Simultaneous motion contrast across space: involvement of second-order motion?

A static or counterphase (target) grating surrounded by drifting (inducer) gratings is perceived to move in the direction opposite that of the inducers. We compared the relative magnitudes of these simultaneous motion contrasts generated by both first-order and second-order stimuli. The first-order stimuli were sinusoidal luminance-modulations of a uniform field, and the second-order stimuli were sinusoidal contrast-modulations of a random-dot field. When the target was a static grating, the second-order stimuli induced little motion contrast, while the first-order stimuli of the same effective contrast produced clear motion contrast. When the target was a counterphase grating, both first- and second-order stimuli produced clear motion contrast. These results are discussed in relation to the involvement of second-order motion pathways in the relative-motion processing, and the two types of motion aftereffects obtained with static and dynamic test stimuli.