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Onkar H Pirdankar, Harold E Bedell, Vallabh E Das; Fixation Stability in Monkeys with Strabismus.. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2574. doi: https://doi.org/.
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To assess the effect of fixation target parameters on fixation stability of the viewing and fellow eyes in normal and strabismic monkeys.
One normal and three exotropic monkeys were presented with four differently shaped fixation targets (x, disk, disk + cross, and %), with diameters of 0.5°, 1° or 2°, during monocular or binocular viewing. Fixation targets were white on a black background or vice versa. Eye movements were recorded using the magnetic search coil technique and fixation stability quantified by calculating the Bivariate Contour Ellipse Area (BCEA). Multi-factorial ANOVA was performed to analyze the effects of target shape, size, background and viewing condition on BCEA.
BCEA was greater (more fixation instability) in the three strabismic monkeys compared to the normal monkey. The BCEAs of the covered (deviated) eye of the strabismic monkeys was significantly greater than the BCEA of the fixating eye (paired t-test p<0.001), but not in the normal monkey. Target shape and size significantly affected fixation stability in all monkeys. A disk-shaped target resulted in significantly higher BCEAs than other target shapes in the viewing eyes of the normal monkey and 2/3 strabismic monkeys (p<0.001). A similar target-shape effect also was observed in the covered eye (p<0.03). Best fixation was elicited with a 0.5° target in the normal monkey and a 1.0° target in the strabismic monkeys, both in the viewing (p<0.005) and the covered eye. Background effects were idiosyncratic among the monkeys. Fixation stability was similar during monocular and binocular viewing in all monkeys. Analysis of the BCEA ratios for the viewing and covered eyes revealed no significant effect of target shape in any monkey and no significant effect of target size except in one monkey, suggesting proportional changes in the stability of both eyes due to shape and size.
The difference in fixation stability between normal and strabismic monkeys is likely due to underlying amblyopia, abnormal drifts and nystagmus eye movements. Target parameters (shape and size) that influence fixation stability in a normal animal also affect fixation stability in strabismus. The influence of target parameters on fixation likely functions via conjugate mechanisms, as proportional effects were observed in both eyes of all monkeys.
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