The influence of motion-defined form on the perception of spatially-defined form.

It is well established that the visual system is sensitive to the global structure--or "form"--of objects defined exclusively by spatial or motion cues, but it remains unclear how form perception combines spatial and motion cues if these are presented concurrently. In the present study, we introduce a novel class of stimuli where spatial-form and motion-form can be superimposed and manipulated independently. In both the spatial and motion domains, global structure consisted of radial-frequency (RF) contours defined by a virtual circle of Gabor elements whose positions and/or drift speeds were sinusoidally modulated at a specified frequency of polar angle. The first two experiments revealed that observers encode the global structure of spatial-RF and motion-RF contours presented in isolation. In a third experiment, observers detected a spatial-RF modulation superimposed on a motion-RF pedestal of identical radial frequency: results showed little facilitation at low pedestal amplitudes but significant masking at higher pedestal amplitudes, especially if the RF modulations of test and pedestal were in anti-phase. Additional experiments demonstrated that masking of the spatial-RF test is abolished if the global structure of the motion-RF pedestal is altered or destroyed while local motion cues are preserved. We argue these results cannot be explained by local neural interactions between spatial and motion cues and propose instead that data reflect higher-level interactions between separate visual pathways encoding spatial-form and motion-form.