Monocular detection of local luminance increments was studied psychophysically, during fusion, rivalry, and nonrivalry, Visibility of contrast flashes occurring less than 0.5 deg away from the center of the fovea, was affected by ipsilateral masking contours in such a way as to suggest that the contrast probe enhanced detectability in the tested eye. Visibility of luminance increments, whether flashed or continuously given in one monocular field, was best when a few contours were present in the tested eye but absent (or suppressed) in the partner eye. On the average, detection was poorest during rivalry. In fusion, detection was intermediate between rivalry and monocular dominance. It is proposed that background luminance summation is the specific mechanism in fusion.