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P. Satgunam, N. Fogt; Vergence Dynamics After Sustained Convergence in Convergence Anomalous Subjects. Invest. Ophthalmol. Vis. Sci. 2008;49(13):133.
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Vergence-anomaly is the absence of vergence response to briefly exposed disparate targets in otherwise normal subjects. Defective coarse disparity detectors have been proposed to be the mechanism responsible for vergence anomaly. In the current study we show that vergence anomaly can disappear after a period of sustained convergence.
Computer generated concentric square targets (1.6º-1.0º) were displayed haploscopically on front surface mirrors at 40 cm. The edges of the computer screen were masked using a black screen and the entire testing procedure occurred in a dark room to avoid fusional artifacts. Subjects’ head movements were minimized using a chin rest. Eye movements were recorded using the ISCAN infrared eye tracker goggle (120 Hz). Subjects viewed a central fixation cross for 2s, followed by a 12º convergence target for 5s. From the 12º vergence position, subjects then made a divergence or convergence movement of 4º (0.2s exposure). The 12º vergence target was again shown for 5 minutes and the 4º vergence movement was re-measured. 20 trials (10 convergence and divergence) were measured (one trial each day).
3 of 20 subjects were identified as convergence anomalous. In 39% (11/28) of the analyzable trials on these subjects, convergence movement was absent to the transient stimulus before sustained vergence and a vergence movement of amplitude1º ± 0.2º was present after sustained vergence. In 54% (15/28) of the trials subjects showed vergence movement to the transient stimulus both before and after sustained vergence. In one subject on one trial no vergence movement was observed before and after sustained vergence, and in the same subject vergence movement was absent only after sustained vergence on one trial.
Following sustained vergence, vergence anomalous subjects were sometimes able to respond to the transient convergence target. The result suggests that sustained vergence can bring about an improvement in disparity detection.
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