Binocular Summation at the Absolute Threshold of Peripheral Vision*

Binocular probability of seeing, P(B), 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. P(B) was greater than the value that would be predicted if the two eyes were independent detectors [P′(B)] for interstimulus intervals below 100 msec. Two peak values of P(B) were observed, one occurring close to zero and the other close to 90 msec. P(B) and P′(B) 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.