How optical illusions work and what they reveal

05 June 2018

By Nick Chater

Swedish graphic artist Oscar Reutersvard (1915-2002) spent his career creating beautiful but baffling ‘impossible objects'.

Each looks like a coherent 3D object when viewed as a whole, but on closer inspection, we find that the different parts of the picture don’t make sense: our brain’s interpretation of the 3D parts of the image can’t be joined together to make a unified 3D image of the whole.

These innocent-looking pictures provide a powerful metaphor for the nature of our thought.

First, they show there is something badly wrong with our common sense idea that our mind is a mirror of the world around us: here we imagine we are seeing a 3D object in our minds - but this must be an illusion, because no such 3D object can possibly exist!.

Secondly, Reutersvard’s impossible objects imply that our brain ‘grasps’ different aspects of the image at different times. We scan the different parts of the figures, and find that each, considered in isolation, has a perfectly coherent 3D depth.

But we don’t notice except after careful scrutiny – that these different 3D fragments don’t fit together as a whole. And we don’t notice because the brain can’t grasp the ‘whole’ at once instead, we perceive the world fragment by fragment.

Baffling: Oscar Reutersvard's impossible objects

The third lesson is the the power of the illusion of depth. Looking at them we have the sense that these are 3D objects, albeit peculiar ones.

But this feeling of solidity is completely misguided – we are actually looking at a flat image that has no possible 3D interpretation.

This illusion of depth is both literal, but also provides a powerful metaphor for our own minds. The mind itself, with all its apparent teeming stories, justifications, explanations and motives, is itself an impossible object – each individual explanation seems coherent – but they don’t fit together!

Disappearing act: the 12 dots illusion created by Jacques Nino

Although you almost certainly can’t see them all at once, there are actually 12 black dots arranged in this image above, in three rows of four.

On a white background they can be seen clearly, but when placed on the grid they seem to only appear when you are paying attention to them. The dots we are not attending to are somehow ‘swallowed up’ by the grey lines.

This illusion illustrates the ‘narrowness’ of our vision. It contradicts our sense that we can see everything around us. In fact almost our entire visual field is blurry and black and white, apart from a small spot in the centre of the retina called the fovea - a dense pit of specialised colour-detecting cone cells, which your eye points at any item of interest.

So the dots ‘disappear’ and yet we typically only have the vaguest sense of which part of an image or scene we are looking at directly, sensing we can grasp the entire visual scene.

Even looking at this page of words is an illusion. You sense you can see words everywhere, but experiments have shown that you can see little more than one word at a time.  

As soon as you ask yourself about the identity of a word, your eye shifts to look at it, and you read it rapidly - the whole process is so speedy that we have the compelling illusion that we are aware of many words at once.

Chaos? Van Nostrand Reinhold's illusion left and another from the University of Illinois

At first the two pictures above seem like a jumble of speckles, marks and smudges. But if you spend a minute or two inspecting them closely suddenly their interpretation ‘pops out’.

After a baffling period order emerges from chaos as a picture of a Dalmatian dog and a cow’s head appear – and once seen, they can’t be unseen. We have no sense of getting ’warmer’, after floundering around the image suddenly hits us like a ‘bolt from the blue’.

These sudden flashes of ‘visual insight’ are similar to the Eureka moments scientists have. Many attribute these sudden flashes of thought down to an hidden ‘unconscious’ mind whirring away in the background, before suddenly conjuring up the right answer.

But such insights are no more mysterious than normal perception: we are never conscious of the brain processes that lead to understanding, only to the understanding itself.

 

To the point: Idesawa’s spiky sphere

The sphere above made out of cones is an amazing product of our perceptual imagination. We almost immediately see a smooth white billiard ball, radiating black conical spines.

Some of the black spines loom somewhat ominously towards us; others point away from us. Yet this entire construction is pure interpretation.

The figure is no more than a few flat black geometric shapes on a white background, if they are placed randomly - as they are next to the 'sphere' - there is no discernible shape.

We have no introspective insight into how our brain finds this ‘spherical’ interpretation - our conscious thoughts are our brain’s interpretation of the input from our senses - and never the brain processes that mysteriously create these interpretations.

These illlusions are taken from Nick Chater's new book The Mind is Flat - The Illusion of Mental Depth and The Improvised Mind published by Penguin.

Nick Chater, Professor of Behavioural Science, lectures on the DBA and teaches Behavioural Sciences for the Manager on the Executive MBA and Executive MBA (London). He also teaches Principles of Cognition on the suite of MSc Business courses and Emotions in Business on the Undergraduate programme.

Follow Nick Chater on Twitter @NickJChater

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