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G. A. Gibson, M. C. M. Dunne, L. N. Davies; Ocular Component Contribution to Accommodation by Vergence Analysis. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1123.
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© ARVO (1962-2015); The Authors (2016-present)
To examine ocular component contribution to accommodation in terms of vergence analysis.
Using a recently published schematic eye, a step-along reverse ray-trace from the retina to the far point was created. The outcome refractive status and vergence contribution of each axial distance and surface radius were examined. The age of the schematic model was varied between 20 and 50 years, and the accommodation level was modified from 0.0 to 4.0 D in 1.0 D steps. The results were displayed as physical biometric change (mm), vergence change (D) and Vergence Contribution Factor (VCF; D/mm) for each ocular component. Further, the model was modified to simulate axial ametropia by increasing and decreasing axial length (AL) by ±2.0 mm.
The data behave in a similar manner to previous work with regard to axial distance and surface radii changes with accommodation. In terms of vergence changes, axial distance variations and separations of ocular components in a 20 year old eye reduce the output refractive status (by -1.8 D for a 4.0 D stimulus), whilst all surface curvature changes enhance accommodation (by a total of +6.3 D for a 4.0 D stimulus). For the same stimulus, anterior chamber and lens thickness changes with accommodation reduce the output response by -1.2 D and -0.2 D, respectively. A smaller degree of myopia is produced (2.69 D/mm in a 20 year old eye) than hyperopia (3.17 D/mm) when altering AL by the same magnitude. Interestingly, myopic eyes produce more accommodation due to damping of the negative output produced by changes in axial component distances.
The data suggest that quantifying the accommodation response mechanism in terms of physical biometric changes alone provides insufficient information. Analysis of vergence change, however, highlights the role that each ocular component plays in the overall accommodative output; thus, enhancing our understanding of the mechanism of accommodation.
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