Response to interrupted hyperopia after restraint of axial elongation in tree shrews

To determine if early restraint of axial elongation in response to plus lenses increases the subsequent response to interrupted hyperopia in tree shrews.

The normal interrupted hyperopia group (n = 5) had normal visual exposure until 24 days of visual experience (VE). Then, from 24 to 45 days of VE, the animals wore binocular -4-diopter (D) lenses, which shifted the refractive state of the eyes in the direction of hyperopia. Interrupted hyperopia was produced by removing the lenses for 2 hours per day. The early-restraint interrupted hyperopia group (n = 5) wore binocular +4-D lenses continuously from 11 to 24 days of VE, becoming emmetropic with the lenses in place and hyperopic when they were removed. Then, from 24 to 45 days of VE, the lenses were removed 22 hours per day and replaced for 2 hours per day. This created the same initial regimen of interrupted hyperopia as in the normal interrupted hyperopia group. A plus lens control group wore binocular +4-D lenses (n = 5) continuously from 11 to 45 days of VE to assess the stability of the refractive compensation.

In the normal interrupted hyperopia animals, 2 hours of relief from the imposed hyperopia was sufficient to prevent myopia development. In the early-restraint interrupted hyperopia animals, 2 hours of relief from the hyperopia did not prevent myopia development; the eyes became myopic while wearing the lens. The control animals compensated for the +4-D lenses and maintained a stable with-the-lens emmetropia through 45 days of VE, demonstrating that the myopic shift in the early-restraint group was caused by the interrupted hyperopia.

Compensation for plus lenses, involving slowed axial elongation, increases the response to subsequent interrupted hyperopia. Similar to previous reports of an eye size factor in elongated eyes, these data provide evidence for an eye size mechanism operating, in this case, in eyes that have restrained their axial length.