Perception and Eye Alignment Control: Do People Think the Same Way Their Brain Thinks?
Purpose: This thesis compares two distinct but related binocular disparity processing phenomena— stereoscopic depth perception and reflexive disparity vergence. Stereoscopic thresholds typically improve with increasing target contrast, but when only one eye’s target contrast increases, the stereoscopic thresholds are worse than for equal low contrast. A similar effect occurs for motion and vernier thresholds when the target components being compared have different contrast. The effect occurs especially for middle and low spatial frequencies. This study compares observers’ stereoscopic depth perception with vertical disparity vergence to understand if this ‘contrast paradox’ affects reflexive eye alignment control. Methods: Four binocular contrast combinations were tested, (OD/OS= 0.125/0.125 ‘low/low’, 0.125/0.5 ‘low /high’, 0.5/0.125 ‘high/low’, and 0.5/0.5 ‘high/high’), at 0.5cpd and at 1.0cpd spatial frequencies. Left/right image pairs were fused with the aid of a four-mirror haploscope and nonius lines were superimposed on the image to allow the subject to monitor eye alignment. Responses to imposed vertical disparity were detected indirectly as observers reported the direction of shifts in nonius alignment. Stereoscopic thresholds were measured by detection of a difference in horizontal disparity between upper and lower halves of the stereogram. A staircase method was used to find the disparity that produced 75% correct performance in each condition, taken as the mean of the last five staircase trials. Results: Results from 5 subjects show strikingly similar effects for stereoscopic depth and vertical eye alignment, including the detrimental effect of mixed contrast. Conclusion: These results imply that both stereopsis and eye alignment may be impaired for individuals with unequal retinal image contrast, as for example in unilateral cataract or monovision prescription.