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.