Transient increases in choroidal thickness are consistently associated with brief daily visual stimuli that inhibit ocular growth in chicks.

In chickens, transient changes in choroidal thickness are found in conditions in which the eye is slowing its growth in response to visual episodes that prevent excessive elongation. To test the hypothesis that the choroidal and ocular growth responses are linked, we used a variety of "brief daily" stimuli known to ameliorate the development of myopia and assessed the concurrence of the responses. If the hypothesis is true, they should always be correlated. Form deprivation w/vision or strobe. Diffusers were worn for 5 days and removed for 2h of "vision" each day in: (a) one block of 2-h (n=16); or (b) two 1-h periods (n=10). Strobe. Birds were given 0.5h episodes of 12 Hz strobe at dawn and dusk (12h apart, n=11). Negative lenses w/vision or strobe. Lenses (-10D) were worn for 5 days and removed for 2h of vision each day (n=14). Strobe. Same as above (n=11). Darkness/brief vision or myopic defocus. Birds in constant darkness were given 2 daily 0.5h episodes of light 12h apart (n=6) or one daily 0.5h episode of +10D myopic defocus (n=6) for 4 days. Darkness/"frequent" or "infrequent" myopic defocus. Birds in constant darkness were given frequent (2 min x 14) or infrequent (1 min x 7) episodes of +10D myopic defocus for 4 days. In all experiments a control group had the myopia-inducing treatment but did not receive the visual stimulation. High frequency ultrasonography was done at the start and end of the experiment, and on the last day immediately prior to and 1h after the period of stimulation. Refractive errors were measured using a Hartinger's refractometer at the end of the experiment. We found that in 7 of the 8 conditions the development of myopia was inhibited. Form deprivation: vision or strobe vs control: -1.2 and -1.8 vs -9.8D. Negative lenses: vision or strobe vs control: -1.2 and -4.3 vs -8D. Constant dark: vision or myopic defocus vs control: -0.7 and 1.8 vs -1.8D. Constant dark: frequent myopic defocus vs control: 4.8 vs -0.4D (p<0.05 for all comparisons). In all the effect was axial with growth rate being significantly inhibited. In all cases the choroids showed significant transient increases in thickness as well. Form deprivation: vision or strobe vs control: 58 and 15 vs -3 microm. Negative lenses: vision or strobe vs controls: 74 and 17 vs -17 microm. Dark: vision or myopic defocus vs control: 56 and 46 vs 11 microm. Dark: frequent vs control: 103 vs 5 microm. In the "infrequent myopic defocus" condition eyes did not compensate to the defocus, however they did not become myopic. The choroidal response was not significant. These results support the hypothesis that these brief choroidal responses may play a role in ocular growth inhibition.