Perception

[Steve Axford]

Colour blindness comes in various forms. The most common is where the red cones are missing and the person only sees blue and green. They cannot see the colours that contain red. Green or blue cones can also be missing giving rise to other forms of colour blindness. It has little to do with our construction of colour except that the signals from those cone cells are transmitted to our brain in order to construct colours. Missing signals lead to missing colours.
Complete lack of cone cells, ie fully colour blind, is very rare but does occur and is quite debilitating as just having rod cells makes us far to sensitive to light. Oliver Sachs wrote a book on this called "The Island of the Colour Blind".

Cones are concentrated in the fovea, rods predominate in the periphery of our eyes.

[ameerat42]

...or, The Evolution Of Walking - Why Kangaroos Don't Use Zebra X-ings:

(Please report missing )

Or just interesting

[Steve Axford]

Very very cool. I haven't seen that one.
The information contained in our visual field is enormous and our brains could not make sense of it if we tried to process it all. So we only attempt to process that which is of use to us and throw away the rest. It is an approximation of the reality that is important to us. Our brain uses many tricks (rules of thumb?) so as to avoid huge computation tasks that would waste time and energy. Of course approximations are sometimes wrong, hence those optical illusions.
Just think of the processing problems associated with crossing the road. We have to recognise where the road is, what the cars are, when the cars will arrive at our crossing point and when they won't. Then we have to estimate how long will it take us to cross the road and will a car arrive before we do that. With practice we can do that easily - and while talking and chewing gum at the same time. I am always amazed at how few people are killed crossing the road or driving cars.