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P. Satgunam, N. Fogt; Vergence Dynamics After Vergence Adaptation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):888.
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The vergence controller contains a fast fusional component (FF) that moves the eyes in response to a disparity, and a slow component that maintains vergence posture. The slow component usually develops over several minutes of sustained vergence, and the neural innervation for the slow component usually dissipates slowly upon monocular occlusion. Thus, prolonged viewing of a near target can change the phoria position of the eye, a phenomenon called vergence adaptation. The purpose of this experiment is to determine whether the sustained innervation from the slow vergence component interacts with the burst of innervation associated with the fast vergence component.
5 subjects were enrolled. Computer generated anaglyphic targets (1.5º) were displayed at 65 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 and head rest. Eye movements were recorded using the IOTA Orbit Eyetrace (ver 1.71) infrared eye tracker goggle (500 Hz). Red (right eye) and blue filters were mounted in front of the eyes. Two sessions, no adaptation (NA) and adaptation (A) were performed on different days. Subjects viewed a central fixation cross for 2s, followed by a 6º convergence target (5s exposure for NA and 3 min for A trials). From the 6º vergence position, subjects then made a divergence or convergence movement of 4º (10s exposure). FF dynamics to the 4º target were measured.10 trials (5 convergence and divergence) in each session were measured on 4 subjects. 2 trials were measured in each session on 1 subject. A 3-5 min break was given between trials.
In the A trials, comparisons of the phoria before and after 3 minutes of sustained vergence showed that vergence adaptation was present in 76% of the analyzable trials in spite of the FF to the 4º target. Latencies, amplitudes, and velocities were calculated for the FF movements to the 4º target. Average differences between the convergence latency, amplitude and velocity for A and NA trials were 126ms, 0.8º and 9.6º/s. That for divergence was 45.7ms, 0.4º and 2º/s. Non-parametric Mann-Whitney test was performed on FF dynamics between NA and A trials. No statistically significant (p>0.1) differences were found.
There were no differences in vergence dynamics between the NA and A session. Since the peripheral apparatus of the vergence system is the same for the fast and slow components of the vergence controller, our result suggests that there is no interaction between these components. The output of the slow vergence component may be temporarily gated during a fast fusional vergence movement.
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