A model is presented which assumed that visual detection of a motion depends on two separate Systems having different processing requirements. The Movingness Analyzing System (MAS) would process on the basis of spatiotemporal summation of the impinging energy, while the Displacement Analyzing System would process changes in positions, comparing the actual position of the object with the trace of a previous position. Hypotheses are developped which provide criteria for making distinctions between conditions in which one or the other System is the prominent processor. The MAS would predominate in peripheral vision, at low levels of luminance, short exposure times, fast velocities and small areas of the object. The DAS would predominate in foveal vision, when the visual motion contains stationary positions, stationary references close enough to the motion track. Data are reviewed which can be handled in the present model.