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Shaheen P. Karim, Joseph Demer; Facilitation of Human Vertical Vergence by Vertical Saccades. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4693.
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While both horizontal and vertical saccades are known to facilitate horizontal vergence, effects of saccades on vertical vergence have been unstudied. We investigated the effect of vertical saccades on prism-induced vertical vergence in humans.
Eye movements of 9 young adults with normal binocular vision and stereopsis were studied using scleral magnetic search coils. Subjects viewed 400 cm distant LED targets at eye level, and 15° above and below. Vertical vergence was induced by a 1° prism placed over the right eye for three 10 s intervals alternating with equal periods of normal vision. For trials combining vertical vergence and saccades, prism was placed only over the eccentric target, with saccades at 10 s intervals. Vertical vergence amplitudes and time constants (τ) were computed from exponential fits.
Mean vertical vergence amplitude during fixation was 0.98° ± 0.26 (± SD), and with saccades 1.03° ± 0.24 (p > 0.50), in each case corresponding to prism power. For down saccades and prism base up, mean τ was 0.64 ± 0.30 s in 5 subjects who had sufficient data, significantly shorter than τ during fixation of 1.11 ± 0.22 (p < 0.03). For down saccades and prism base down, mean τ was 0.65 ± 0.44 s in 7 subjects, significantly shorter than fixation τ of 1.44 ± 0.50 s (p < 0.01). Many subjects could not achieve vertical vergence during up saccades, and those few who could do so exhibited greater variability than for other conditions. With prism base up, τ during fixation was 3.63 ± 3.37 s in 3 subjects, compared to 1.04 ± 0.57 s during saccades (p < 0.12). With prism base down, τ during fixation was 2.91 ± 2.69 s in 5 subjects, longer than average value during saccades, 0.52 ± 0.34 s (p < 0.20).
Vertical saccades speed up prism-induced vertical vergence in humans, although significant directional asymmetries exist. Down saccades speed vertical vergence regardless of prism base up or down orientation. These findings extend those of Enright (1984) and Zee (1992), who found facilitation of horizontal vergence by both horizontal and vertical saccades. Upward saccades have paradoxical effects, interfering with the ability of many subjects to achieve vertical vergence at all, but speeding it in those subjects who can.
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