Binocular probability of seeing, <i>P</i>(<i>B</i>), was measured as a function of the time between onsets of 2-msec, 35-min visual angle flashes to corresponding locations 7° horizontally displaced from the foveas of both eyes of dark-adapted subjects. <i>P</i>(<i>B</i>) was greater than the value that would be predicted if the two eyes were independent detectors [<i>P</i>′(<i>B</i>)] for interstimulus intervals below 100 msec. Two peak values of <i>P</i>(<i>B</i>) were observed, one occurring close to zero and the other close to 90 msec. <i>P</i>(<i>B</i>) and <i>P</i>′(<i>B</i>) were indistinguishable for interstimulus intervals above 100 msec. It is concluded that the largest barrier to seeing at threshold lies in a pathway common to the two eyes.
LEONARD MATIN, "Binocular Summation at the Absolute Threshold of Peripheral Vision," J. Opt. Soc. Am. 52, 1276-1285 (1962)