It is known that when hyperopic or myopic defocus is imposed on chick eyes by spectacle
lenses, they rapidly compensate, becoming myopic or hyperopic respectively, by altering
the depth of their vitreous chamber. Changes in two components--ocular length and
choroidal thickness--underlie this rapid compensation. With monocular lens treatment,
hyperopic defocus imposed by negative lenses resulted in substantially increased ocular
elongation and a slight thinning of the choroid, both changes resulting in myopia;
myopic defocus imposed by positive lenses resulted a dramatic increase in choroidal
thickness, which pushed the retina forward toward the image plane, and a slight decrease
in ocular elongation, both changes resulting in hyperopia. The refractive error after
5 days of lens wear correlated well with vitreous chamber depth, which reflected the
changes in both choroidal thickness and ocular length. The degree of compensation
for lenses was not affected by whether the fellow eye was covered or open. Both form-deprivation
myopia and lens-induced myopia declined with age in parallel, but wearing a -15 D
lens produced more myopia than did form deprivation. The spectacle lenses affected
the refractive error not only of the lens-wearing eye, but also, to a much lesser
degree, of the untreated fellow eye. At lens removal refractive errors were opposite
in sign to the lense worn, and the subsequent changes in choroidal thickness and ocular
length were also opposite to those that occurred when the lenses were in place. In
this situation as well, effects of the spectacle lenses on the fellow eyes were observed.
Eyes with no functional afferent connection to the brain because of either prior optic
nerve section or intraocular tetrodotoxin injections showed compensatory changes to
imposed defocus, but these were limited to compensation for imposed myopic defocus,
at least for the eyes with optic nerve section. In addition, optic nerve section,
but not tetrodotoxin treatment, moved the set-point of the visual compensatory mechanism
toward hyperopia. Optic nerve section prevents myopia in response to negative lenses
but not to diffusers, suggesting that compensation for hyperopia requires the central
nervous system.