The final pattern of left-eye, 
right-eye alternation in 
cortical layer 4C develops 
normally even if both eyes are 
sewn shut, indicating that the 
appropriate wiring can come 
about in the absence of 
experience. We suppose that 
during development, the 
incoming fibers from the two 
eyes compete in layer 4C in 
such a way that if one eye has 
the upper hand at any one 
place, the eye's advantage, in 
terms of numbers of nerve 
terminals, tends to increase, 
and the losing eye's terminals 
correspondingly recede. Any 
slight initial imbalance thus 
tends to increase progressively 
until, at age one month, the 
final punched-out stripes 
result, with complete 
domination everywhere in 
layer 4. In the case of eye 
closure, the balance is 
changed, and at the borders of 
the stripes, where normally the 
outcome would be a close battle, 
the open eye is favored and 
wins out, as shown in the 
diagram to the left.

This competition model 
explains the segregation of 
fourth-layer fibers into eye-
dominance columns. At birth 
the columns have already 
begun to form. Normally at any 
given point if one eye 
dominates even slightly, it ends 
up with a complete monopoly. If 
an eye is closed at birth, the 
fibers from the open eye still 
surviving at any given point in 
layer 4 take over completely. 
The only regions with 
persisting fibers from the 
closed eye are those where that 
eye had no competition when it 
was closed.

    We don't know what causes 
the initial imbalance during 
normal development, but in 
this unstable equilibrium 
presumably even the slightest 
difference would set things off. 
Why the pattern that develops 
should be one of parallel 
stripes, each a half-millimeter 
wide, is a matter of speculation. 
An idea several people espouse 
is that axons from the same eye 
attract each other over a short 
range but that left-eye and 
right-eye axons repel each 
other with a force that at short 
distances is weaker than the 
attracting forces, so that 
attraction wins. With 
increasing distance, the 
attracting force falls off more 
rapidly than the repelling 
force, so that farther away 
repulsion wins. The ranges of 
these competing tendencies 
determine the size of the 
columns. It seems from the 
mathematics that to get parallel 
stripes as opposed to a 
checkerboard or to islands of 
left-eye axons in a right-eye 
matrix, we need only specify 
that the boundaries between 
columns should be as short as 
possible.

    One thus has a way of 
explaining the shrinkage and 
expansion of columns, by 
showing that at the time the 
eye was closed, early in life, 
competition was, after all, 
possible.