The effect of manipulations to target contrast on emmetropization in chick

Emmetropization is dependent on visual feedback and presumably some measure of the optical and image quality of the eye. We investigated the effect of simple alterations to image contrast on eye growth and refractive development. A 1.6 cyc/deg square-wave-grating target was located at the end of a 3.3 cm cone, imaged by a +30 D lens and applied monocularly to the eyes of 8-day-old chicks. Eleven different contrast targets were tested: 95, 67, 47.5, 33.5, 24, 17, 12, 8.5, 4.2, 2.1, and 0%. Refractive error (RE), vitreous chamber depth (VC) and axial length (AL) varied with the contrast of the image (RE diff: F10,86 = 12.420, p < 0.0005; VC diff: F10,86 = 8.756, p < 0.0005; AL diff: F10,86 = 9.240, p < 0.0005). Target contrasts 4.2% and lower produced relative myopia (4.2%: RE diff = −7.48 ± 2.26 D, p = 0.987; 2.1%: RE diff = −7.22 ± 2.77 D, p = 0.951) of similar amount to that observed in response to a featureless 0% contrast target (RE diff = −9.11 ± 4.68 D). For target contrast levels 47.5% and greater isometropia was maintained (95%: RE diff  = 1.83 ± 2.78 D; 67%: RE diff = 0.14 ± 1.84 D; 47.5%: RE diff = 0.25 ± 1.82 D). Contrasts in between produced an intermediate amount of myopia (33.5%: RE diff = −2.81 ± 1.80 D; 24%: RE diff = −3.45 ± 1.64 D; 17%: RE diff = −3.19 ± 1.54 D; 12%: RE diff = −4.08 ± 3.56 D; 8.5%: RE diff = −4.09 ± 3.60 D). We conclude that image contrast provides important visual information for the eye growth control system or that contrast must reach a threshold value for some other emmetropization signal to function.