The second example of the 
unpredictability of binocular 
effects has direct bearing on 
stereopsis but involves retinal 
rivalry, which we allude to in 
our discussion of strabismus in 
Chapter 9. 

You cannot fuse this pair in the 
way you can fuse other pairs, 
such as the first circle 
stereopairs shown earlier. 
Instead, you get "retinal 
rivalry"--a patchwork quilt of 
vertical and horizontal areas 
whose borders fade in and out 
and change position.

If two very different images are 
made to fall on the two retinas, 
very often one will be, as it 
were, turned off. If you look at 
the left black-and-white square 
in this diagram with the left 
eye and the right one with the 
right eye, by crossing or 
uncrossing your eyes or with a 
stereoscope, you might expect 
to see a grid, or mesh, like a 
window screen. Actually, it is 
virtually impossible to see both 
sets of orthogonal stripes 
together. You may see all 
vertical or all horizontal, with 
one set coming into view for a 
few seconds as the other fades, 
or you may see a kind of 
patchwork mosaic of the two, in 
which the patches move and 
blend in and out from one 
orientation to the other, as 
shown by the figure to the left. 
For some reason the nervous 
system will not put up with so 
different simultaneous stimuli 
in any one part of the visual 
field--it suppresses one of 
them. But here we use the word 
suppress as a short way of 
redescribing the phenomenon: 
we don't really know how the 
suppression is accomplished or 
at what level in the central 
nervous system it takes place. 
To me, the patchy quality of the 
outcome of the battle between 
the two eyes suggests that the 
decision takes place rather 
early in visual processing, 
conceivably in area 17 or 18. (I 
am glad I do not have to defend 
such a guess.)