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S Chen, SB Kaye, L McCloskey, H Mfazo, A Rubin, WF Harris; The Effect of Induced Astigmatism on Stereoacuity Thresholds . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4679.
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Purpose: To investigate the effect of induced astigmatic power and axis on stereopsis. Methods: Four healthy adults were studied. Astigmatism was induced initially in one eye by introducing cylinders of increasing power in 0.75 dioptre increments (range: -8.85 to +11.25 dioptres) at axes varying from 0 to 90 degrees, leaving the fellow eye unaffected. Each measurement was repeated in triplicate. This was then repeated using cylinders, then Jackson cross cylinders (JCC) before both eyes. The combined threshold of depth discrimination and monocular thresholds were measured at near using the Frisby, TNO and Titmus stereoacuity tests and at distance using the Variable Distance Stereoacuity Test (VDS).[1,2] LogMAR visual acuity was measured at near and at distance. Results: The combined (CT), monocular (MT) and binocular (BT) thresholds of depth discrimination increased with increasing induced astigmatism under all test conditions, with a similar reduction in VA. With monocularly induced astigmatism there was no significant dependency of CT on axis. With binocularly induced astigmatism however, the least reduction in stereopsis and visual acuity occurred with a horizontal axis, compared to a vertical (p<0.001) or oblique axis (p<0.01). The degradation of stereothresholds was greatest for oblique astigmatism, although this effect depended on the stereoacuity test used (TNO, p=0.05, Titmus p=0.03, Frisby, p=0.004, VDS, p=0.2). Similarly using a JCC, the overall reduction in stereopsis was maximal with oblique astigmatism (p=0.04). There were significant correlations between the CT and VA as the degree and axis of the induced astigmatism was varied (0.86<r<0.998, p<0.001). Conclusion: The retention of reasonable stereoacuity with induced vertical astigmatism (horizontal axis) and hence vertical disparity, reflects the lesser contribution of vertical disparity to depth discrimination compared to horizontal disparity. Conversely the greater reduction in stereopsis with horizontally induced astigmatism, reflects the greater contribution of horizontal disparity to stereopsis. Oblique astigmatism, affects both horizontal and vertical disparity, and may account for the greater effect on the threshold of depth discrimination noted. References: 1 Kaye SB, et al. A variable distance stereotest. Invest Ophthalmol Vis Sci 1997; 38:110 2 Kaye SB, et al. Monocular and binocular depth discrimination thresholds. Optom Vis Sci 1999;76:770
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