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Sarah J Waugh, Monika A Formankiewicz, Hannah Warner; Visual acuity performance for luminance-modulated and contrast-modulated Cs and letters in the periphery: what crowds best?. Journal of Vision 2013;13(9):572. doi: 10.1167/13.9.572.
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Foveal visual acuity is ~3x worse for contrast-modulated (CM) than for luminance-modulated (LM) Cs and shows greater magnitude and extent (in arcmin) of contour interaction, which is more robust to blur (Hairol et al, VSS2010; Waugh et al, VSS2011). In agreement with Wong et al (2001), these findings suggest that CM stimuli may be useful for earlier detection of anisometropic amblyopia. In this study, we examine contour interaction and crowding on visual acuity for foveal and eccentric (2.5, 5 and 10 deg) viewing, giving insight into strabismic amblyopia (Levi & Klein, 1982). Depths and extents of contour interaction and crowding were assessed by comparing performance for a C with and without surrounding bars, and for HOTV letters with and without surrounding bars or letters, as per Flom (1963). With eccentric viewing, the isolated letter generating 80-90% performance for CM stimuli was ~3x bigger than for LM stimuli, but to discriminate the direction of the C it was larger than to recognize HOTV letters, particularly eccentrically. When Cs and letters were surrounded by bars, performance dropped 20-45% across eccentricity; for Cs, slightly more for CM than LM stimuli. When letters were surrounded by letters, performance dropped more (40-60%), "bottoming out" at small eccentricities. Extents (arcmin) of interaction were greater at the fovea for CM stimuli, however with eccentric viewing, they were similar for LM and CM stimuli at roughly 0.5x the eccentricity, following Bouma’s Law. These results suggest that C and letter acuities tap into different underlying processes; the C reflects resolution and positional uncertainty; the letters reflect shape discrimination, extracted at a later common stage for LM and CM stimuli. Letters surrounded by letters near the visual acuity limit show stronger crowding effects, but they are similar for LM and CM stimuli, at least in normal peripheral vision.
Meeting abstract presented at VSS 2013
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