Three experiments on tachistoscopic perception of visual stimuli demonstrate that the visual system is sensitive to global topological properties. The results indicate that extraction of global topological properties is a basic factor in perceptual organization. (Author)

Training improves the ability of human observers to discriminate between two similar directions of motion. This gradual improvement is specific to the direction on which an observer is trained, enduring for several months. Improvement does not affect motion perception generally, nor does it depend on recognition of details of the movement. (Author)

Reports effects of context that are entirely perceptual. Visual discrimination was enhanced when line segments were flashed in a region that was perceived as a figure. Discrimination was substantially degraded when the same region was seen as ground although the physical stimulus remained identical throughout figure-ground reversals. (Author/JN)