Optical illusions and schizophrenia

You might already know that possible symptoms of schizophrenia can include delusions and hallucinations. But did you know that in some situations people with schizophrenia actually have a better grasp on reality that those without the disorder?

Specifically, people with schizophrenia seem not to be susceptible to some kinds of optical illusions. Researchers have long known that schizophrenia is associated with fundamental changes in perception, and there’s now mounting evidence that these changes can be detected in something as trivial as how people see optical illusions.

Here are 4 illusions that people with and without schizophrenia tend to perceive differently:

1. Hollow Mask Illusion

When presented with a concave mask, people tend to see it as convex rather than concave – that is, they see it as a normal mask rather than the hollow mask it really is. The idea of a concave face doesn’t make a lot of sense, so people’s brains will generally choose to reinterpret a hollow mask as a more familiar convex face.

See this video for an animation of what the hollow mask illusion loHollow Mask Illusionoks like.

People with schizophrenia, however, aren’t fooled by this illusion. If you show them a hollow mask, they tend to see it for what it is – a hollow mask.

A 2009 study came up with dramatic results on just differently people with and without schizophrenia perceive this illusion. The 16 participants without schizophrenia mistook 99 percent of the hollow faces they saw as convex faces, but the 13 schizophrenia patients perceived the hollow faces as convex only 6 percent of the time.

The study also suggested a possible explanation: it may be that people with schizophrenia experience differences in brain connectivity that lead them to process things in a more “bottom-up” way – whereas people without schizophrenia process things in a more “top-down” way, which leads them to conclude based on previous experience that seeing a hollow face is unlikely.

2. Chubb Illusion

Chubb illusions take advantage of how we perceive contrast – in particular, the fact that we tend to see something as lower-contrast if it’s placed on a high-contrast background. For example, take a look at the following picture:

Chubb Illusion

Which of the small circles around the edge of the larger circle matches the small circle within the larger circle? On average, people will say that the small circle at 10:30 o’clock bears the closest resemblance to the small central circle.

However, a 2005 study found that people with schizophrenia estimate the circle at 6:00 o’clock as being most similar to the central circle. And in fact, the people with schizophrenia are correct – the high-contrast circle at the bottom is the same as the inner circle.

As with the hollow mask illusion, one possible explanation is that people with schizophrenia make less use of context and process things in a more bottom-up manner, which in this case leads them to see the image as it really is.

3. Müller-Lyer and Ponzo Illusions

Although people with schizophrenia aren’t tricked by some illusions, they’re also more susceptible to others.

A 2011 study showed that they’re worse at guessing the lengths of the lines in the Müller-Lyer illusion (the lines are the same length):

Müller-Lyer Illusion

Whether people with schizophrenia are fooled by an optical illusion can also depend on how long they’ve had the disorder. Paradoxically, the same study that looked at the Müller-Lyer illusion also discovered that people who haven’t had schizophrenia for very long are less susceptible to the Ponzo illusion than people without schizophrenia, but people who’ve had schizophrenia for a while are more susceptible than people without schizophrenia (again, the lines are the same length):

Ponzo Illusion

4. Three-Flash Illusions

Some optical illusions take place not just in space, but in time as well. In the three-flash illusion, people will often perceive two flashes of light displayed in quick succession as three distinct flashes.

As for how schizophrenia fits into the picture, well, a 2008 study found that in this case the devil’s in the details.

When the flashes of light are between 90 and 110 ms apart, people with schizophrenia are less susceptible to the illusion – that is, they’re more likely to correctly perceive the two flashes of light as two flashes of light than people without schizophrenia.

When the flashes are between 130 and 310 ms apart, though, people with schizophrenia are more likely to mistakenly perceive three flashes. To put it another way, both people with and without schizophrenia are susceptible to this illusion, but for people with schizophrenia it takes place on a larger time scale.

Exactly why things work out this way is anyone’s guess for now, but the researchers suggested that certain aspects of time perception may be more “dilated” for people with schizophrenia.

Screening for Schizophrenia With Optical Illusions

So does all this mean a diagnostic test for schizophrenia based on optical illusions is in the cards? Well, not yet – none of these illusions discriminate perfectly between people with and without schizophrenia.

But some of the results are dramatic, and it’s possible that this line of research could lead to improved screening procedures or at least a better understanding of how schizophrenia has to do with changes in the way people see the world.

Time will tell, but one thing we do know for now is that optical illusions, which we often think of as little more than parlor tricks, could hold some of the secrets to how our brains work, how we see the world and what happens when our thoughts and perceptions go awry.

Title image: Freeimages.com/ruchi rao

Hollow mask image: Flickr.com/Atom

Chubb image: Dakin et al.

Müller-Lyer image: Wikipedia.org/Fibonacci