SHARP VISION - A PREREQUISITE FOR COMPENSATION TO MYOPIC DEFOCUS IN THE CHICK

Purpose. Compensatory responses to focusing errors imposed by spectacl e lenses in chicks, tree shrews and primates leave little doubt chat a ctive emmetropization can occur, and debate is now centered on whether this process is uni-directional or bi-directional in nature, To provi de further insight into this emmetropization process. the studies repo rted in this paper addressed the question of whether access to sharp v ision is necessary for compensation to myopic defocus in the chick. Me thods. Two different experimental paradigms were used to address the a bove question: (A) Myopic defocus was imposed, either with +15 or +40 D lenses alone or with +15 D lenses on eyes made myopic by 7 days of f orm deprivation; these treatments result in a shift in the plane of fo cus of the eye (far point) to 6.67, 2.5 cm and approximately 3.5 cm re sp., with only objects at or closer than these planes being in focus. The addition to the lenses of stand-off cones, either 2.5 or 5 cm in l ength, further limited access to (or precluded) sharp vision by contro lling how closely the chicks could approach objects. One group that ha d sharp vision precluded also underwent optic nerve section. (B) A ran ge of positive lenses (+15 to +65 D) were used on their own to impose myopic defocus; far the high power lenses, access to sharp vision was very restricted because of the close proximity of the new far paint (1 .54 cm for +65 D lens). Refractive errors and axial ocular dimensions were measured in all experiments. Results. In the first study (A), pre clusion of sharp vision not only prevented compensation but resulted i n increased eye growth and myopia. This myopia, like form-deprivation myopia, was unaltered by optic nerve section surgery. Limiting but not precluding sharp vision resulted in partial compensation. In the seco nd study (B), good compensation was observed with the +15 D lens but c ompensation progressively declined for higher powers, with the +50 D l ens having no apparent effect on eye growth and refraction and the +65 D lens inducing myopia instead of hyperopia. Conclusions. Together th ese results argue that some sharp vision is fundamental to compensatio n to imposed myopia. The significance of this new finding in relation to the processes underlying active emmetropization is discussed.