How do visual illusions work?

17 May 2016



Why can I choose how I perceive some illusions but not others? With some visual illusions I can choose to shift my focus e.g. with the face/vase illusion I can see the face then refocus and see the vase (
Other illusions like the spinning dancer I have to close my eyes or look away and then look back if I want to see the alternate illusion (

What's going on here? Is it because the illusions are 'happening' in different parts of my brain? It because I'm dealing with motion vs static images?
P.S. best of luck discussing visual illusions on the radio.


Neuroscientist Kate Storrs answered this deceptive question...

Kate - So yes, they're certainly happening in different parts of your brain. That's not necessarily the explanation for why some are switchable and some are not though. So, just to give a bit of background on why somethings have two interpretations. It's because they're fundamentally ambiguous similarly. There are cues to two different interpretations both within the image.

So this face/vase illusion that most people have probably seen. You can either see it as two black faces sitting on a white background or a white vase sitting on a black background. We know that in our visual system we have cells that respond to different bits of the visual field that respond to, for example, a white surface on a black background or a black surface on a white background. So, if I look at it and by chance the cells that like white things on black backgrounds are a little bit more active when I first glance at it, they have excitatory connections to other cells that have similar interpretations so they're going to win and form this coalition that's kind of self-reinforcing and then that will feed to higher layers and that will be interpreted as a vase.

Chris - Does it have the converse where when those ones get a bit more excited, as well as helping to excite other cells, they also turn off the cells that want to instead of a vase - they want to see a face?

Kat - Yes, absolutely - spot on. Because seeing as a white thing on a black background is incompatible with seeing it as black on white and so you have inhibitory connections between the cells that have those two different interpretations.

Chris - Dan.

Dan - The first time I saw that spinning dancer illusion. Where it's an animation of a dancer spinning and it appears to be spinning in one direction and then suddenly, for me anyway, she switched directions and I felt really nauseated when that happened. Is that common or.?

Kate - Yes. There's something very unnerving about your interpretation that had felt so solid and real suddenly completely switching.

Dan - Yes, your brain doesn't like that, I guess?

Kate - No.

Chris - The other one is that. Ah Gerry, go.

Gerry - It's an interesting cosmological consequence of this as well is that for pretty well all norther hemisphere cultures we talk about the Milky Way, so there's this white on dark sky. Yet, for the Aboriginal people of southeast Australia, they talk about dark on white. It's the dark bands through the middle of the Milky Way that form a gigantic Emu shape and they really do actually.

Chris - And don't they use that to know when it's in a certain position in the sky to know when to go and get the eggs?

Gerry - Absolutely, yes. It's cultural and it works very well but it's a nice inversion of black and white and which one you decide is the signal and which one you decide is the background is clearly just a cultural presumption.

Chris - Kate - one of the things which I've noticed when you look at that hollow mask illusion is that it's impossible to force yourself, even though you know, and by this I'm talking about if you see a mask spinning round, and you're seeing it from the front, and it turns round, and you see it from the back, at some point you know you're looking into the inside of the mask but you can't help yourself but to see it coming out at you. I know with this illusion where we've got this vase or the faces, I can cause myself to flick between the two but, with the hollow mask one, I just can't overcome that.

Kate - Yes. Your visual system takes a lot of prior knowledge into account and our experience with faces is that they're always convex objects, and they're such familiar patterns, we just see them compulsorily as being convex objects even if know, at some kind of intellectual level, that they're not.

Chris - So basically, it's your brain saying, I know what the world's all about and, therefore, I'm going to force you to see it one way regardless of whether that actually is the reality?

Kate - Yes. So to answer Cameron's question. I think the difficulty of voluntarily switching depends on how strongly these coalitions that represent the different interpretations are suppressing one another for some stimuli. The cues in the image seem to be such that there isn't very strong evidence for either interpretation and you can tilt them with eye movements or attention, whereas others like the spinning dancer, they're just totally compelling and antithetical interpretations.

Chris - Some people have found that individuals who have schizophrenia can't see these illusions - they don't experience them.

Kate - Yes, the hollow mask illusion in particular. There's evidence that schizophrenics see it veridically as being a hollow mask.

Chris - Why?

Kate - We don't know. Possibly something to do with a failure to incorporate prior knowledge as effectively into your interpretation of the world as healthy brains do.


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