That we experience retinal 
rivalry implies that in cases in 
which the visual system cannot 
get a sensible result out of the 
combination of the two sets of 
inputs from the two eyes--
either a single fused flat scene 
if the images are identical or a 
scene with depth if the images 
differ only in small horizontal 
disparities--it gives up and 
simply rejects one of the two, 
either outright, as when you 
look through a monocular 
microscope, keeping the other 
eye open, or in patchy or 
alternating fashion, as in the 
example described here. In the 
case of the microscope, 
attention surely plays a role, 
and the neural mechanisms of 
that role are likewise 
unknown.

    You can see another example 
of retinal rivalry if you attempt 
to fuse two patches of different 
colors, say red and green, 
instead of vertical and 
horizontal lines as just 
described. As I will show in the 
next chapter, simply mixing 
red and green light produces 
the sensation of yellow. On the 
contrary, when the two colors 
are presented to separate eyes 
the result is usually intense 
rivalry, with red predominating 
one moment and green the 
next, and again a tendency for 
red and green regions to break 
up into patches that come and 
go. The rivalry however 
disappears and one sees yellow 
if the brightnesses of the 
patches are carefully adjusted 
so as to be equal. It seems that 
color rivalry is produced by 
differences in brightness 
rather than differences in hue.

    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.