title: Accounting for the phase, spatial frequency and orientation demands of the task improves metrics based on the visual Strehl ratio.
abstract: Advances in ophthalmic instrumentation have allowed high order aberrations to be measured in vivo. These measurements describe the distortions to a plane wavefront entering the eye, but not the effect they have on visual performance. One metric for predicting visual performance from a wavefront measurement uses the visual Strehl ratio, calculated in the optical transfer function (OTF) domain (VSOTF) (Thibos et al., 2004). We considered how well such a metric captures empirical measurements of the effects of defocus, coma and secondary astigmatism on letter identification and on reading. We show that predictions using the visual Strehl ratio can be significantly improved by weighting the OTF by the spatial frequency band that mediates letter identification and further improved by considering the orientation of phase and contrast changes imposed by the aberration. We additionally showed that these altered metrics compare well to a cross-correlation-based metric. We suggest a version of the visual Strehl ratio, VScombined, that incorporates primarily those phase disruptions and contrast changes that have been shown independently to affect object recognition processes. This metric compared well to VSOTF for letter identification and was the best predictor of reading performance, having a higher correlation with the data than either the VSOTF or cross-correlation-based metric.
authors.affiliation: Department of Experimental Psychology, University of Oxford, 9 South Parks Road, Oxford OX1 3UD, UK. laura.young@psy.ox.ac.uk
authors.forename: Laura K
authors.initials: LK
authors.lastname: Young
authors.forename: Gordon D
authors.initials: GD
authors.lastname: Love
authors.forename: Hannah E
authors.initials: HE
authors.lastname: Smithson
mesh_terms: D015350:Contrast Sensitivity / Q000502:physiology*; D057108:Corneal Wavefront Aberration / Q000503:physiopathology; D006801:Humans; D008962:Models, Theoretical; D055095:Optics and Photonics / Q000379:methods*; D011932:Reading; D012029:Refraction, Ocular / Q000502:physiology*; D013028:Space Perception / Q000502:physiology*; D014786:Vision Disorders / Q000175:diagnosis*
keywords: Letter identification; Ocular aberrations; Optical distortions; Reading; Visual Strehl ratio; Visual performance metrics