Foveal spatial summation was investigated in light-adapted subjects. Stimuli were luminance increments or decrements of two 1.5′ square targets separated by 2.5′ to 15′. Stimuli included increment-increment, decrement-decrement, decrement-increment and increment-decrement. For a separation of 2.5′, increment-increment combinations were detected most easily. At a separation of 7.5′, increment-decrements were most readily detected. At the 15′ separation all stimulus combinations were equally detectable. While perfect spatial summation was not found at any separation, summation was most efficient at the smallest separation. Inhibitory effects, which were inferred from the greater sensitivity to increment-decrements at 7.5′, were large even at 2.5′. A model based on excitation and inhibition was proposed to explain the observed interactions including the inhibition. It postulates two complementary channels, one that sums quanta-over a small region of the retina and another that compares the quantum sums at two small regions and signals their difference. The model also explains inhibition phenomena observed between increments and decrements separated by time or by wavelength.