will start to materialize. The image will become a bit fuzzy or grainy. And if you put your nose 2 inches away from the painting (as long as some museum steward doesn’t get nervous and ask you to leave) then you’ll be looking at a pattern of splotches that has almost no meaning whatsoever! It’s completely without context, and it’s more than a little disconcerting.
This is why, in my opinion, pointillism is one of the best studies of perspective in the art world. And this is why Shepherd uses it as a metaphor to explain how flavor (and particularly smell images) get more and more refined as they make their way through the flavor processing systems of the brain.
So, getting back to our step 1: form a pointillist image. How does that work at the flavor level.
Well, the individual dots and dabs, in flavor perception, are really individual electrical signals from our taste buds and olfactory receptors. And although these signals are similar, we’re going to focus on the olfactory receptors because of how much more complex they are than taste buds, of which there are only 5 types: (sweet, salty, bitter, sour, and umami (or glutamate)). Olfactory receptor types, however, number in the hundreds.
Think about this:
Our eyes have photoreceptors called cones, and there are three types that all have different sensitivity to red, green, and blue light. But as a result, we can perceive over 10 million individual colors.
Now imagine if you were dealing with hundreds of types of receptors instead of just 3. Yeah. That’s the olfactory system. The world of smell is so diverse, it’s like walking around in a world where we can see hundreds of times more colors than we see now…and we sometimes forget it’s even there.
Kinda sad if you think about it.
So, when smell molecules find their way to a receptor site in the nasal cavity, that site sends an electrochemical signal to a structure called the “olfactory bulb,” and the collection of those electrochemical signals is our pointillist image. Basically, it would be like standing right up next to the pointillist painting—just a collection of dots and dabs, and not a whole lot of context if you sample any individual grouping.
Step 2: Process it Locally
This is where the olfactory bulb takes information from the smell receptors about things like smell intensity and codes those into the representation of the smell image. This can be compared to adding depth perception to a visual experience. You go from a 2-dimensional image, to something with a bit more nuance and depth. Something that looks more similar to our actual experience of the world.
Step 3: Format it Globally
This is where the smell image gets prepped to be fused with information from the taste and touch receptors in the mouth and on the tongue. But to use a computer analogy, a smell file and a taste file need to be re-formatted to a common language before they can be successfully integrated.
This is what happens in an area of the brain just north of the Olfactory bulb called the “olfactory cortex.” And after this brain system is done doing its magic to the image, you can think of it as having taken several steps back from our pointillist painting to where you can still see the individual dots, but the image is largely comprehensible.
Once the pointillist image is formatted globally in the olfactory cortex, we can talk about the last three steps as working together, as complementary processes:
Represent it in memory, enhance it with emotion, and perceive it consciously.
This all happens in the neocortex, which is the very front part of the human brain—the part that evolved last and is most developed in humans, compared to other animals. The neocortex is where taste and smell info combine with things like texture, temperature, and visual inputs to form the overarching sensation of flavor.
And not only that, but flavor is assembled basically right next door to all the brain structures responsible for our memories and emotions, so when we’re consciously perceiving a flavor, it’s not just a sterile image assembled by our senses—it’s a rich and emotionally charged experience informed to a large degree by our memories of other experiences in the past.
And this is what usually resonates with us when we have a really noteworthy flavor experience. You walk into a room and immediately get whisked off to your childhood with memories of your mom’s pumpkin pie, or your grandfather chopping fresh vegetables.