Recovering the velocity of objects moving in the
visual field requires both the integration and segmentation
of local neuronal responses elicited by moving 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. Measuring the ability of human
observers to discriminate the global direction of motion
displays composed of spatially distributed patches of drifting
gratings whose motion is locally ambiguous, we provide
psychophysical evidence that linking component motion across
space is facilitated at low contrast and high patch density.
Furthermore, direction discrimination depends on the spatial
frequency of component gratings and is more accurate for
spatial configurations that contain “virtual”
L junctions as compared to configurations composed of “virtual”
T junctions. We suggest that the conditions yielding global
motion coherence can be accounted for by the existence
of anisotropic cooperative/competitive, contrast-dependent,
long-range interactions among oriented direction-selective
units. In addition, we bring evidence that motion segmentation
processes rely upon the processing of moving local spatial
discontinuities. The results are discussed in the light
of recent psychophysical and physiological evidence that
long-range excitatory and inhibitory interactions within
primary visual cortex modulate perceptual linking.