Sebastiaan Mathôt’s elegant experiments

Your brains can tweak what you see

If there’s one thing that makes experimental psychologist Sebastiaan Mathôt happy, it’s simple, elegant research. He found his niche by focusing on the secrets of the senses. ‘How far can our brains go in adjusting our perception?’
23 November om 10:16 uur.
Laatst gewijzigd op 23 November 2020
om 10:37 uur.
November 23 at 10:16 AM.
Last modified on November 23, 2020
at 10:37 AM.
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Door Christien Boomsma

23 November om 10:16 uur.
Laatst gewijzigd op 23 November 2020
om 10:37 uur.
Avatar photo

By Christien Boomsma

November 23 at 10:16 AM.
Last modified on November 23, 2020
at 10:37 AM.
Avatar photo

Christien Boomsma

Achtergrondcoördinator en wetenschapsredacteur Volledig bio » Background coordinator and science editor Full bio »




You may not have noticed it, but your pupils constricted just the tiniest bit when you read those three words just now. Just by thinking about light or light-adjacent things, you set processes in motion in your body that adjust your pupils as efficiently as possible. The adjustment is subtle, but undeniable. Experimental psychologist Sebastiaan Mathôt says the whole thing is pretty spectacular.

He discovered the phenomenon a few years ago, when he was working as a post-doctoral candidate in Marseilles. Until then, his research was focused on attention and eye movements. He wanted to find out how the eye manages to turn everything it sees into a single whole, like when you quickly move your head. The research was fun and interesting, but not particularly innovative.

Mathôt wanted something for himself. Something no one else was doing, that would allow him to contribute something new, to make a difference. But, he says, it’s not that easy. ‘So much has already been done.’

Fully automatic

Then he found something strange in a dataset. Test subjects had done a task in which they would look at light and dark surfaces. But there was something weird about the data. ‘It looks as though whenever the test subjects were about to look at something bright, their pupils constricted beforehand. And whenever they were about to look at something dark, they dilated.’

So much research has already been done

Mathôt’s first thought was that this was impossible. Until then, scientists were convinced that pupils could only respond in reflex. When someone looks at the sun, their pupils constrict to no more than a quarter of their maximum size. At night, or whenever a person enters a dark room, the opposite happens: their pupils dilate in an effort to absorb as much light as possible. When someone is startled or becomes excited, their eyes also expand, in an effort to see as much of what’s happening in their surroundings.

All of this happens completely automatically. No one can force their pupils to dilate or constrict. They’re not like muscles, where people can tell them to squeeze harder or touch something very gently. Pupil response is a simple, automatic reflex. Yet the data suggested otherwise, namely a process in the brains that made the pupils react to the expectation of light or dark. 

Piece of the puzzle

He decided to investigate further. A simple experiment resulted in the same effect. ‘I kept repeating the experiment in different settings. Whenever people directed just their attention to something light, their pupils constricted. Whenever they imagined something light, read a word that’s associated with light; it all resulted in pupil constriction.’

People optimise their eyes for certain tasks

The effect was robust, and a classic example of the simple and elegant research he so loves. ‘It’s the most satisfying to me’, he says. ‘We have a hypothesis: can people prepare for their eye movements? We performed a simple test, consisting of clear tasks. We measured the results. And now we’ve discovered this new piece of the puzzle.’

But the results also led to more questions. When pupils dilate or contract, the effect is similar to a camera’s aperture. A small pupil makes the picture sharper, allowing people to focus. A large pupil makes one’s vision more sensitive, allowing people to see vague shadows better. ‘I suspect that when people adjust their pupils, they optimise their eyes for a task.’

But how big is this effect? How does it work exactly? Does it only happen in eyes, or does it work with other senses as well?

Sensory tuning

It was the start of his quest into sensory tuning, which is what he named his hypothesis that our senses are active organs that the brain controls. ‘How far can our brains go in tweaking our senses?’ he wants to know. ‘How strict are these rules?’ Now that research financier NWO has awarded him a VIDI grant worth 800,000 euros, he’s finally able to fully delve in.

He’s once again going for the ‘simple and elegant’ approach; clear hypotheses and simple experiments meant to answer a plain, open question. He will instruct his test subject to perform tasks in which they will either have to focus intently or cast their eye a little wider. Mathôt will measure their pupil size to see if it adjusts to the task at hand.

I believe in little things coming together to form one big thing

But the second part of his research is possibly even more interesting. He will be using an fMRI scanner, a variation on a regular brain scanning machine. ‘We know that in people who looked at a blue stimulus, their pupils remained small for a while afterwards. But in people who looked at a red stimulus, the effect is reversed’, eh says.

Mathôt wants to use this knowledge to manipulate pupil size in his test subjects and have them perform the earlier tasks again, but this time in an fMRI scanner. He then looks at how the brains process the visual information. ‘When people’s perception is improved, an fMRI scan will show that more clearly’, Mathôt explains.

‘But I also want to know which level of the brains processes the information. Is it visible right at the beginning, when the stimulus enters the brain? Or is it a little bit at the beginning, with the information escalating – which means the stimulus becomes stronger as it’s being processed.’


Mathôt wants to do the same experiment for hearing. It turns out that people’s ears can also adapt to their circumstances. ‘In what’s known as the acoustic reflex, a muscle in the middle ear tenses up whenever there is a loud noise. This filters out low frequencies, preventing damage to the eardrums’, he says.

The muscle also tenses when people themselves talk, filtering the vibrations through their jaw and skull, allowing them to focus on the conversation. ‘I’d really love to find out if there’s some form of subtle control happening here, too’, says Mathôt.

There is plenty to do in the field. While psychologists have done a lot of research into visual reflexes, it was medical doctors that covered acoustic reflexes, focusing mainly on hearing damage.

If he can discover sensory tuning in the context of acoustic reflexes, he says, that would be a clue that our brains influence our sensory perception on various levels. And once we know that, we can use it to improve cochlear and visual prostheses, among other things.

Sure, all this is very far away. And no matter how much NWO wants to know about the social impact of his research, that’s not why he does it. ‘I believe in proper, innovative research. In how a bunch of little things can come together and form one big thing. That’s how we can make a difference.’