Fechner's paradox in binocular contrast sensitivity.

It is generally accepted that binocular spatial contrast sensitivity in normal observers is higher than monocular sensitivity by some 42% across all spatial frequencies, an amount predictable on the basis of neural summation of the two monocular responses. Such summation predicts that a reduction of sensitivity in one eye would result in a fall in binocular sensitivity to a level approaching, but never lower than, that of the other eye. We present evidence that reduction in monocular sensitivity caused by reduced luminance can, in some subjects, lower binocular sensitivity to a level below that of the other eye, an analogue of Fechner's brightness paradox. In other subjects the expected summation occurs and binocular sensitivity always remains at or above the monocular level.