Have you looked for your diary on a cluttered desk just before bed, and not been able to find it? Yet the next morning you get up and there it is. Why didn't you see it, when it was there all the time? The explanation rests in how the human visual system works.
One part, the dorsal stream, works "online". It provides moment-to-moment subconscious analysis of the visual scene, informing the mind of where everything is to enable vision to be used for accurate guidance of all our movements. We take this for granted but it's a miracle. The "picture" is in the mind, but the world through which we move is seen as being out in front.
In children, this dual function - subconsciously analysing what is being seen, despite the clutter, and using vision to guide movement - is progressively developing. This is recognised by book printers. Large clear pictures and text give way to cluttered pages with many small pictures as children progress through school. But is the degree of clutter on the printed page correctly matched to the age of children, so that they can access it all?
In Glasgow, we have seen hundreds of children whose vision does not allow them to handle the complexity of a visual scene. And there may well be many more whose capacity to handle the density of data they are presented with is limited. Such children are often condemned as being clumsy because they bump into things and trip.
They easily get angry, upset or distracted and are labelled as being difficult. But they cannot cope with too much aural or visual "noise". To cap it all, they get told off when they are struggling to do their best. It is not surprising that some can react with explosions of anger.
Who are these children and what is the problem? They have often been born prematurely. They can't see something in the distance because the further things are away, the more there is to see. They can't find Mum in the playground so she always has to stand in the same place and call out.
They stand at the side in the playground and don't make friends - if they can't find Mum, what chance do they have of finding a friend in a moving melee of children? They can't find an item of clothing in a pile of clothes. They can't find a toy in a toy box (Mum has learnt this and has separated everything out).
They can't go to parties because it is too distressing to cope with the crowd (Mum has learnt to go early for a short while). They suffer in open-plan classrooms more than everyone else. They are slow to read; but de-clutter the text, or mask surrounding text, and they often read more easily. Fishing with Dad, on the other hand, is wonderful.
These children may have dorsal stream dysfunction. A pathway in the brain, which allows us automatically and unconsciously to move through the 3D visual world and sorts out the visual scene to find things, has been damaged.
Is this a common problem? We think it may be. There are those in mainstream education with significant damage to the brain who have these difficulties as part of their overall picture. And there are likely to be many other children in mainstream education with mild unrecognised damage - although they may have a diagnosis of dyslexia or a behavioural disorder. We see about 30 such cases a year but this may be the tip of an iceberg.
Then there are "normal" children for whom the amount of clutter may be impairing their access to information in a variety of ways but whose difficulties go unrecognised. This is at the moment just an hypothesis: our preliminary work to determine the frequency of mild perceptual dysfunction in children is underway. But it is significant that adult literacy learners we have been studying commonly have difficulty handling the complexity of a visual scene.
What can be done to help children whose visual systems do not allow them to deal with the mass of data assailing them every day? The most obvious approach is for school designers and teachers to reduce clutter in the classroom.
How is it that museums learned long ago that single exhibits attract more attention, yet schools still crowd all their walls, allow open plan working, have small, cramped text - and expect all children to cope?
Professor Gordon N Dutton is consultant paediatric ophthalmologist at the Royal Hospital for Sick Children, Glasgow.