Binocular summation in vernier acuity.

Monocular and binocular abutting line vernier acuities were measured as a function of contrast. Over a range of contrasts from near the line-detection threshold to approximately 20 times threshold, binocular vernier thresholds are lower (better) than monocular thresholds by approximately 50-60%, similar to the binocular improvement found for the detection of both a thin line and a dipole. At higher contrasts the binocular advantage diminishes, apparently as a result of saturation.