HARD CONTACT-LENSES ALTER ACCOMMODATIVE GAIN BUT DO NOT PREVENT REFRACTIVE ADAPTATION IN CHICKS

This study compared the compensatory response to hyperopic defocus imp osed on chicks in two different ways: (1) with -10 D spectacle lenses, and (2) with piano hard contact lens. The hyperopia seen with the con tact lenses in situ was a consequence of their flat profile relative t o the chick cornea, resulting in a negative fluid lens of approximatel y 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 n eutralizing the cornea, the contact lenses also had two other importan t effects: (1) a reduction in refractive astigmatism to almost negligi ble levels, and (2) a reduction in accommodative gain. The latter effe ct reflects the loss of the corneal component of the chick's accommoda tion and was estimated to be of the order of 40 to 57%, based on measu rements made using topically applied nicotine to stimulate accommodati on. Thus any estimate of the imposed hyperopic defocus based on accomm odative effort required to overcome such errors will be too large. Chi cks 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 c hanges seen in the spectacle lens group thus represent poorer compensa tion, 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 spect acle lens, suggests that neither accommodation nor astigmatism are fun damental cues for emmetropization as modeled here.