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Maria Lev, Jian Ding, Uri Polat, Dennis Levi; Two eyes are not better than one with crowded targets. Journal of Vision 2019;19(10):66. doi: https://doi.org/10.1167/19.10.66.
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Introduction: For a broad range of near threshold stimuli and tasks, two eye are better than one. Binocular summation (BS) is ubiquitous, resulting in improved sensitivity by a factor of ~1.4. However, we note that most experiments and models of BS have focused on local targets. Here we explored the effect of flankers on BS. Methods: We measured both letter identification and contrast detection in healthy young adult observers with fully corrected vision. Letter identification was measured with low contrast (30%) E targets flanked by an array of E flankers presented for 40 ms at the fovea. Target-flanker separation was either one or 0.4 linter-letter-spacing (“crowded”). Contrast detection was measured for a Gabor patch, either in isolation or flanked by two high contrast flankers, positioned either in collinear or orthogonal configurations. Target-flanker separation was varied to measure the effect of spatial interactions, both in the fovea and at an eccentricity of 4 degrees. For both experiments, monocular (each eye) and binocular stimuli were randomly interleaved using shutter goggles (NVIDIA 3D Vision, 120 Hz, background luminance for non-stimulated eye was 40 cd/m2)Results: for single targets (letter or Gabor) and orthogonal configurations, binocular performance was ≈ 25 - 30% better than monocular performance. Similar summation was found for the collinear configuration at large target-flanker separations. However, there was no binocular advantage for letter identification in the crowded condition, or for contrast detection in the collinear configuration for short target-flanker separation of 3 lambda (non-overlapping). Interestingly, for orthogonal flankers, BS is found for all target-flanker separations. Conclusions: Our results show that nearby flankers completely eliminate binocular summation for letter identification and contrast detection, but only when the configuration is collinear. Current models of binocular combination need to be updated to explain the effects of spatial interactions on binocular summation.
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