Hard contact lenses alter accommodative gain but do not prevent refractive adaptation in chicks

This study compared the compensatory response to hyperopic defocus imposed on chicks in two different ways: (1) with-10 D spectacle lenses, and (2) with plano hard contact lens. The hyperopia seen with the contact lenses in situ was a consequence of their flat profile relative to the chick cornea, resulting in a negative fluid lens of approximately 16 D at day 2 and 9 D by day 10. This decrease with age reflects the corneal flattening that accompanies normal eye growth. By optically neutralizing the cornea, the contact lenses also had two other important effects: (1) a reduction in refractive astigmatism to almost negligible levels, and (2) a reduction in accommodative gain. The latter effect reflects the loss of the corneal component of the chick's accommodation and was estimated to be of the order of 40 to 57%, based on measurements made using topically applied nicotine to stimulate accommodation. Thus any estimate of the imposed hyperopic defocus based on accommodative effort required to overcome such errors will be too large. Chicks wearing either lens type on a continuous basis from hatching to 10 days only partially compensated for the imposed hyperopia through an increase in vitreous chamber growth. However, the effects were smaller in the spectacle lens group (e.g., a mean myopic shift of -4.1 +/- 2.3 D compared to -6.3 +/- 2.4 D for the contact lens group at day 10), although both groups experienced similar amounts of hyperopic defocus around day 10 (effective power of -10 D spectacle lens: -9.4 D). The changes seen in the spectacle lens group thus represent poorer compensation, i.e., 44 vs. 71% of the imposed error. However, overcompensation is the predicted effect, if any, of the accommodative deficit imposed on the contact lens group, and this was not seen. That compensation, albeit incomplete, occurred with the contact lens as well as the spectacle lens, suggests that neither accommodation nor astigmatism are fundamental cues for emmetropization as modeled here.