Binocular fusion regarded as simple summation of monocular photopic luminances

Binocular (B) and monocular (BM) increment intensity at threshold was measured on binocularly fused backgrounds of luminance I (30 cd/m2), or 2I (60 cd/m2). Simple summation (S) predicts S = BM(I)/B(2I) = 1. Mean results yield S = 0.99. The mean was similar in the 4 observers, and it did not change with absolute intensity at threshold, which decreased tenfold, as duration of the increment was increased from 3 and 240 msec. Simple summation of monocular luminances occurring during binocular fusion is the most likely explanation of the results.