Spatial frequency and myopic defocus detection in chick eye in a closed visual environment.

Purpose:While it is known that the chick eye can compensate for a wide range of lens induced myopic defocus accurately under unrestricted visual environment, the exact vergence requirement and spatial frequency where the eye can correctly decode the sign of defocus are unclear. We sought to determine the effective range of emmetropization for myopic defocus and its relationship with spatial frequency under closed visual environment. Methods:Lens–cone–target devices were monocularly applied to 12–day–old chicks for 4 days. The power of the lens was manipulated to produce myopic defocus of 0, 5, 10, 15, 25 or 35D. At all defocus levels, one of the two targets was presented (target A: a typical low spatial frequency Maltese cross pattern; target B: a Maltese cross filled with mixed mid to high spatial frequency pattern up to 4.0 cyc/deg). Refractive error (RE) was measured by retinoscopy. Vitreous chamber depth (VCD) and choroidal thickness (CT) were measured by A–scan ultrasonography under isoflurane anaesthesia. Treatment effect was defined by the differences between the treatment and control eyes. Results:Target B consistently induced significant hyperopia according to the measured parameters (p<=0.05) at defocus up to 25D. In contrast, target A did not induce significant hyperopia. At 15D defocus level, myopia instead of hyperopia was induced. Comparing with chicks under the same amount of defocus with target B, there were significant differences between them in all the measured parameters (target B group vs target A group: RE: +5.1D vs –1.4D, p=0.043; VCD: –0.46mm vs –0.06mm, p=0.010; CT: +0.35mm vs –0.08mm, p=0.005; n =4). Conclusions:Under closed visual environment where object distance is fixed, the eye can decode myopic defocus up to 25D provided that the retina received mid to high spatial frequency images. When only low spatial frequency content is available, the eye can not emmetropize correctly and becomes myopic, which probably represents a response to form–deprivation.