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Suraj Upadhyaya, Mythri Pullela, Santoshi Ramachandran, Samuel Adade, Anand C Joshi, Vallabh E Das; Effect of target parameters on fixational saccades in strabismic monkeys. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3441. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Fixation in strabismic monkeys and humans is unstable. We have shown earlier that fixation stability as measured by a bivariate contour ellipse area (BCEA) metric is influenced by target shape and size parameters in normal and strabismic monkeys. One component of fixational eye movements that may contribute to fixational instability is fixational saccades. The purpose of this study was to evaluate the effect of target parameters on fixational saccades in strabismic monkeys.
Three juvenile prism-reared strabismic monkeys, implanted with binocular search coils, were presented with fixation targets of two different shapes (‘%’optotype and Disk) and two different sizes (0.5° and 2°) during monocular and binocular viewing. Each of the twelve combinations were presented 5 times for 60sec. A microsaccade detection algorithm modified from an unsupervised clustering method published by Otero-Millan and colleagues was used to detect saccades within the fixation data. Amplitude, frequency and direction of fixational saccades were calculated and analysed using multi-factorial ANOVA. Strabismic monkey data were compared with data from two monkeys with normal eye alignment.
We analysed ~21000 fixational saccades in the three strabismic monkeys. When compared to normal monkeys, amplitude of fixational saccades in both the viewing and non-viewing eye was greater in one strabismic monkey but frequency of fixational saccades was greater in all strabismic monkeys for at least one viewing condition. Target shape and size effects on amplitude of fixational saccades in the viewing eye were inconsistent across the strabismic monkeys. Target shape effects on frequency of fixational saccades were inconsistent but larger targets showed increased frequency in the viewing eye. Unlike in the normal monkey, a polar histogram of saccade directions did not resemble target shape.
The increased fixational instability observed in the strabismic monkeys could be partially due to increased amplitude and/or frequency of fixational saccades. Increase in frequency of fixational saccades is largely due to the quick phases of nystagmus. Target shape and size effects on fixation stability were not replicated when analysing fixational saccade amplitudes. Unlike in the normal monkey, directions of fixational saccades are dominated by nystagmus direction and therefore does not help in extracting salient feature of objects.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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