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Mengyuan Ke, Marsha L Kisilak, Laura Emptage, Ian Andrews, Melanie C W Campbell; Retinal blur is not significantly different from cone spacing in chick following normal emmetropization or lens induction of myopia. Invest. Ophthalmol. Vis. Sci. 2016;57(12):5511.
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© ARVO (1962-2015); The Authors (2016-present)
Emmetropization is an active process involving retinal feedback linked to the amount of image defocus on the retina. We wish to determine, in chicks, the relationship between the retinal blur due to defocus and cone spacing during normal emmetropization and during the lens induction of myopia.
Twelve Ross Ross chicks were acquired on the day of hatching in 3 sets. Axial length was measured using A scan ultrasound. Aberrations and defocus were measured in a custom built Hartmann-Shack aberrometer. Equivalent defocus blur was calculated from the wavefront aberration due to defocus. Eyes were imaged in an adaptive optics corrected scanning laser ophthalmoscope modified for small animal use that had supplemental lenses for defocus correction. All measurements were performed on awake birds. After measurements on the initial day, the right eyes were goggled with -15D lenses. Measurements were repeated on subsequent days until day 14. All measurements were taken close to the optical axis and the anatomical position of the area centralis. Angular cone densities were measured directly. Linear cone spacings and linear blur on the retina were calculated from angular measurements and published schematic eye models modified for individual eye lengths and assuming hexagonally packed cones.
By day 14, goggled eyes were on average 15D myopic. Cones were successfully imaged on all days. As previously reported, the angular density of cones was not significantly different between control and goggled eyes (p>0.2) on any day. However, because of the ocular elongation in response to the goggle, the linear cone spacing in the goggled eye was significantly larger than in the control eye (p<0.05). Initially, in both the control and goggled eyes, the radius of the blur on the retina (angular and linear), was much larger than the cone spacing prior to completion of emmetropization. By day 14, defocus blurs of 4.4 arcmin and 5.9 arcmin in the control and goggled eyes respectively did not differ significantly and were not significantly different from the angular cone spacings.
Normal emmetropization and lens induction of myopia both produce an active reduction of angular and linear retinal blur due to defocus to values not different from the measured cone spacings, suggesting an explanation for the refractive error remaining when emmtropization is complete.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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