Sunday, 27 September 2009

Why you can't see the colour of the car that hit you

The visual system is one of the best understood in all neuroscience (not that there aren't plenty of mysteries about it!), and its most salient features is its economy. The brain is an expensive organ, evolutionarily - pound for pound, it consumes 22 times more energy than muscle (which is itself rather expensive), and all this has to be repaid by eating more in reply. Therefore, any bit of it that doesn't absolutely have to be around won't make the cut as the aeons roll by.

One way you could have designed the retina would be to cover it densely with both cones (for high acuity colour vision) and rods (for high sensitivity night vision). In this way, no matter where the photons landed, you would get the best image possible.

But the eye chooses a more elegant system. It concentrates almost all its cones in and around the fovea centralis, a region near the centre of the eye. To some extent, it has put almost all its eggs in one basket, since any images that are attempting to form themselves elsewhere on the retina are left to the low quality rods to fumble with. The visual system makes up for this by constantly rotating the eyeball so that the centre of our visual field falls on it; in this way, whatever we are 'looking at' can be rendered in the highest possible quality.

Furthermore, our brain hides this flaw from us by rapidly darting the eye around a particular scene, and patching the numerous, small high quality images together. It may surprise you to find out just how bad your peripheral vision actually is. The only way to do this is to keep your eyes absolutely still and then try to make out the details of something in the periphery of your vision.

Reading is a good example. Place something with large letters in near to you. (Take care not to peek at what the text says, or the brain will cheat, as I've mentioned. One way of minimising this likelihood is to ask a friend to write something down instead.) If you are stringent about keeping your eyes staring straight ahead, it won't be possible to make out any letters even a few degrees from centre.

Furthermore, since cones are used for both high-acuity vision and colour vision, you won't be able to make out the colour of an object seen only with your peripheral vision. If anything, this fact is even more surprising, but if you're as strict with your eyeball as before, you can easily confirm this.

Note also how clever the brain is at filling in the details. After you have seen an object with your central (cone-dominated) visual apparatus, the brain will give the illusion of still seeing that object in colour long after it has moved to your peripheral vision, where this is no longer possible! Its amazing how good the virtual reality generator is...

One thing the peripheral vision really is good at, though, is detecting movement. (It isn't hard to see how this might be beneficial.) The slightest movement, and the eyeballs rotate to lock the powerful central vision on to the object.