Cooperative and competitive spatial interactions in motion integration

Recovering the velocity of objects moving in the visual field requires both the integration and segmentation of local neuronal responses elicited by m oving stimuli in primary visual cortex. Herein, we investigate the effects of the contrast, density, spatial proximity, spatial frequency, and spatial configuration of component motions on these complementary processes. Measu ring the ability of human observers to discriminate the global direction of motion displays composed of spatially distributed patches of drifting grat ings whose motion is locally ambiguous, we provide psychophysical evidence that linking component motion across space is facilitated at low contrast a nd high patch density. Furthermore, direction discrimination depends on the spatial frequency of component gratings and is more accurate for spatial c onfigurations that contain; "virtual" L junctions as compared to configurat ions composed of "virtual" T junctions. We suggest that the conditions yiel ding global motion coherence can be accounted for by the existence of aniso tropic cooperative/competitive, contrast-dependent, long-range interactions among oriented direction-selective units. In addition, we bring evidence t hat motion segmentation processes rely upon the processing of moving local spatial discontinuities. The results are discussed in the light of recent p sychophysical and physiological evidence that long-range excitatory and inh ibitory interactions within primary visual cortex modulate perceptual linki ng.