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A. F. Macedo, M. D. Crossland, G. S. Rubin; The Effects of Fixation Instability on Peripheral Visual Acuity. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5843.
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© ARVO (1962-2015); The Authors (2016-present)
People with central scotomas must use peripheral areas of the retina in order to look at an object. Eye movement control is impaired during this eccentric viewing, causing fixation instability. It is unclear whether counteracting this fixation instability improves vision in the peripheral retina. The purpose of this study was to measure peripheral visual acuity (PVA), with and without crowding, whilst modulating fixation instability by using gaze-linked stimuli.
Six observers with normal or corrected-to-normal vision participated in the study. Peripheral visual acuity was measured with a Landolt "C" target with and without crowding bars. The target was presented in four retinal positions at 5º and at 10º eccentricity. Gaze position was monitored throughout using an infrared eyetracker (SMI Eyelink I). The target was presented for up to 500 msec, either with no movement relative to gaze position (zero instability), with reduced movement relative to eye movements (damped instability) or with increased movement relative to gaze position (increased instability). Data were analysed using linear mixed models.
Without crowding PVA was slightly better with increased instability compared with damped instability (mean improvement = 0.04 logMAR; p < 0.05) but did not differ significantly from zero instability (mean difference = 0.01 logMAR; p > 0.05). PVA was equal for damped and zero instability conditions (p < 0.05). In contrast to the previous result, under crowded conditions PVA decreased markedly with increased instability compared with the other two conditions (mean reduction 0.13 logMAR, p < 0.001).
Increased fixation instability improves peripheral visual acuity for uncrowded tasks yet reduces visual performance in the presence of crowding. For people with unstable fixation, peripheral visual acuity measured with single (uncrowded) targets is not likely to be a good predictor of performance in real visual tasks. These results provide further evidence for the importance of fixation stability on reading ability.
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