Alan Woodruff

Key points

Did you know that schizophrenics may be less prone to visual illusions than the rest of us? A number of studies showing this effect now exist, and researchers are trying to use this information to help us understand what is different about the schizophrenic brain. Before getting into this, let’s use a visual illusion to quickly introduce some concepts for how perception works – it may not be in the way you think!

Look at the image above, which shows the Cornsweet illusion, one that I really like. There’s a dark grey surface joined to and on top of a white surface. A blue sky is in the background, and the shape is resting on a patterned ground. Seem reasonable? Look a bit more closely and you can conclude that there’s a light source, probably the sun, shining down on the dark grey surface and casting shadows on the ground and on surfaces that don’t get direct light.

Now the fun part: cover up the region where the dark and light surfaces join, maybe with a finger held horizontally against the screen…crazy, right?? I’m going to assume that you can hardly believe the dark grey surface is the same shade as the underside of the white surface. What’s more, even after knowing they’re the same, they still don’t look the same in the presence of all those contextual cues!

What’s going on? What our brains are doing is taking into account all of the available visual information, combining that with our past experience, and saying “Based on what I remember and what I can see right now, the most likely thing is X.” In the example above, the brain recognises a blue sky, a horizon and a shadow, and concludes the sun must be shining from above. The grey surfaces on top and below then make sense – the top surface is something normally dark, seen in sunlight, and the bottom surface is something white, but in shadow. This is how perception works, combining sensory input with prior knowledge or expectations to gauge the most likely way a stimulus is produced. More broadly, it’s probably the major function of the brain: to use prior knowledge and current sensory input to plan and carry out interactions with our environment. Tap your desk. Did it moo when you hit it? No, and you would have been pretty shocked if it did, because based on past experience, your brain knows that the movements that lead to fingers hitting desks do not tend to evoke a moo from a tiger…no, cow. These predictions of what happens when we interact with our environment are happening all the time, subconsciously, and are just what our brain does, naturally – combine prior knowledge with sensory input, automatically, to predict what comes next. But there’s evidence that in schizophrenia, this process goes awry.

Schizophrenia and visual illusions

Look at the image below. The task here is to judge the relative contrast of the central target circle (bounded by red), measured against reference patches of varying contrast (such as on the left of the figure). Subjects judge the contrast of the small central target either when a surrounding circle of high contrast is present (as in the image) or when the central target is presented alone, with no surrounding circle. Have a go – what value contrast do you think the central circle in the image is?

This task was presented to people with schizophrenia and to 2 control groups – either those with no known mental disorder, or to non-schizophrenic mental patients. The researchers found that when no surround circle was present, all subjects were able to accurately judge the true contrast of the central circle. However, when the high contrast surround circle was present, both sets of controls were pretty inaccurate, underestimating the contrast values of the central circle (in the image above, the true contrast of the central circle is (supposedly) 40%, but controls judged it to be roughly 20–30%). Surprisingly, SZ subjects were pretty accurate at this task! It was as though the surrounding circle hardly had any effect on their perception; in other words, the context didn’t really matter. Compared to controls, schizophrenic subjects seemed to put relatively more weight on the sensory input they received.

The results from the above experiment aren’t isolated. Using different visual illusions that test different aspects of visual processing, researchers have similarly found that people with SZ are less prone to these illusions; they more accurately judge the true nature of a stimulus, as it would be seen on the retina. What does this tell us about schizophrenics? It appears that something is different about the way they integrate sensory input with context or prior knowledge – they seem to take the sensory input more or less at face value, neglecting to use context to guide their perception. I kind of wish we could temporarily see the world more or less at face value, without prior knowledge biasing what we perceive, just to see how crazy everything would look and how much we take for granted the importance of context and experience on visual perception. I guess that, in a way, babies must be seeing this (un)warped version of reality?

What’s going on in the brain?

Perception is a combination of ‘bottom-up’ and ‘top-down’ processes. In this terminology, the bottom is where neural activity is driven mainly by sensory inputs, whereas the top is where activity is driven mostly by internal motivations or processes. Thus processing sensory inputs is bottom-up, whereas using context and prior knowledge is top-down. As evidenced by the very existence of illusions, top-down processes – such as the use of context or prior knowledge – can interact with and modulate bottom-up sensory processing.

In the schizophrenic brain, too little emphasis is given to top-down processes. This idea is backed up by neuroimaging studies performed on subjects viewing the hollow mask illusion.

Analysis of schizophrenic subjects, most of whom were immune to the illusion (i.e. they could see the back of the mask), revealed that their brain scans were consistent with dominant bottom-up connectivity. The opposite was true for controls – top-down connectivity was greater. Seeing inside-out faces isn’t really a big part of our past experience, which is why control subjects just couldn’t visualise that – instead they saw eyes, a nose and a mouth and turned it into a regular convex face. The schizophrenics seemed not to care that inside-out faces don’t exist. They saw the visual stimulus for what it was, unhindered by context and prior experience. The diminished top-down connectivity in schizophrenics appeared to be responsible for them taking the stimulus at face value (sorry).


There are really 2 takeaway messages from all of this. First, perception isn’t just the raw visual stimulus – it’s a combination of sensory input and our brain’s biases based on context and past experience. Our brains operate like this as an adaptive measure to make sense of all the ambiguities that are present in our sensory world (for more info on this, check out this TED talk). Second, the schizophrenic brain seems to have diminished top-down functionality, meaning less influence of prior experience and contextual cues. Visual illusions have been used to help demonstrate this. Interestingly, some researchers believe that aberrant top-down function may also account for some better known schizophrenic symptoms, such as delusions and hallucinations.

A couple of notes…if you happen to see the back of the mask, don’t worry! I have it on good authority from someone who assures me they aren’t schizophrenic that they also saw the back of the mask. They also weren’t really, really drunk or stoned, which is apparently another way for non-schizophrenics to see through the illusion. Also, although the two are commonly associated, resistance to visual illusions is not diagnostic of the disorder!