THE PERCEIVED SPEED OF 2ND-ORDER MOTION AND ITS DEPENDENCE ON STIMULUS CONTRAST

Speed matches were obtained, using a spatial two-alternative forced-choice task, between a second-order motion stimulus and a first-order motion stimulus. The second-order motion stimulus was composed of contrast-modulated noise [produced by multiplying two-dimensional (2-d), static noise by a drifting, one-dimensional (1-d) sinusoid]. The first-order motion stimulus was composed of luminance-modulated noise (produced by summing, rather than multiplying, 2-d noise and a drifting sine grating). In Expt 1, the relationship between the perceived speed of first- and second-order motion was examined. The motion stimuli had the same spatial frequency (1 or 3 c/deg) and were equated for visibility by presenting them at the same multiple of direction-identification threshold. Over a range of physical speeds, the perceived speeds of the first-order and second-order motion stimuli were identical when their physical speeds were the same. In Expt 2, the effect of varying stimulus ''contrast'' (contrast modulation depth) on the perceived speed of second-order motion was examined. The contrast of the first-order motion stimulus was fixed and speed matches were obtained for second-order motion stimuli at several contrast modulation depths. The motion stimuli had the same spatial (1 or 4 c/deg) and temporal (5 or 20 Hz) frequencies. It was found that the perceived speed of second-order motion was approximately linearly related to log modulation depth. In agreement with previous studies we also confirmed that the perceived speed of first-order motion is similarly dependent on stimulus contrast (luminance modulation depth). The results are discussed in the context of current models of second-order motion perception.