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Pedro Miguel Serra, Michael J Cox, Catharine M Chisholm, Angel Sanchez-Trancon; Predicting letter and word discrimination with spherical and astigmatic blur. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2111.
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© ARVO (1962-2015); The Authors (2016-present)
Visual Acuity (VA), a measure of visual detail discrimination, depends on the optical quality of the eye and the integrity of the visual pathways. VA can be estimated using the intersection of the modulation transfer function (MTF) and the retinal threshold function (RTF). The effectiveness of this approach was demonstrated for average populations but evidence of intra-individual comparisons of subjectively measured and predicted VA is limited. This work uses anatomical and ocular wavefront data measured on pseudophakic eyes to calculate the MTF and compares it with subjectively measured VA and reading acuity (RA) to estimate the predictability of subjective data.
High contrast VA was measured monocularly at 3.0 (far) and 0.33 (near) metres in a group of 59 pseudophakes (median age: 68, [38; 78]) and RA was determined during a near reading task in a subgroup (n=54) of the participants. A repeated measurements protocol was used to measure VA and RA under various supplementary refractive conditions after participants had been optically corrected to distance emmetropia (Far: 0.00; +2.00x180; +2.00x90; +1.00, Near: 0.00; +2.00x180; +2.00x90; +3.00). The VA predictions were performed using individually constructed physiological model eyes. These included the wavefront data calculated for the pupil diameter measured whilst performing the task. The refractive conditions and low order aberrations correction were implemented using paraxial lenses. The MTF cut-off was considered as the average of the intersection of the sagittal and tangential MTF with the average RTF of a population.
Average VA predicted for far and near distance tended to be poorer than the measured VA and RA, with the differences being dependent on the level and type of defocus. For the in-focus condition at distance or near, prediction differed by ~0.1 logMAR, whereas for the induced refractive error conditions differences ranged between 0.01 (far: +2.00x180) to 0.49 logMAR (near: 0.00). Intra-individual analysis between predicted and measured visual performance showed weak (lower: R=0.004) to moderate (higher: R=0.51) correlations.
The results show that inferring VA or RA based on individual MTF data requires some caution. The prediction protocol only accounts for the optical part of the process omitting the different abilities of the participant to recover detail in blurred or clear conditions.
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