Friday, 1 February 2013

How Do Visual Illusions Work?

Tricks Of The Eye
In this illusion, the grey lines between the black and white squares appear to be bending. Now get a straight edge and test them, you will find that they are all straight and parallel!
Source: Austin Hackney (author)
Optical Illusions
Visual illusions - sometimes more popularly known as optical illusions - are images that deceive the eye into misinterpreting what is really there and seeing something that isn't.
More precisely, they play on the perceptive process in the brain in order to create a perception which is at odds with measurable, demonstrable reality.
Shortly, we'll look at some of the main types of visual illusions and how they work. They're great fun to play with as well as useful tools for understanding how our brains and perceptive processes have evolved and function.
There are two fundamental keys to the creation of a visual illusion. These are:
·         optical/visual deception
·         manipulation of expectations
Both of these factors have a powerful influence over how and what we see - or think we see! The latter, the manipulation of expectations, is also known as misdirection and is widely used by stage and sleight-of-hand magicians in order to create their illusions.
The larger part of magic is psychology. So, to answer the question, how do visual illusions work? it is to psychology that we must first turn.
Perception as an Active Process
Perception is not actually the passive process that it seems to be. In fact it is because our perception is a carefully filtered and organized version of the reality around us and not a direct and true experience of all the possible stimuli bombarding our senses at any one time, that we are able to function at all.

This diagram shows the range of the known electromagnetic spectrum. The visible light spectrum - that part of the range that we can see - is a tiny fraction of what is there.
                                                            Source: Penubag CC-BY-SA 2.5 via Wikimedia Commons
We have evolved to operate in a certain way and across a certain range of possibilities and our perceptive processes have clearly evolved too to support those needs. For example, we can only see a very small slice of the light spectrum - the colours of the rainbow - which is a tiny fraction of the actual light frequencies flashing through the cosmos all the time. If we had a direct experience of reality in its totality we would be in a state of abject confusion in a world in which everything was constantly changing size and form, colour and speed. It would be totally disorientating and leave us unable to function for survival.
People who have taken the drug LSD - which in part serves to break down the natural filtering system of the brain - report exactly this sort of experience.
However, perception is not a passive process of receiving stimuli it is an active process of eliminating and organizing stimuli. In consequence of that process, we are able to go about our daily lives in a world that seems stable, known and relatively predictable.
Sounds a bit complicated but we can get a clearer grasp on this by considering in a bit more detail the three main principles of perception. They are:
·         selection
·         inference
·         organization
Let's look at them each in turn.
The Principles of Perception
It has been estimated that a mere 1/2000th of all the sensory information that bombards the senses is actually used to formulate our perceptions. The rest is invisible and intangible to our conscious minds.
This is absolutely necessary to our well-being. Not only is there a structural limitation on the types of stimulation that we can register, there is a constant process of adjustive selection taking place.
Imagine how difficult life could be if you were constantly and equally aware of every physical sensation in your body, every movement and fluctuation in the physical world around you in a continuous simultaneity of experience.
We have evolved to have a very narrow focus of attention and a limited range of available stimuli at any one time according to what has been useful for our survival. So, our perception is built from a carefully selected set of information.
A Thought Experiment
To understand better the way in which your brain actively selects which data it will retain and use to build your perception in any given moment, try this thought experiment:
·         sit comfortably in a busy place - a park, a shopping mall, maybe your family home!
·         close your eyes and relax
·         listen to the sounds around you - try and be aware of each individual sound simultaneously
·         Now select one sound quite close to you and concentrate on it
·         Now move your attention to a sound further away
You will find that it is impossible to give your attention to all the sounds at once - even though physically you must be able to hear them. It is easier to listen to one sound at a time.
Have you ever been so absorbed in some activity that you have not noticed it getting dark, or the time passing, or someone walking into the room? That is how your brain works - selectively!

Paradoxically, the brain, once it has filtered out most of the available information that it could use to build a perception, then frequently finds itself lacking sufficient data to 'see' the whole picture and so has to go beyond the information given in order to infer some aspect of reality.
Take, as an example, the true shape of things compared to the retinal image that they project.
We continue to perceive doors as rectangular - even though they appear trapezoid to the eye from certain distances. Or a person seen from a few yards off is still perceived as normal height, even though the sensory image suggests they are a fraction of their real height. A magician's assistant, lying in a box with feet and head exposed, is not seen as just a pair of feet and a head but as a whole person.
Can you see the white triangle? Maybe you can - but it ain't there! Your brain infers the presence of the triangle in order to create wholeness - and on this occasion it makes a mistake.
Source: Austin Hackney (author)
We also often attribute knowledge that could only come from one sense to another that we are actively using, such as 'seeing' a sweet apple, when we would have to taste it to know it was sweet.
This function is known as constancy. The brain 'fills in the gaps' to keep the world continuous and to have everything 'make sense.' The brain also adjusts for the fact that the retinal image is two-dimensional and upside down, yet we perceive the world in three dimensions and the right way up!
Perception is clearly organized.
The brain has evolved to perceive things according to principles of wholeness or completeness. The brain will always seek to complete the perception by making the simplest, unified form possible - even if the individual sensory elements are not in fact related.
The brain seeks to create images of a world that is simple, symmetrical, uniform and stable. The world isn't actually like that - but it is useful for us to perceive it that way.

Visual Illusions and Art
The facts of our perceptive processes have been exploited not only by magicians and conjurers down the centuries but most notably also by artists.
Early medieval paintings frequently fail to create convincing illusions of comparative size or proportion.
An early panel painting showing holy characters from approximately 1156

These functions had not yet been understood. This was still a time when it was thought the eyes worked by projecting light out onto the world, that consciousness was a substance, the Earth sitting comfortably at the center of a divinely ordered Universe set up as the moral playground of man. Primitive, pre-scientific times. The results in art often looked like the picture opposite.
Clearly, the results are pleasing - but very childish and cartoon-like. The over-sized heads and disproportionate bodies, the flat landscape behind the figures and the skewed, odd-ball perspective of the book all show a considerable naivety.
By the late Renaissance, however, with the advent of humanism, scientific observation and a greater attention to the detailed study of nature, things had come on quite a bit.
There were many great painters of the period who studied perception and perspective. The results are clear to see in this famous work The School of Athens by Raphael. This painting gives a fair illusion of depth and space and proper proportion.
The School of Athens by Raphael. A fine study in perspective and proportion to create the visual illusion of space and distance.
Behind a visual illusion of this complexity, there is a great deal of mathematics and geometry. This is evidenced beautifully in this surviving sketch by Leonardo da Vinci.

Trompe d'Oeil
Trompe d'Oeil is a form of artistic technique which uses visual illusion and the laws of perspective to create realistic images to the effect of a powerful optical illusion designed to truly convince the viewer of the reality of the painted form - and that it exists in three dimensions.
It is simply the French for 'a trick of the eye.'
Here is a great early example entitled Escaping Criticism, painted in 1874 by Pere Borrell del Caso.

In this early example of Trompe d'Oeil, the painting depicts a boy apparently attempting to escape from the painting!
And here is a more contemporary example, a mural by Meursault. Neither the window on the left, the lady nor the shutters are anything more than visual illusions. Seen from the street below the image is utterly convincing.

The Importance of Visual Illusions
The importance of visual illusions should now be clear - and also the importance of understanding how they work.
Not only does this understanding enable us to create elaborate architecture, beautiful art and surprising fun and games, but it also enables us to open the door a little wider on what we really are, how we evolved and why we see, think and react in the way that we do.
It also enables us to realize that we live in only one possible world among an almost infinite variety of other experiential worlds which remain closed to us because of the limitations of our perceptive powers.
Other animals have evolved slightly differently and live in a correspondingly different experiential world. Bats, for example, who navigate a world made up of multidimensional soundscapes at frequencies we cannot even hear.
I would caution against leaping on this information as some kind of proxy justification for a belief in the supernatural.
I've heard it suggested that maybe spirits/angels/ghosts/fairies etc. inhabit these 'unseen realms.'
However, that is to misunderstand what it means. We already know most of what it is we can't directly sense - from radio waves (the oldest form of light, incidentally) to dark matter. There are no gaps for ghosts to jump into! In any case, I can't see it so that proves it's there is the precise fiction in the brain that leads to the experience of illusions. And illusions remain just that - illusions.
There is no suggestion in this of the supernatural. Just that, once again, nature is even more complex, beautiful, mysterious and surprising than we sometimes give ourselves pause to think.
So to round off, one last illusion and a personal favorite. Is this a spiral or a series of circles?
Source: Austin Hackney (author)


  1. In researching the symbols on Pictish symbol stones and making a connection with a version of Roman Mithraism, I have found what looks to be a very example of illusion of depth which is a type of Trompe d'Oeil in a carving popularly called the Z-Rod and Double Disc. I suggest that the double disc is a bisected annulus (dough ring cut across) which comprises (typically) concentric circles with curved lines connecting the out circles. Examples can be seen at in Appendix A.
    I would be interested in a view of an artist regarding the correct term to use for this illusion which tends to suggest that the connected circles are a sliced through ring.

  2. Thanks for your comment, Norman!

    There is a Mithraic temple (or the remains thereof) not so far from where I live and a reconstruction of the site in a nearby museum.

    You are welcome to promote your website here - it's very interesting.

    Kind regards,

    Austin. :)


All comments and contributions to the discussion are welcome! And I always reply to comments, too.