Have you ever been walking down the street when you stopped dead in your tracks and thought “OH GOD OH GOD HOW DO I TASTE THINGS?!! HOW DOES MY BRAIN LET ME TASTE SALTY PRETZELS UNNNGHHHH OH GOD I DON’T KNOW FFFFFFFFFFFFFWHAT DO I DO?”. I bet you have. I haven’t, but that’s because I do know (in a very rough approximation), although I imagine that if I didn’t know, I would be shrieking and my eyes would be bulging out of their sockets and I would possibly be convulsing in the middle of a road somewhere. Such is life.
So that’s it. It’s neuroscience time. None of this namby-pamby psychology stuff with people reading lists of words and acting ever so slightly differently as a result. We’re getting on a little nano-rocket and riding into the neuron metropolis. I’m a neuroscientist and I’m bringing my A-game. Are you? Yes? Is it folded up in your backpack? Ok.
Right then. After your tastebuds have detected food and the basic information about the food has been transmitted along nerves to the more fancy parts of the brain, what does the brain do? How do the actual neurons in the brain respond, and what sort of information do they respond to?
I’m going to concentrate on just one part of the brain for now – the primary taste cortex, consisting of sections called the insula and the frontal operculum. This primary taste cortex is place where the brain starts to integrate all the different bits of information about food. After this complex processing has begun, the primary taste cortex interacts with a huge range of other brain areas that are involved in the processing and perception of vision, smell and touch (not surprising, given the multisensory experience that is eating).
The other senses also have primary cortices where this sort of processing of sensory information goes on at a higher level, and taste perception shows some striking similarities to perception in the other senses.
For instance, in the primary visual cortex, there are neurons that only fire when you see very particular things. For example, there are neurons that fire in response to a vertical line in your field of vision (the side of house, a flagpole, etc) but don’t fire or decrease their firing below normal in response to a horizontal line (the horizon, the top of a desk, etc), and likewise, there are some neurons that only fire in response to horizontal lines and they’re not so responsive when presented with vertical lines. Neurons can be very specifically tuned and only respond to a very distinct stimulus, or they can be more general and fire in response to a range of stimuli. The same is the case in the primary taste cortex.
In the primary taste cortex, neurons respond to lots of different properties of food. Verhagen and colleagues looked at neurons in the primary taste cortex and how these neurons fired in response to food of varying taste, temperature, grittiness, viscosity and fat texture. They found that neurons can have a very specific profile of stimuli that they actually fire in response to.
53% of the neurons tested fired in response to the thickness or viscosity of food. When the researchers tested different viscosities (using carboxymethylcellulose), some neurons fired more in response to very thick consistencies whereas others fired in response to a runnier consistency.
8% of the neurons tested responded to grittiness as a consistency. So I guess these neurons will be firing like mad if you eat porridge with sand in it.
Another 8% of neurons tested actually responded to fat, but they identified fat by its texture, not by any chemical method. The researchers figured this out because the same neurons fired in response to non-fat oils that had the same texture as the fats tested.
35% of neurons tested responded to the temperature of whatever was in the mouth. The researchers used water at different temperatures (10°C for a chilled drink, 42°C for a warm drink, 37°C for body temperature and 23°C for room temperature) and some neurons fired more in response to particular temperatures than others.
6% of the neurons tested responded to capsaicin, the hot compound of chilli peppers. The interesting thing here is that these neurons didn’t respond to the warmest water temperature (42°C). So even though capsaicin is experienced as heat, it didn’t result in firing of the neurons that had fired to a warm liquid. However, it turns out that 42°C might have just been an unlucky temperature to choose as the upper limit in the experiment, as capsaicin’s hot effect is achieved through a particular type of receptor that only responds to temperatures greater than… 43°C. So if a hotter liquid had also been used in the experiment, then maybe neurons would have been found that fired in response to the hot liquid and capsaicin.
And of course, a fair few neurons responded to taste. 56% of neurons tested responded to taste, which was tested using blackcurrant juice (for sweetness), table salt (for saltiness), weak hydrochloric acid (for sourness), quinine (for bitterness) and MSG (for umami).
Now, these neurons didn’t always just fire preferentially for taste or temperature or viscosity or whatever — about half of them fired in response to combinations of these classes of stimuli. For example, 23% of them fired in response to both taste and temperature. A couple of them fired in response to taste and temperature and viscosity and fat. So there was overlap between the groups of neurons involved in responding to the different properties of the food, which allows for a much more complex and nuanced representation of food in the brain.
The interesting thing was that these neurons in the primary taste cortex did not fire in response to odor or the sight of food. This tells us that it isn’t until a later stage that taste information and visual and olfactory information get integrated. And that is indeed what the pathways of sensation in the brain tell us (as seen in the diagram below that I hastily knocked together). It’s rather complicated but hey, that’s the brain for you:
So don’t worry about understanding this whole mess (simply stand back and appreciate the complexity of that soft lumpy thing inside your skull). Just know that arrows indicate the flow of information, and blunt-ended lines indicate an inhibitory effect where the flow of information is dampened. And you can see that in the pink taste pathway, by the time information has gotten to the primary taste cortex from the taste receptors in the tongue, it hasn’t interacted with any of the other pathways. But in the next step, when information flows into the orbitofrontal cortex and amygdala, it has its first chance to interact with information from the other sensory modalities, vision (green), smell (gold) and touch (blue).
The orbitofrontal cortex is also the part of the brain responsible for the representation of the palatability and pleasantness of food, which means that how enjoyable food is probably results from an interplay of visual, taste, olfactory and touch factors. So no matter how delicious that pie tastes, if it happens to look like horse manure then the orbitofrontal cortex just isn’t going to assign as much of a pleasant experience to it.
So there you go. The basics of how the brain processes taste. But as is always the case, it’s a whole lot more complicated than that. When Verhagen and colleagues were looking at the firing properties of neurons in the primary taste cortex, they only reported on the neurons that responded to at least one of the stimuli in the experiment, whether it be a particular taste or viscosity, the presence of grittiness or fat, water of a particular temperature, and so on. In all, they tested 29 different stimuli to see if neurons fired in response. They found 62 neurons that did this. They found 1,060 that didn’t.
What are these other 1,060 neurons doing? What are the other hundreds of thousands of neurons that weren’t tested in the primary taste cortex doing? What do they respond to? What’s their job?
The science continues!
And in the meantime, while you wait, have a HobNob shake. It’s based on the recipe for the Max Brenner cookie shake, lord among shakes that it is. It looks good, it tastes good, it smells good and… well I guess it has a good texture, in the scheme of things. Your orbitofrontal cortex is going to assign awesomeness to this via the striatum so hard.
Read on for the recipe for a HobNob shake.