Diether Sigrid, Wildsoet Christine F
Section of Neurobiology of the Eye, University Eye Hospital Tübingen, Tübingen, Germany.
Invest Ophthalmol Vis Sci. 2005 Jul;46(7):2242-52. doi: 10.1167/iovs.04-1200.
The bidirectional nature of emmetropization, as observed in young chicks, implies that eyes are able to distinguish between myopic and hyperopic focusing errors. In the current study the spatial frequency and contrast dependence of this process were investigated in an experimental paradigm that allowed strict control over both parameters of the retinal image. Also investigated was the influence of accommodation.
Defocusing stimuli were presented through lens-cone devices with attached targets. These devices were monocularly applied to 5-day-old chickens for 4 days. Defocus conditions included: (1) 7 D of myopic defocus, (2) 7 D of hyperopic defocus, and (3) a combination of the two. Two high contrast target designs, a spatially rich, striped Maltese cross (target 1) and a standard Maltese cross (target 2) were used, except in some experiments where target contrast or spatial frequency content was further manipulated. To test the role of accommodation, the treated eye of some chicks underwent ciliary nerve section before attachment of the device. Refractive error (RE) was measured by retinoscopy and axial ocular dimensions measured by A-scan ultrasonography, both in chicks under anesthesia.
With imposed myopic defocus and high contrast, target 1 elicited significantly better compensation than did target 2. With imposed hyperopic defocus, both targets elicited near normal compensatory responses. Reducing image contrast to 32% for target 2 and to 16% for target 1 precluded compensation for myopic defocus, inducing myopia instead. The low-pass-filtered target also induced myopia, irrespective of the sign of imposed defocus. With competing defocus and intact accommodation, target 1 induced a transient hyperopic growth response, whereas myopia was consistently observed with target 2. When accommodation was rendered inactive, both targets induced myopia under these competitive conditions.
Compensation to myopic defocus is critically dependent on the inclusion of middle to high spatial frequencies in the stimulus and has a spatial frequency-dependent threshold contrast requirement. With competing myopic and hyperopic defocus, the former transiently dominates the latter as a determinant of ocular growth, provided that the stimulus conditions include sufficient middle to high spatial frequency information and that accommodation cues are available.
如在幼雏中观察到的那样，正视化的双向性意味着眼睛能够区分近视性和远视性聚焦误差。在本研究中，在一个允许对视网膜图像的两个参数进行严格控制的实验范式中，研究了这一过程的空间频率和对比度依赖性。还研究了调节的影响。
通过带有附着目标的透镜 - 视锥装置呈现散焦刺激。这些装置单眼应用于5日龄雏鸡，持续4天。散焦条件包括：（1）7D近视性散焦，（2）7D远视性散焦，以及（3）两者的组合。使用了两种高对比度目标设计，一种空间丰富的条纹马耳他十字（目标1）和一个标准马耳他十字（目标2），除了在一些实验中目标对比度或空间频率内容被进一步操纵的情况。为了测试调节的作用，一些雏鸡的受试眼在装置附着前进行了睫状神经切断。在麻醉状态下的雏鸡中，通过检影法测量屈光不正（RE），并通过A扫描超声测量眼轴尺寸。
在施加近视性散焦和高对比度时，目标1引发的补偿明显优于目标2。在施加远视性散焦时，两个目标都引发了接近正常的补偿反应。将目标2的图像对比度降低到32%，目标1降低到16%，会阻止对近视性散焦的补偿，反而诱发近视。低通滤波后的目标也会诱发近视，与施加散焦的符号无关。在竞争性散焦且调节功能完好时，目标1诱发短暂的远视性生长反应，而目标2则持续观察到近视。当调节功能失效时，在这些竞争条件下两个目标都会诱发近视。
对近视性散焦的补偿严重依赖于刺激中包含中到高空间频率，并且具有空间频率依赖性的阈值对比度要求。在竞争性近视性和远视性散焦情况下，只要刺激条件包括足够的中到高空间频率信息且调节线索可用，前者作为眼生长的决定因素会暂时主导后者。
