Effect of Rapid Alternating Binocular Flicker on Vergence and Stereopsis

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Purpose: Rapid, alternating occlusion therapy is one of several binocular treatment options for amblyopia. Recent studies suggest it may improve stereoacuity and letter acuity in some amblyopic subjects. Little is known about its impacts on binocular functions like stereopsis and vergence. Here we examine the effect of alternating (out-of-phase) flicker and in phase flicker of different frequencies on stereopsis and vergence eye movements. Methodology: Stereoscopic disparity thresholds were measured using a 30 trial, 1 up 2 down staircase beginning at 10 arc minutes disparity. Five staircases were obtained from each of five healthy adult subjects for all combinations of contrast (100%, 50%, 25%) and flicker (2, 4, 8, 16 and 32Hz, in-phase and out of phase binocular flicker, and no flicker). The stimulus was a spatially filtered noise pattern (1 cpd, 1 octave bandwidth), presented for one second. Vergence eye movements were measured using a binocular dual Purkinje image eye tracker. The vergence target was a 30-degree diameter filtered noise pattern of spatial frequency either 0.25 or 2 c/deg with a bandwidth of 1 octave. Stimulus contrast ranged from 3% to 100%. The trials were 2 seconds long. Results: Flicker elevated stereoscopic thresholds in most subjects. The flicker did not abolish stereopsis, however: subjects were able to detect a few arc minutes of disparity at all but the lowest frequencies alternating phase flicker. Lower frequencies of flicker impaired but did not abolish stereoscopic thresholds in most subjects. Vergence responses did not show any difference at higher contrast stimuli for all the flicker frequencies. At lower contrasts, latency, velocity, and final amplitude accuracy were lower for lower frequency compared to high frequency flicker. Conclusion: When it comes to stereopsis, results show that thresholds are elevated by both alternate and in phase flicker but are mildly impacted only at the lowest temporal frequencies of 2 and 4Hz. The results of vergence measures show that there are minor differences in vergence responses between out of phase and in phase flicker and the vergence responses showed a trend of decrease in vergence performance compared to vergence responses with in-phase flicker.

Stereopsis, ocular convergence, ocular divergence, alternating flicker