In the car the other day, I think it was the Steve Wright Show on Radio 2 , in the regular titbits of information they give, the question was "The eye is responsible for how much of a person's sight?" The answer they gave was 25%. This was on the grounds that the eye is primarily just a lens. The image projected by that lens needs to be processed by the brain, in order to have some meaning and recognition. This seems sensible to me, and explains why some people with dementia - my Jan included - lose their sight, when their eyes are perfectly healthy. Damage to their brain prevents it deciphering the images. Amazing where these things come out.