Donders Wonders

The tinted glasses of your brain

This post is also available in Dutch.

Did you know that we don’t perceive the world exactly as it is? Neural activity can show us what the differences are between our perception and the external world.

We trust that what we see in the world around us is how things actually are. After all, if we’re about to cross the street, we don’t start walking if there’s a car coming, and that’s a good thing. But the external world is not exactly the same as the way we perceive it, as we can see from multiple visual illusions such as this one:

The Müller-Lyer illusion: the top line appears longer than the bottom one even though they are exactly the same in length.

Image source: Quora (CC-BY-3.0)

In this example, the visual system in our brain tells us that these two lines have different lengths when in fact they are the same. This is a clear mismatch between the way the external world is and how we perceive it.

Researching how the brain perceives the world

Nowadays we can actually study the brain to discover how it perceives the world and how that is different from reality.

In a recent paper published in the prestigious journal Nature Communications by researchers at the Donders Institute, Matthias Ekman and his colleagues found something remarkable. They showed participants a dot that moved from left to right across the screen and recorded the observers’ brain activity with fMRI. Participants saw this moving dot many times, so they came to expect its movement from left to right.

Later, the researchers only presented the dot in its starting position and again recorded brain activity. This time the dot didn’t move across the screen. Strikingly, brain activity in the visual cortex, where visual information is interpreted, was similar regardless of whether the dot actually moved or not: it looked as though the brain was pre-playing the dot’s motion because it expected the dot to move across the screen.

Left: Brain activity in response to each position of the dot as it moved across the screen. Right: Brain activity when the dot was only presented in the left-most position without moving across the screen. The brain pre-plays the dot’s motion across the screen.

Image source: Ekman et al. (2017) Nature Communications (CC-BY-4.0)

What Matthias Ekman and his colleagues found is fascinating because it means that the brain pre-plays what it expects to see even if that doesn’t actually happen. In other words, the brain “sees” what it expects to see even if that thing is not actually there. This is the first time that researchers have seen this effect so clearly.

Neural activity reveals the differences between our perception and the external world

This gives us more insight into the way our brain interprets the world. Neural activity can show us what we predict will happen in the future even when something else takes place instead. When we are about to cross the street and we look left and right, our brain may briefly pre-play the image of a car coming by even if in fact the street is empty. This may help us to recognize a car more quickly once it is there, allowing us to cross the street safely.

Written by Marisha. Edited by Roselyne.

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