title: Metamorphopsia and letter recognition.
abstract: Acuity is the most commonly used measure of visual function, and reductions in acuity are associated with most eye diseases. Metamorphopsia--a perceived distortion of visual space--is another common symptom of visual impairment and is currently assessed qualitatively using Amsler (1953) charts. In order to quantify the impact of metamorphopsia on acuity, we measured the effect of physical spatial distortion on letter recognition. Following earlier work showing that letter recognition is tuned to specific spatial frequency (SF) channels, we hypothesized that the effect of distortion might depend on the spatial scale of visual distortion just as it depends on the spatial scale of masking noise. Six normally sighted observers completed a 26 alternate forced choice (AFC) Sloan letter identification task at five different viewing distances, and the letters underwent different levels of spatial distortion. Distortion was controlled using spatially band-pass filtered noise that spatially remapped pixel locations. Noise was varied over five spatial frequencies and five magnitudes. Performance was modeled with logistic regression and worsened linearly with increasing distortion magnitude and decreasing letter size. We found that retinal SF affects distortion at midrange frequencies and can be explained with the tuning of a basic contrast sensitivity function, while object-centered distortion SF follows a similar pattern of letter object recognition sensitivity and is tuned to approximately three cycles per letter (CPL). The interaction between letter size and distortion makes acuity an unreliable outcome for metamorphopsia assessment.
authors.affiliation: Massachusetts Eye and Ear Infirmary, Boston, MA, USA Department of Ophthalmology, Harvard Medical School, Boston, MA, USA Institute of Ophthalmology, University College London, London, UK.
authors.forename: Emily
authors.initials: E
authors.lastname: Wiecek
authors.affiliation: Institute of Ophthalmology, University College London, London, UK Biomedical Research Centre, Moorfields Eye Hospital, National Institute for Health Research, London, UK Department of Optometry and Vision Science, University of Auckland, New Zealand.
authors.forename: Steven C
authors.initials: SC
authors.lastname: Dakin
authors.affiliation: Massachusetts Eye and Ear Infirmary, Boston, MA, USA Department of Ophthalmology, Harvard Medical School, Boston, MA, USA Department of Psychology, Northeastern University, Boston, MA, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Bex
mesh_terms: D000328:Adult; D015350:Contrast Sensitivity / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D010364:Pattern Recognition, Visual / Q000502:physiology*; D013037:Spatial Behavior / Q000502:physiology; D014786:Vision Disorders / Q000503:physiopathology*; D014792:Visual Acuity / Q000502:physiology; D055815:Young Adult
keywords: clinical vision; letter recognition; macular degeneration; metamorphopsia; spatial vision; visual acuity