Detection rates were determined for multiple flashes (20-msec duration, 7.5 arc min dia) presented foveally to one eye while the other eye had either a uniform target (the "monocular" condition), a congruent grating (fusion), or an orthogonal grating (rivalry). Average detection rates were highest in the monocular condition, and they were about the same in fusion and rivalry. In all conditions, the typical error (some 70% of all errors) was one stimulus missed out of several simultaneously presented. It was concluded that sensitivity fluctuates over the fovea, even in the absence of contours in the other eye. In two observers, the ratio of detection rates for the two eyes was about the same for all conditions; in two other observers, this ratio decreased (eye dominance increased) in fusion, and especially in rivalry, relative to the monocular condition. The question is raised whether a common link exists between spontaneous fluctuations of sensitivity and those induced by dichoptic contours.