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Chris Chase, Stefanie Drew, Amy Escobar, Chunming Liu, Efrain Castellanos, Lawrence Stark, Eric Borsting; Larger Tonic Accommodation Correlates with Better Focus at Near for Symptomatic Graduate School Students. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4260. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Under stimulus-free conditions, tonic accommodation (TA) adopts a resting posture of about 1.5 D, although studies have shown considerable individual variability. Models developed in the 1980’s suggested that TA contributes little to the closed-loop accommodation response (AR), but empirical research on this issue has been sparse. This study examines the relationship between TA and AR in symptomatic and asymptomatic graduate students.
Students from Western University of Health Sciences completed three tasks using an open-field WAM-5500 infrared autorefractor under monocular viewing conditions. First, continuous 2-min recordings were made to assess AR at 0, 2, 3, 4, and 5 D. Second, after 5-min of dark adaptation, TA was recorded for 2-min. Third, continuous recordings were made while reading 20/50 text for 10-min at 3 D. Participants were screened for normal visual acuity, no significant ocular pathology, no strabismus, normal stereopsis, and no significant uncorrected refractive error. Near-work symptoms were assessed by the Convergence Insufficiency Symptom Survey (CISS). The sample was divided into High (N=22) and Low (N=17) symptom groups based on the CISS adult cut-off score of 21. The average AR for each stimulus was compared between groups and correlated with the average TA.
Figure 1 shows the High and Low symptom groups had similar AR for targets (F(4,132)=.54, p=.71). Groups also had the same AR while reading (t(32)=.51, p=.61). TA values were the same for both groups (t(37)=.11, p=.91), but the sample averaged a significantly larger TA value (1.89 ± 1.09 D) than previously reported for college students (t(38)=2.14, p=.04). TA was significantly correlated with all AR measures for the High symptom group but not for the Low Symptom group (see Table 1). A linear regression between TA and accommodative error during reading showed that error increased 0.4 D for every 1 D decrease in TA below the text viewing distance of 3 D (Y = 1.5 - 0.4*X, R2 = 0.54, p=.0002).
These results suggest TA plays a significant role in the AR of symptomatic graduate students. Those with more TA have better accommodative focus at near work distances. Possible weaknesses in the accommodation system of the symptomatic students may be compensated for by tonic accommodation.
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