APPARENT MOTION PRODUCED BY TEMPORALLY MODULATED BRIGHTNESS CONTRAST AND ASSIMILATION

When regions containing a counterphasing sine-wave grating are present ed side by side and in spatial and temporal quadrature phase, a transp arent perception of motion results. This occurs even though none of th e stimulus parts is moving. The two percepts of motion in these displa ys are in opposite directions, one analogous to brightness contrast, t he other to brightness assimilation. If the regions are separated by a gap, the contrast and assimilation motions remain visible for separat ions up to 0.5 and 1 period, respectively. Both motions occur at tempo ral frequencies from 1 to 16 Hz. The perceived motion analogous to bri ghtness assimilation is easily modeled with elongated receptive fields that integrate flux along the long axis, such as simple cells. The pe rceived motion analogous to brightness contrast can be accounted for b y receptive fields that subtract the flux in one region from the flux in another region. Examples are center-surround subunits such as are f ound in the elaborated Reichart model [W. Reichardt, in Sensory Commun ication, W. A. Rosenblith, ed. (MIT Press, Cambridge, Mass., 1961), pp . 303-317; J. P. H. van Santen and G. Sperling, J. Opt. Sec. Am. A 2, 300-321 (1985)]. The dual perceived motion suggests that more than one kind of motion channel (distinguished by the two-dimensional receptiv e field of the front-end filter) is present in the human visual system .