Varikuti PhD thesis final.pdf - Anglia Ruskin Research Online

McMonnies and Ho, 2000) is significant to determining visual acuity, inaddition to the progression of letter sizes between rows (Bailey and Lovie,1976; McGraw and Winn, 1993), the spacing between the adjacent letters(Liu and Arditi, 2000; Shah, Laidlaw, Brown and Robson, 2010; Norgett andSiderov, 2011) and the accuracy of fixational and saccadic eye movements(Flom, 1991).Optotype identification is more difficult when surrounded or crowded byother features or targets (Ffooks, 1965; Keith, Diamond and Stansfield,1972; Hilton and Stanley, 1972; Youngson, 1975; Friendly, 1978). Visualcrowding is the phenomenon whereby visual acuity is adversely affectedwhen optotypes are presented together instead of in isolation (Stuart andBurian, 1962). Flom (1991) suggested that crowding was due to the effect ofcontour interaction (the detrimental effect of visual acuity due to theinfluence of neighbouring optotypes), eye movements (fixational eyemovements needed to fixate on each optotype that needs to be identifiedand saccadic eye movements needed to fixate from one optotype toanother) as well as an attentional component (attention needed to separatea target optotype from the flanking optotypes while identifying eachoptotype in a linear or whole optotype chart).Although crowding is observed with other visual tasks such as stereopsis(Butler and Westheimer, 1978), vernier acuity (Levi, Klein and Aitsebaomo,1985) and moving targets (Bex, Dakin and Simmers, 2003), this thesis islimited to the study of the effect of crowding on visual acuity measurementand its clinical implications.2