Spatial Frequency Tuning of Myopia–Prevention in Chicks by Lens–Cone Gratings

Abstract

Abstract: : Purpose: Diffusers induce form–deprivation myopia (FDM) in chicks, which is prevented by a few hours/day of normal vision. Understanding how FDM is induced or prevented by visual processing/signaling in the retina could lead to new anti–myopia therapies. Studies with spatially filtered images or gratings on a rotating drum have suggested that spatial frequencies ∼1 cycle/degree (c/d) efficiently prevent FDM in the chick. Here we refined the spatial tuning function for prevention of FDM with simplified stimuli, which can now be used to identify the retinal neurons that use them to control eye growth. Methods:White Leghorn cockerels were kept on a 12:12 hour light:dark cycle. On day 5 after hatching (P5) one eye was covered with a removable diffuser; the other eye remained uncovered. On P6, P7 & P8 the diffuser was replaced with a test target for 2 hours during the light phase, then the diffuser was put back on. The target was a high–contrast printed transparency of a square or sine wave grating (0.03 – 10 c/d) backed by a white diffuser, or a uniform grey of equal luminance, 3.3 cm from the eye on a conical support and focused with a +30D lens. On P9 both eyes were refracted, axial dimensions measured by 30 MHz A–scan, and the eyes excised and weighed. Results: Uniform grey targets for 2 hours had little or no effect on FDM. Square wave gratings, used initially because they were easiest to make, diminished FDM optimally at 1 c/d but were ineffective (<20% of maximum) at 0.03, 3 and 10 c/d. The 0.1 and 0.3 c/d square waves were partially effective (∼70–80%), but 0.1 and 0.3 c/d sine waves were ineffective. The function, effect (vitreal chamber growth) vs. frequency, resembles spatial frequency tuning functions for retinal ganglion cells in chicks (Miles, 1972) and quail (Uchiyama & Barlow, 1994). Conclusions: These observations confirm that the retinal FDM–preventing mechanism in chicks is tuned optimally to spatial frequencies ∼1 c/d (Schmid & Wildsoet, 1997), suggesting that the retinal regulatory neurons have receptive field centers ∼0.5 c/d or ∼50 µm in diameter. The substantial effect of 0.1–0.3 c/d square wave gratings is probably due to the effectiveness of higher–frequency harmonics. Optimal stimuli are now being used to probe for responsive retinal cells by the activity–dependent marker method.

Keywords: emmetropization • receptive fields • amacrine cells