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Mythri Pullela, Suraj Upadhyaya, Santoshi Ramachandran, Anand C Joshi, Samuel Adade, Vallabh E Das; Effect of target parameters on fixational saccades in normal monkeys. Invest. Ophthalmol. Vis. Sci. 2017;58(8):751.
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Previously others and we have shown that target shape and size parameters can influence the stability of fixation as measured by a bivariate contour ellipse area (BCEA) metric. Eye movements during fixation of a stationary target are primarily made up of fixational saccades (fast) and drift (slow) components. The present study aims to study the influence of target parameters on fixational saccades in normal monkeys.
Two juvenile rhesus monkeys (M1, M2), implanted with binocular scleral search coils, were presented with fixation targets of two different sizes (2° and 0.5°), and two different shapes (optotype - ‘%’ and disk) during monocular and binocular viewing. Each combination of target size, shape and viewing condition was presented 5 times and duration of each trial was 60sec. A saccade detection algorithm modified from an unsupervised clustering method published by Otero-Millan and colleagues was used to detect fixational saccades within the fixation data. Amplitude, frequency and direction of fixational saccades were recorded and compared across different stimulus conditions using multi-factorial ANOVA at a significance level of 0.05.
We analyzed ~5200 fixational saccades in the two normal monkeys. The mean amplitude of fixational saccades in M1 was 0.42±0.24° and in M2 was 0.87±0.47. Mean frequency of fixational saccades was 1.38 saccades/sec in M1 and 2.37 saccades/sec in M2. Fixational saccade amplitude was significantly influenced by target shape and size in both monkeys. Fixational saccade frequency increased with target size in M1 only. A polar histogram developed from fixational saccade directions resembled target shape (‘%’ optotype or disk) for the larger targets. The relationship between BCEA and fixational saccade amplitude could be modeled as an exponential rise to maximum suggesting that fixational saccade amplitude was the primary determinant of the BCEA for stimulus conditions that resulted in high fixation stability.
Target parameters such as size and shape influence fixational saccade amplitude with larger target sizes resulting in larger amplitude fixational saccades. The distribution of fixational saccade directions shows that they play a role in scanning the features of the presented target. Fixational saccade amplitude is a limiting factor when fixation stability is high while drift components likely contribute significantly when fixation stability is poor.
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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