How can altered visual experience in one eye influence the normally
regulated growth of the fellow nontreated eyes? Several possible
mechanisms have been suggested. It has been speculated that interocular
effects could reflect a humoral growth influence from one eye to the
other.
5 For example, in chicks, in which the medial walls
of the two orbits are essentially juxtaposed, growth factors could
easily cross from one orbit to the other. However, the inconsistencies
in the direction of the refractive alterations observed in the
nontreated eyes of chicks argue against this idea.
5 21 Moreover, in monkeys, in which the orbital separation is more
substantial, it seems less likely that such a mechanism could explain
interocular effects. Alternatively, the nontreated eye effects
could come about as a result of some central influence on the
regulation of early eye growth.
5 26 34 40 For example, in
chicks, tree shrews, and monkeys, changes in choroidal thickness appear
to precede vision-dependent changes in ocular
growth.
5 24 32 41 42 It is possible that the
innervation of the choroid or some other critical ocular structure has
a binocular component that is somehow altered by monocular form
deprivation and that these innervational changes somehow modulate the
normal growth of the fellow eye in monocularly treated animals.
However, the nature of this input and exactly how it could influence
growth is unknown. In monkeys, in which accommodation in the two eyes
is closely yoked, interocular effects have been associated with
alterations in the fixation behavior of the nontreated eye. For
example, infant monkeys reared with low-powered positive lenses in
front of one eye, but zero-powered lenses in front of the fellow eye,
posture their accommodation for the eye viewing through the positive
lens. Consequently, the fellow nontreated eye experiences hyperopic
defocus and relative axial myopia develops.
9 10 On the one
hand, given the degree of image degradation produced by our diffuser
lenses and the likelihood that the treated eyes were
amblyopic,
43 it is very unlikely that our monkeys,
particularly the animals that wore the diffuser lenses continuously,
attempted to fixate through the treated eyes. On the other hand, it is
reasonable to suppose that the relative hyperopic errors found in the
nontreated eyes represent a vision-dependent alteration in ocular
growth.