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Shrikant R. Bharadwaj, Vaishnavi Raghuram, Vivek Labhishetty; Impact Of Lens-induced Anisometropia On Accommodative Responses To Step And Ramp Stimuli In Adults. Invest. Ophthalmol. Vis. Sci. 2011;52(14):832.
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Despite large inter-ocular differences in retinal image quality induced by anisometropia, adult accommodative responses to ramp changes in stimulus position (i.e. for gradual temporal changes in retinal blur) are only marginally smaller with induced anisometropia than habitual binocular viewing. To determine if this immunity to induced anisometropia also exists for step changes in stimulus position (i.e. for large instantaneous changes in retinal blur), this study compared accommodative performance to step and ramp stimuli in the presence of transiently induced myopic and hyperopic anisometropia.
Individually calibrated accommodative responses from both eyes of 8 emmetropic adults (19.2 to 28.4yrs) were recorded using a dynamic (30Hz) infrared photorefractor. Subjects watched high-contrast 20/40-sized letters on an LCD screen under four conditions: i) habitual binocular viewing, ii and iii) with +3.25D or -3.25D lens over right eye (induced myopic and hyperopic anisometropia, respectively) and iv) monocular viewing (right eye covered with an IR filter). In the ramp condition, the LCD screen moved between 55 and 30cm (1.5D demand) at 0.2D/s, with 4s stable period at each distance. In the step condition, the stimulus was switched electronically between two LCD screens placed at 55cms and 30cms, with a 4s stable period at each distance. Both eyes were dilated with 5% PHCl to maintain pupil size within the operating range of photorefractor.
Mean (+1SD) left eye’s accommodative response was largest under binocular condition (Ramp: 1.08+0.22D; Step: 1.10+0.23D), followed by monocular condition (Ramp: 0.96+0.29D; Step: 1.00+0.29D), -3.25D aniso condition (Ramp: 0.92+0.26D; Step: 1.01+0.26D) and +3.25D aniso condition (Ramp: 0.88+0.32D; Step: 0.95+0.34D). 2-factor ANOVA showed no statistically significant main effect of viewing condition (p=0.18) or stimulus movement (p=0.06). Accommodative responses of both eyes were consensual under all viewing conditions in 7 of the 8 subjects.
Marginal reduction in the size of ramp accommodative responses with +3.25D of induced anisometropia is similar to earlier observations. Similar accommodative responses to ramp and step stimuli indicates that the rate of change of retinal blur has no significant influence on how induced anisometropia impacts accommodative performance. Consensual accommodation with induced anisometropia suggests that most subjects did not employ a monovision-type focusing strategy for either the step or ramp stimuli.
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