Nevin S T, Schmid K L, Wildsoet C F
Center of Eye Research, Queensland University of Technology, Brisbane, Australia.
Curr Eye Res. 1998 Mar;17(3):322-31. doi: 10.1076/ceyr.17.3.322.5220.
Compensatory responses to focusing errors imposed by spectacle lenses in chicks, tree shrews and primates leave little doubt that active emmetropization can occur, and debate is now centered on whether this process is uni-directional or bi-directional in nature. To provide further insight into this emmetropization process, the studies reported in this paper addressed the question of whether access to sharp vision is necessary for compensation to myopic defocus in the chick.
Two different experimental paradigms were used to address the above 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 form deprivation; these treatments result in a shift in the plane of focus of the eye (far point) to 6.67, 2.5 cm and approximately 3.5 cm resp., 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 length, further limited access to (or precluded) sharp vision by controlling how closely the chicks could approach objects. One group that had sharp vision precluded also underwent optic nerve section. (B) A range of positive lenses (+15 to +65 D) were used on their own to impose myopic defocus; for the high power lenses, access to sharp vision was very restricted because of the close proximity of the new far point (1.54 cm for +65 D lens). Refractive errors and axial ocular dimensions were measured in all experiments.
In the first study (A), preclusion of sharp vision not only prevented compensation but resulted in 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 second study (B), good compensation was observed with the +15 D lens but compensation progressively declined for higher powers, with the +50 D lens having no apparent effect on eye growth and refraction and the +65 D lens inducing myopia instead of hyperopia.
Together these results argue that some sharp vision is fundamental to compensation to impose myopia. The significance of this new finding in relation to the processes underlying active emmetropization is discussed.
对雏鸡、树鼩和灵长类动物中由眼镜镜片施加的聚焦误差的代偿反应，无疑表明主动正视化是可以发生的，现在的争论集中在这个过程本质上是单向的还是双向的。为了进一步深入了解这个正视化过程，本文报道的研究探讨了雏鸡对近视性离焦进行代偿时清晰视觉是否必要的问题。
使用两种不同的实验范式来解决上述问题：（A）单独使用+15或+40 D镜片，或在因7天形觉剥夺而近视的眼睛上使用+15 D镜片施加近视性离焦；这些处理分别使眼睛的焦平面（远点）移动到6.67、2.5 cm和大约3.5 cm，只有位于或比这些平面更近的物体才能清晰聚焦。在镜片上添加长度为2.5或5 cm的支撑锥体，通过控制雏鸡接近物体的距离，进一步限制（或排除）清晰视觉。一组被排除清晰视觉的雏鸡还接受了视神经切断术。（B）单独使用一系列正镜片（+15至+65 D）来施加近视性离焦；对于高屈光度镜片，由于新远点很近（+65 D镜片为1.54 cm），清晰视觉的获取受到很大限制。在所有实验中测量屈光不正和眼轴尺寸。
在第一项研究（A）中，排除清晰视觉不仅阻止了代偿，还导致眼轴增长和近视增加。这种近视，与形觉剥夺性近视一样，不受视神经切断术的影响。限制但不排除清晰视觉导致部分代偿。在第二项研究（B）中，使用+15 D镜片时观察到良好的代偿，但随着屈光度增加代偿逐渐下降，+50 D镜片对眼轴增长和屈光没有明显影响，而+65 D镜片反而诱发近视而非远视。
这些结果共同表明，一定程度的清晰视觉对于近视性离焦的代偿至关重要。讨论了这一新发现与主动正视化潜在过程的关系。
