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S. Shankar, K. T. Ball, R. Suryakumar, W. R. Bobier; Can Accommodative Differences Between Young Hyperopes and Emmetropes Account for an Observed Lag in Emergent Literacy Skills?. Invest. Ophthalmol. Vis. Sci. 2007;48(13):963.
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We have previously shown that habitually hyperopic children (HHYP) who have never received a correction manifested lags in the acquisition of emergent literacy skills as compared to their peers with normal refractive errors (EMM). The purpose of this study was to determine if the higher accommodative demand of these hyperopes in anyway limited their ability to focus on print at near.
Four HHYP (mean equivalent sphere, OU= 2.09±0.61D, mean age = 6.75±0.5yrs) and 6 EMM (mean equivalent sphere, OU=0.95±0.34 D, mean age =6.33±0.52 yrs) participated in the study. Monocular accommodative responses were measured with a PowerRefractor (Multichannel systems, Germany) while subjects fixated on a high contrast target set at 1, 2, 3, 4 & 5 D demands. Accommodative stimulus-response curves (S-R) and root mean square (RMS) deviations at each stimulus demand were compared between the two groups. The main sequence of accommodation (peak velocity vs. amplitude) was measured from 3 HHYP and 5 EMM with a digital high speed photorefractor (sampling at 75Hz) while the subjects changed focus from a far target fixed at 1m to various near positions (2, 3 & 4D).
The S-R curves between HHYP and EMM were significantly different, with the HHYP showing a steeper function with greater accommodative response at the highest stimulus demand and significantly less accommodative response at the lower stimulus demands (H-HYP: y= -0.87x+1.2, r2=0.99, p<0.01; EMM: y= -0.66x+0.4, r2=0.99, p<0.01, F(1,6)=14.23,p=0.01). There were no significant differences for other parameters, though differences in tonic accommodation (HHYP=1.60±0.62, EMM= 0.67±0.77, t (8) = 2.02, p=0.07) and main sequence (F (2, 66) =2.99, p=0.056) were close to significance.
The HHYP appear to show a steeper S-R curve than EMM, showing more accommodation at higher stimulus demands. Interestingly, this stimulus- response pattern appears to be similar to that seen in corrected hyperopic adults1 where the steeper function for hyperopes vs. emmetropes is associated with higher tonic accommodative levels. The dynamic characteristics of this response appear to be similar between the two groups. These results are not suggestive that the static accommodative response is in any way reduced in these HHYP children, and thus may not directly be contributing to their lags in emergent literacy skills.1McBrien, N, A., Millidot, M. (1996) Ophthal. Physiol.Opt. Vol 6, 145-149.
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