The Mind's Eye
Our sight, the most complicated sense we possess, has been fooling the mind behind our eyes for millions of years. We believe that we have wide angle, high detail color vision, but the reality is our accurate color vision is concentrated inside a tiny part of our visual field. The rest is simply movement sensing, mostly black-and-white awareness.
The brain fools itself into pretending that we can see as well away from the center of our vision as we can at our point of focus. This allows us to use much less mental processing capacity than would otherwise be needed. We fixate on the details of our focus and safely ignore the surroundings until our concentration moves in that direction.
The picture you hold in your head that you assume to be an accurate live feed from your eyes is really a composite image created in your mind's eye from the information you are recieving from your senses. This would be similar to looking at a wall where a photographer is putting up photos to create a collage of the pictures taken from the window.
For more perspective about the mind's eye, the January, 2005 issue of New Scientist magazine contains an article about a Turkish painter who has always been blind.
How We See the World
As you look at an object, light is reflected off the object into your eye through the lens. You detect this light when it falls on the retina at the back of the eye.
In the middle of your retina is your fovea. The fovea is the focus point of your vision and it has much greater image resolution. The fovea is just big enough to see a circle about the size of your thumbnail at arms length. It contains many more color detectors or cones, than the rest of your eye, allowing it to distinguish more detail.
The rest of the retina contains a few cones but is mostly rods. These are simple light detectors and only able to separate black and white. However, we have these rods in vast numbers; this gives us very efficient peripheral vision, which enables us to easily see any object that moves near us.
Figure 1: The left picture shows the difference between how much light you detect in the middle with your fovea and the rest of your retina on the outside. A series of snap shots (fixations) like this are combined in the brain to create the scene as we see it, shown in the right picture.