Emmetropization in chicks uses optical vergence and relative distance cues to decode defocus

Abstract

When visual information is confined to one object plane, the emmetropization end-point is adjusted in accord with the corresponding incident optical vergence at the eye [Proceedings of the 7th International Conference on Myopia (2000) 113]. We now report the effect of adding extra visual information beyond the target plane. Visual conditions were controlled using a cone-lens system: black Maltese cross targets on white opaque backgrounds (OMX) were attached to the open faces of 2.5 cm translucent cones fitted with either 0, +25 or +40 D imaging lenses. An alternative target (TMX) was made by substituting the opaque target background for a transparent background, which allowed access to visual information beyond the target plane. The imaging devices were applied to 7-day-old chicks and worn for 4 days. Prior to this treatment, on day 2, some chicks underwent ciliary nerve section (CNS) to preclude accommodation. All treatments were monocular. Refractive errors and axial ocular dimensions were measured using retinoscopy and A-scan ultrasonography under halothane anesthesia. Treatment effects were specified as mean ( +/-S.D.) interocular differences. Eyes with the OMX/+40 D lens combination remained emmetropic (+0.73 +/-3.57 D), consistent with the target plane being approximately conjugate with the retina. Switching to the TMX caused a hyperopic shift in refractive error (+3.78 +/-3.41 D). This relative shift towards hyperopia in switching from the OMX to the TMX target also occurred for the other two lens powers. Thus, the OMX/+25 D lens induced myopia (-7.00 +/-5.88 D), corresponding to the imposed hyperopic defocus (target plane now imaged behind the retina), and switching to the TMX resulted in a reduction in myopia (-1.73 +/-5.36 D). The OMX/0 D lens combination produced the largest myopic shift, and here, switching to the TMX condition almost eliminated the myopic response (-15.50 +/-6.62 D cf. -0.56 +/-1.24 D). This relative hyperopic shift associated with switching from the OMX to the TMX target was eliminated by CNS surgery. Thus, the two CNS/TMX groups were both more myopic than the equivalent no CNS/TMX groups (+40 D lens: -2.66 +/-2.34 D; +25 D lens: -7.97 +/-6.87 D). When the visual information is restricted to one plane, incident optical vergence appears to direct emmetropization. Adding visual information at other distances produces a shift in the end-point of emmetropization in the direction of the added information. That these effects are dependent on the integrity of the accommodation system implies that accommodation plays a role in emmetropization and represents the first reported evidence of this kind.

Ocular and Scleral Alterations in Gene-Targeted Lumican-Fibromodulin Double-Null Mice

“…Abnormal ocular growth, axial elongation, and scleral changes are hallmark features of high myopia of long-standing duration and experimental animal models for myopia. 11,13,14,30,[32][33][34][35][36][37][38] In the exper-FIGURE 7. Scleral fibril diameter distributions.…”

Stimulus Requirements for the Decoding of Myopic and Hyperopic Defocus under Single and Competing Defocus Conditions in the Chicken

“…The design of the lens-cone devices has been described in detail. 13,14 The cones were made from white, translucent polyethylene and were attached by Velcro ring supports. They were designed to provide a field of view of approximately 45°.…”

“…In the present study, we made use of a cone-shaped imaging system (lens-cone device) that allows strict and sustained control over the visual information presented as well as retinal image defocus. 13,14 We took advantage of its flexibility in allowing visual information to be presented in one or more planes, at different levels of defocus, with control over both spatial frequency and contrast. For some of these conditions, we also added ciliary nerve lesioning surgery by way of testing the influence of accommodation.…”

“…14 In the present study, we investigated the effect of manipulating the spatial frequency and contrast content of retinal images on the eye's growth response to both single and competing defocus stimuli. We find that compensation for myopic defocus has both spatial frequency and contrast requirements.…”

Human eyes do not need monochromatic aberrations for dynamic accommodation

“…This knowledge of the accommodation mechanism and its stimulus could improve our understanding of the emmetropisation mechanism, since studies on animals and humans show that the process of long‐term growth and development of the eye (emmetropisation) responds to the direction of defocus, i.e., the same optical signals that control short term defocus adaptation (dynamic accommodation) also could control long‐term emmetropisation 1, 11, 30, 31, 32, 33, 34…”