ABSTRACT
Superimposing random dot patterns translating with sufficiently different speeds or directions, or superimposing motion pattern depicting optic flow components can lead to the perception of transparency, i.e. two or more structures or patterns which are moving through each other (e.g. Gibson et al, 1959, van Doom et al 1985, De Bruyn and Orban 1993). Transparency of complex motion patterns has been questioned with respect to a possible simultaneous processing, the rôle of foreknowledge, and its uniformity over the visual field (De Bruyn and Orban 1993; De Bruyn 1997, 1998). Perceptual transparency also exists in the stereo domain where stimuli can depict multiple patterns lying across one another and, as in the motion domain, the different spatially superimposed patterns are readily picked up by human observers (Julesz and Johnson, 1968). Surprisingly many studies which compare unidirectional and transparent motion use displays which are generated by superimposing unidirectional motions moving in opposite directions whilst keeping the speed of the individual components constant. The tacit presumption here is that the two opposite motions are independent of one another: the speed of transparent motion is presumed to be equivalent to the speed of the unidirectional components. To test this assumption a speed matching experiment was carried out. Observers were asked to match the speed of transparent and unidirectional motion and the ‘point of subjective equality’, i.e. when the two displays were perceived as having the same perceptual speed, was measured.
