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Tom C. A. Freeman, Andrew J. Kolarik, Tom H. Margrain; Accuracy and precision of tracking eye movements as a function of age. Journal of Vision 2008;8(6):668. doi: 10.1167/8.6.668a.
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The accuracy of pursuit and optokinetic nystagmus is known to decline with age. However, only a few studies have compared these two types of ocular following in the same observers and those that do confound stimulus size with type of eye movement (small for pursuit, large for nystagmus). Moreover, measures of precision have not been reported, despite the detrimental effect noisy eye movement may have on sensitivity to motion and visual detail. Eye movements were therefore recorded in younger (age 20–34) and older (age 60–82) observers who viewed grating patterns (0.6 cpd) moving at speeds of 4.8, 9.6, 19.2, and 38.4 deg/s. In the deliberate condition, they were asked to ‘follow’ the gratings and in the reflexive condition, they were asked to ‘stare’. Gain was used to assess accuracy by dividing mean eye speed by target speed. Precision was assessed by partitioning motion variability into Drift (standard deviation across a set of slow-phase means) and Shake (median across a set of slow phase standard deviations - the distribution was skewed). This is analogous to partitioning the sums-of-squares in ANOVA. Drift reflects long-term variability across slow-phases. Shake reflects short-term variability within each slow-phase. We found that eye movements were less accurate in older observers by around 10–15%, while drift was similar across age groups. Shake increased with stimulus speed, slightly more so in older observers in the deliberate condition. However, no age-related differences in shake were found in the reflexive condition. A similar experiment using random dot patterns yielded little differences between age groups on any of the measures. This is possibly because features in the dot patterns were easier to track than the repetitive features in gratings. We will also report on how accuracy and precision depend on age for stimuli containing less predictable motion.
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