The Conditions under which Opposing Motion is Seen in Transparency or as Flicker

We present several examples of moving stimuli comprising several harmonic components that can be perceived either as moving independently, or together as a coherent moving pattern. A simple example is two sinusoidal gratings of equal contrast and spatial frequencies moving in opposite directions that are perceived as a single grating modulated sinusoidally over time (counterphase). However, two square waves drifting in opposite directions, while being a superposition of counterphasing pairs, are perceived as two distinct patterns drifting in transparency one over the other. Intermediate situations such as pairs of counterphase gratings can be perceived alternatively as counterphases or as drifting in two directions.

We show that the relative phase of the components plays a fundamental role in whether they group or become transparent. The tendency of a component to become part of a pattern is maximal when the phase relationship is 0 deg, and gradually decreases to a minimum at 90 deg. This agrees quantitatively with previous measurements on different stimuli (paper presented at Del Viva and Morrone, ARVO96), suggesting a common computational mechanism. The data were well modelled by a nonlinear model of motion analysis based on feature tracking, showing sensitivity to the exact degree of nonlinearity.