For centuries, similar hints 
had come from clinical 
neurology. A small stroke, 
tumor, or injury to part of the 
primary visual cortex can lead 
to blindness in a small, 
precisely demarcated island in 
the visual field; we find 
perfectly normal vision 
elsewhere, instead of the 
overall mild reduction in vision 
that we might expect if each 
cell communicated in some 
measure with all other cells. To 
digress slightly, we can note 
here that such a stroke patient 
may be unaware of anything 
wrong, especially if the defect 
is not in the foveal 
representation of the cortex 
and hence in the center of 
gaze--at least he will not 
perceive in his visual field an 
island of blackness or greyness 
or indeed anything at all. Even 
if the injury has destroyed one 
entire occipital lobe, leaving 
the subject blind in the entire 
half visual field on the other 
side, the result is not any 
active sensation of the world 
being blotted out on that side. 
My occasional migraine attacks 
(luckily without the headache) 
produce transient blindness, 
often in a large part of one 
visual field; if asked what I see 
there, I can only say, literally, 
nothing--not white, grey, or 
black, but just what I see 
directly behind--nothing.