Coherent directional motion can be seen if an image is displayed in two sequential frames (F1 and F2), where F2 is a translated version of F1. A similar two-frame sequence can produce metacontrast masking: the visibility of a leading target (F1) is reduced by a trailing, spatially nonoverlapping mask (F2). Strict temporal succession of the stimuli has been considered essential for both motion and masking. This requirement for a minimum stimulus-onset asynchrony (SOA) is known as the SOA law. Contrary to the SOA law, we found that motion and masking can be obtained with simultaneous onsets of the stimuli, provided that F2 outlasts F1. We compared motion and metacontrast with simultaneous onsets of the stimuli (SIM paradigm) with the traditional paradigm in which an interstimulus interval (ISI) is inserted between the leading and the trailing stimuli (ISI paradigm). We studied the effects in light-adapted and in dark-adapted viewing, each over a wide range of stimulus intensities. Homologous results were obtained with the two paradigms, thus disconfirming the SOA law. Models of motion sensors, such as that proposed by Reichardt [in Sensory Communication, W. A. Rosenblith, ed. (MIT Press, Cambridge, Mass., 1961), p. 303], are inherently capable of explaining the motion results obtained with both paradigms. The masking results with the SIM paradigm disconfirm theories based on onset-locked slow excitatory and fast inhibitory responses but can be explained in terms of Bridgeman's network model [Bull. Math. Biol. 40, 605 (1978)]. In light of the results obtained with the two paradigms, we discuss, and tentatively support, the suggestion that motion and metacontrast may be complementary parts of a unitary perceptual system.