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Ashli F. Milling, Paul C. Knox; Dynamic Eye Alignment In Two Types Of Conjugate Saccade Task. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4864. doi: https://doi.org/.
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During and after a conjugate saccade, the challenge to the visuomotor system is to maintain appropriate eye alignment. It has been reported that in adults conjugate saccades are accompanied by a transient divergence of the eyes, followed by a convergent post-saccadic drift. However, details of the proportions of convergent (C) versus divergent (D) responses are rarely provided, and usually sample sizes have been small and small point targets have been used. We compared performance to point targets with performance to letter targets, and found a varied pattern of vergence responses.
After an orthoptic assessment, 11 adult subjects (mean age:24.5y; 3 male; 8 right eye dominant) with normal visual acuity and ocular motility completed two runs of 120 saccade tasks. After a variable fixation period (1-1.5s), a saccade target appeared randomly 5° or 10° to either right or left of fixation. In one run the target was a small point target (a 0.2° black square), in the other it was an uppercase letter (height 0.2°; Arial font). In the letter task, the target letter varied randomly and subjects had to respond with a button-press when it was a vowel. Eye movements were recorded using infra-red oculography. Vergence angle was calculated by subtracting right from left eye position, the direction of the peak vergence response (C vs D) during the primary saccade was recorded and its amplitude measured.
Divergent (D) responses occurred in 67±33% (range 8% to 100%) of trials in the letter task and 72±26% (15% to 100%) in the point task (intersubject mean±SD). However, for the letter task 73±31% were D responses for leftward targets, and 60.5±35% for rightward targets. This pattern was reversed for the point task (L: 63±24%; Rightward 81±25%). In a repeated measures ANOVA with task type (letter vs point) as a between subjects factor, and direction as a within subjects factor, neither factor was statistically significant (both p>0.5) but the interaction term bordered on significance (p=0.053). The mean amplitude of vergence change (ignoring whether it was D or C, collapsed across direction and target amplitude) was 2.3±0.8° for the letter task, and 2.11±0.7° for the point target (t=1.2, p=0.23).
Our results suggest a marked variability between subjects in eye alignment around the time of conjugate saccades that is influenced by task demands.
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