Simon Bill is currently working as artist in residence at Furnace Park. He is making a series of paintings of optical illusions which will be installed on the perimeter fences.

Most people have heard of Optical Illusions, and could probably recognise one or two, but may not realise there are actually hundreds of them, some with names – for example the Fraser spiral, the Necker cube, the Müller-Lyer illusion, Rubin’s face/vase, the Zollner illusion, Jastrow’s duck/rabbit, Kitaoka’s rotating snakes, and the ‘blind spot’ illusion. The effects themselves are diverse – static images can appear to move, identical quantities to differ; the same picture can seem to be pictures of two entirely different things, spots and flashes can appear or disappear, straight lines appear to curve, objects flip alternately between convex and concave, and so on – But where do they come from, and do they mean anything?

The psychologist Richard Gregory defined Optical Illusions as ‘systematic deviations from fact’ – in other words you are seeing something you know isn’t right, but this mistake or misperception will be the reliable outcome of a particular set of circumstances. It isn’t random, and it doesn’t mean there’s anything wrong with you when you see them; quite the opposite.

In fact it’s important when talking about Optical Illusions to make it clear that they are not at all the same thing as hallucinations, nor are they the same as other sorts of ordered mistake in perception such as mirages, or a pencil looking bent when stood in a glass of water (which is caused by the refraction of light). Optical Illusions occur in your mind but are not caused by being drunk or mad. The many hundreds of known Optical Illusions were discovered by (and often named after) psychologists trying to work out how normal vision works. This is not as simple as it might seem on the face of it. What happens when you see feels like the full and immediate disclosure of the world around us – You open your eyes and there it all is. In fact visual perception is not like that at all, and what seems to be happening when you see and what is really happening are very different things. The process of seeing may begin in your eye, but most of the work is carried out in the brain (which is why so much it, about half, is used in processing vision). Our eyes don’t really collect actual images; they can only collect fragmentary and fleeting scraps of data, all of which are fractionally out-of-date by the time they get to the brain. Visual perception is the complicated, and yet seemingly effortless, business of creating the full experience of vision out of this meagre raw sense-data. Think of it as being like creating a whole complicated jigsaw picture from a handful of scattered pieces.

How does the brain do this? – It does it by conjecture, or hypothesis. What you seem to be seeing when you see – what seems like the whole and immediate projection or disclosure of the world in to your head – is actually your brain’s best guess about what’s there, based on knowledge and experience applied to these fleeting fragments of information furnished by your eyes. The fact that these conjectures are almost always correct, and what you seem to see is actually pretty much what is really there, shows that the brain uses reliable rules of thumb to form its hypotheses. But sometimes, in some unusual conditions, these rules are applied wrongly, or in such a way that you notice them being applied. Optical Illusions are the accidental exposure of the mechanisms that must be at work all the time in normal vision.

One example that demonstrates this clearly is the ‘blind spot’ illusion. To observe this effect place two blobs of Blu-tack at eye-level about a metre apart on a pattern wallpapered wall. Stand about four metres away, shut your right eye and look at the Blu-tack blob on the right whilst moving slowly forwards. At a certain critical distance the other blob disappears. This is because that blob has become aligned with your left eye’s ‘blind spot’, which is a patch of the retina that is not light sensitive because it’s where the neural ‘wiring’ from the rest of the retina leaves the eye and goes to the brain. But if this patch of the visual field is blind, how is that you can still see the wallpaper pattern, instead of a dark patch, or just nothing? It’s because the brain is ‘filling in’. Your brain reasons that an area from which it is receiving no information is likely to be the same as the area around it, so it just colours in the blank with an approximation of its surroundings. The blind spot illusion shows that a good part of what we presume to be direct data from the outside world is actually just unconsciously reasoned (and in this case correct) conjecture.

Simon Bill 2013

Simon Bill is an artist, writer and curator based in Sheffield. He studied at Central St Martins and the Royal College of Art, London, and has exhibited internationally. His novel BRAINS, about a painter becoming artist-in-residence at a neurology clinic, was published in 2011 by Cabinet 2. He has been engaged in PhD research on art  and the neuropsychology of visual perception at Kingston University. Simon is represented as an artist by Patrick Painter Inc, Santa Monica, and as a writer by Piers Blofeld of Sheil Land Associates, London.