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P. Thiagarajan, K. J. Ciuffreda, D. P. Ludlam; Vergence Dynamics to Congruent and Non-Congruent Stimulus Visual Fatiguing Tasks. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2546.
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To evaluate vergence dynamics by inducing visual fatigue using congruent and non-congruent vergence tasks.
Binocular horizontal position of the eyes was recorded objectively using the Power Refractor II (12Hz sampling rate). Convergence and divergence dynamics were assessed for a 10.3º stimulus amplitude. Targets comprised the red fixation LED located on the measuring head of the Power Refractor at 100cm and a white LED placed at 25cm, both aligned along the midline. Subjects (asymptomatic, visually-normal adults, n=7) altered bifixation between the targets every 3-4 seconds upon command for 2 minutes. Congruent (C) (vergence stimulus=accommodative stimulus) and non-congruent (NC) (vergence stimulusaccommodative stimulus) tasks of equal vergence demand (4.5MA) were tested on 2 different days. Each session had three phases: Pre-task vergence dynamics, fatigue task (C/NC), and post-task vergence dynamics. C task: Subjects altered bifixation (50 cycles) between a reduced Snellen chart at 2m and 20cm every 3 seconds to the beat of metronome. NC task: Subjects performed 50 cycles of rapid vergence flipper (7Δ BO/BI) while bifixating a reduced Snellen chart at 40cm. Pre- and post- task mean amplitudes, time constants, and peak velocities were compared. In addition, vergence dynamics was recorded in 4 of 7 subjects to 100 cycles of vergence using the C task.
There was no significant difference (p>0.05) for convergence and divergence between the pre- and post- fatigue group mean amplitudes, time constants, and peak velocities for both the C and NC tasks. With the C task, 2 of 7 subjects exhibited a 19-35% decrease, and 4 of 7 subjects exhibited a 5-34% increase, in convergence and divergence peak velocities. With the NC task, 3 of 7 subjects exhibited 7-8% decrease, and 3 of 7 subjects exhibited a 20-30% increase, in peak velocity for convergence and divergence. Only one of 4 subjects exhibited a decrease in peak velocity (41%) with 100 cycles of vergence.
The vergence system did not consistently exhibit velocity-based fatigue effects in the majority of subjects. Decreases in peak velocity suggest vergence fatigue involving transient dysfunction of the velocity pulse controller, while increases suggest oculomotor learning. Further experiments are needed to understand basic mechanism underlying the neurology of vergence fatigue.
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