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AJ Simmers, T Ledgeway, RF Hess, PV McGraw; Contrast and Motion Specific Deficits In Amblyopia Measured Using The Global Motion Paradigm . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3941.
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Purpose: To investigate whether there are additional deficits at the level of the dorsal stream within the extra-striate cortex, we investigated global motion processing for both luminance-defined (first-order) and contrast-defined (second-order) stimuli in a group of adult amblyopes using a method that allows us to assess the influence of the known V1 deficit to contrast sensitivity. Methods: Global motion thresholds were measured using an adaptive staircase procedure, in which a percentage of dots (signal) within a random-dot-kinematogram (RDK) moved in the same direction, the remaining noise dots having random direction. Each image contained 50 dots (dot density of 0.44 dots/°2) the diameter of each dot was 0.47° with a drift speed of 5.7 °/s. For any given presentation, subjects had to discriminate the direction of the coherent global motion of the signal dots. Performance, quantified in terms of the minimum number of signal dots (coherence) required to support direction discrimination, was measured as a function of the modulation depth (visibility) for both first-order and second-order motion RDKs. Results:Similar to normal observers global motion thresholds in amblyopia were found to exhibit asymptotic behaviour at high levels of dot modulation and increase markedly as the magnitude of the dot modulation decreased. However, most amblyopes displayed reduced performance on the task, with the underlying deficit being composed of both visibility and global motion components. The relative contributions of visibility and global motion processing to the overall deficit were not significantly correlated for either first-order (p=0.8) or second-order (p=0.5) stimuli. The global motion deficit being greater for second-order stimuli (p=0.04). There was also no significant correlation between any aspect of first-order and second-order performance. Conclusion: A number of previous studies have provided indirect evidence for a motion processing deficit in amblyopia. These results are the first to use global motion stimuli, to show directly that there is a deficit for global motion in amblyopia unrelated to the contrast sensitivity deficit known to reside in V1.
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