A study uses hot sauce, of all things, to show how what we expect influences what we perceive.

Our perceptions of reality are already fragile things. We perceive the world through our eyes, ears, sensory nerves in our skin, proprioceptive receptors, and integrate all that information into our brains to create a model of reality that allows us, at least, to navigate through it. But these systems are fallible – as simple illusions illustrate, prone to error, and dependent on prior experience which may color our perceptions for the rest of our lives.

And as if that isn’t bad enough – what we perceive as reality is shaped by our very expectations.

If you give a patient a pill, even a sugar pill, and they expect that it will help them, it will help them. If you tell them the pill you gave them often causes nausea, they will become nauseated.

But what fascinates me about this phenomenon is that that it’s not just confirmation bias. People aren’t faking the benefit, or faking the nausea. There are real, measurable changes happening in the brain when expectation and reality meet.

And this week, we are exploring that meeting…. With hot sauce.

Probing the complex interplay of expectation and perception is not easy. I mean, even those terms themselves are rather vague when you think about it.  They are concepts – words we have for some stuff that happens in our brains. If I were to start to think about designing a study to test the effect of expectation on perception – I would probably decide I needed an fMRI machine, which I very much do not have.

But fortunately, others do, and I was struck by the elegant design of this study of expectation and perception, appearing in PLOS Biology.

I’ll get to the results in a minute, but first I think it’s worthwhile to explain how this experiment worked.

A victim – sorry a volunteer was placed inside an fMRI machine which could scan their brain as they sipped fluid out of a tube.

The fluid could be one of three things: regular water, a hot sauce with a low level of spice in it, or a hot sauce with a high level of spice in it. The participants had no idea in what order these fluids would come. In each run of the experiment – thirty sips were delivered, in random order, with a water rinse in between each.

The first run was a control condition.  Prior to each sip, regardless of what fluid was coming, the participant would see an image that looked like this – two grayed-out peppers.

They would take sip, and say how spicy the fluid was, and how much they liked it.

On to the second part of the experiment – and here’s where expectation comes into play. Prior to each sip, those peppers were displayed again. But this time, there were two blue peppers if the squirt would be water, a blue and a red pepper if the sauce would be mild, and two red peppers if the sauce would be hot.

I know – you’re curious what would happen if you tricked people – told them hot was coming and then it was just water. I’m curious about that too, but that’s not how this worked. These were honest icons.

OK so now we have three levels of spiciness, and two conditions – one where no information is available to the patient prior to the squirt, and one where they are “warned” for lack of a better word.

So the question is… what happens.  How does expectation shape perception?

Before I can give you the answer though, we run into an interesting problem – one somewhat unique to spiciness itself.  Some people like spicy, and some people really don’t. That adds a layer of complexity to the whole thing, because someone who is really into spicy food who sees those two hot peppers might get excited about how great and spicy the next sip will be, while someone who hates spiciness might become terrified about it. As such, the authors stratify all their results based on whether people like spicy foods or not and since that’s what they did, that’s what I’ll show you.

Let’s start simply, by looking at how people rated the hotness of the fluid they were exposed to.

The spice-lovers rated the spiciness of the hottest sauce pretty similarly even when primed with those two red peppers. The red/blue pepper combo led to them rating the milder sauce even cooler than they had under the control condition.

The people who hated spice were more or less unaffected by the expectation-priming – at least in terms of rating the heat of the sauces themselves.

But what about the pleasure derived from the sauces?  For those who like spicy stuff – priming with information about the level of spice didn’t do very much.  They liked the spiciest sauce the most, and water the least.

For those who did not like spiciness, the presence of those two red peppers significantly decreased their pleasure ratings.  Same with the one red pepper actually. Basically, they saw that spice was coming, prepared to not like it, and really didn’t like it.

This happens pretty much every night at my dinner table when I beg my youngest to try a food she has never had before. She decides before it even goes in her mouth that she hates it and when, finally, she tries it – she hates it all the more for that expectation.

Of course, the cool part of this study is that this whole scenario is playing out inside an MRI machine. Like, we can literally see which areas of the brain are lighting up when all of these expectations and perceptions are being made.

Using an fMRI, you can actually “see” pain in the brain, by which I mean you can see areas of the brain associated with pain perception becoming more active.

Among those who like spiciness, there was really no interesting differences in pain activity in the brain. The spicy stuff didn’t cause more pain pathways than water, and expectation priming had no effect either. 

But look at the people who don’t like spicy food.

Under the control condition – their pain levels were a bit higher with spicy stuff than with water – but when they were primed with information about what was coming – there was a dramatic difference. Pain pathways were much more active when these individuals were told they were about to get a squirt of something very spicy.

The authors theorize that what is happening is that these pathways are getting pre-excited by expectation. After seeing those two spicy peppers they are on a hair trigger to cause pain perception – and when the spicy squirt comes, they fire. It’s the same reason we try to distract our kids when they need to get a shot – if they see the needle, it will hurt more. Not in some purely psychological way, but literally. The part of their brain that is responsible for feeling pain will be more active.

Is there a clinical use for this?  Can we invent a new type of anesthesia based on the idea of telling a patient – don’t worry this won’t hurt?  If only. Expectation modifies perception – it does not completely control it. We don’t live in the matrix. I think. I’m pretty sure. We’ll see.

In any case, it’s something to think about the next time you add a bit of sriracha to your sandwich. If you like spicy stuff, maybe take a moment to remind yourself just how very spicy it is before you take that first bite.  

A version of this commentary first appeared on Medscape.com.