Abstract
Purpose :
Previous work (Gawne et al., 2022) showed that a small closed cage with a restricted viewing distance produced environmentally-induced myopia in the tree shrew. Presenting tree shrews with Chromatically Simulated Myopic Defocus (CSMD) abolished the myopiagenic effect of the Closed Cage. We set out to determine whether this result would generalize across species to the chicken using the same apparatus, rearing procedure, and stimuli.
Methods :
We reared 30 chickens with a 12/12h light-dark cycle in 28 cm cubical cages with three opaque walls and a digital photo frame (screen: 25.4 by 14.3 cm). Chickens were reared in batches of 5 and presented with either a field of black and white Maltese crosses without (Closed Cage) or with CSMD on the display. The CSMD stimulus was computed using the Tree Shrew LCA aberration function and appeared as a blue-yellow fringe around each cross. Illuminance at the cage floor was approximately 50 lux, and the display had a 100 cd/m2 luminance. All 30 birds' ocular biometry was measured with a LenStar non-contact ocular biometer; only birds in batches 2 and 3 (19 birds total) had their refractive error (RE) measured with an IR Photorefractor.
Results :
Changes in RE between the Simulated Myopic Blur and Closed Cage (n=20) groups were small and highly variable (0.16 D, 95% CI = (-2.37, 2.71), p > 0.05) and not significant. Vitreous chamber depth measures (n=30) showed a main effect of bird batch (F=7.01, p=0.004, eta-squared=0.37) and stimulus (F=2.65, p=0.11, eta-squared = 0.099). Post-hoc Tukey's HSD showed that the Closed Cage birds had deeper vitreous chambers than the Simulated Blur Group (mean = -0.058 mm 95% CI = -0.13 - 0.016, p = 0.12)
Conclusions :
Chromatically simulated myopic defocus does appear to slow axial elongation but not RE in chickens as in tree shrews, although the effect size is much smaller. The reduced effect could be due to differences in behavior in the Closed Cage or differences in the use of chromatic cues. Chromatic cues guide emmetropization, and cross-species replications are necessary to understand their mechanism of action.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.