What is a visual stimulus?

When two stimuli, s 1 and s 2 , having different phase spectra are added we get not only that the amplitudesin (s 1 + s 2 ) will be reduced relative to the amplitudes in s 1 plus the amplitudes in s 2 but also that thedistribution of amplitudes in the spectra (as well as the spectrum of the squares of the amplitudes) may bealtered. Yet their appearance may remain unaltered. When the amplitude spectrum of a stimulus has beenaltered it can no longer be held to be the same stimulus. Visual stimuli can, therefore, not be counted on tobe the same stimuli when they are combined as when they are presented in isolation even if they have thesame appearance. This is due to interference in the stimuli which is unrelated to vision. These observations,it would seem, have implications for a number of areas in visual science among them visual masking, visualcrowding, visual search, and for the adding of noise to stimuli.