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P. M. Riddell, A. M. Horwood; Optimum Stimulus Characteristics for Eliciting Maximum Hyperopic Refractive Error in Non-Cycloplegic Photorefraction. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3988.
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Non-cycloplegic photorefraction is used for vision screening, as part of optometric examination and experimentally when assessing accommodation responses. This study investigated which target condition would elicit maximum manifest hyperopic refractive error in typical infants, children and adults.
Thirty seven infants were assessed longitudinally between 6 and 26 weeks of age with 2-8 visits per infant. An additional sample of 32 older children and adults were also tested. A remote haploscopic off-axis videorefractor was used to measure simultaneous accommodation and vergence (in dioptres and metre angles respectively) to a detailed picture stimulus at 4 target distances (0.3, 0.5, 1 & 2m). Blur, disparity and size change cues were removed selectively by using a DOG patch, remote occlusion, and scaled targets respectively. Participants were tested with all combinations of cues (8 conditions). A sample of 13 infants at 26 weeks and 24 older children and adults were refracted with cycloplegia for comparison.
The maximum hyperopic error was more likely to be elicited by a target moving away from the participant at all ages. Stimuli that contained disparity and/or size change cues were more likely to produce maximum hyperopic error than targets containing blur cues. Correlation between cycloplegic refraction and the maximum hyperopic refraction obtained from non-cycloplegic photorefraction was high (r2 = 0.88, p<0.0001), and there was no significant difference between these measures (t=0.93, p=0.3 (ns); 95% confidence limits ± 1.75D).
The size change cue, as a target moves into the distance, along with retinal disparity but not blur cues, appears to be most effective in relaxing accommodation. Previous studies have shown that disparity is the main cue to accommodation in adults. In the same study we also found that looming was a very weak cue to vergence and accommodation in adults. However, these findings seem to suggest that reduction in target size, as the target moves away, might assist in relaxation of accommodation providing an estimate of manifest hyperopic refractive error, at least in the population tested here.
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