Visual stimuli that are spatially removed from a neuron's classic reception field and which they elicit no response can strongly affect the magnitude of a response to stimuli within the classic field. It is thought that lateral connections among neurons mediate the interaction. Behaviorally, the presence of surround stimuli can profoundly affect the ability to perform fine spatial discriminations between similar center patches. In the proposed research, we test recent findings from the neuroscience literature in the psychophysical domain. An understanding of the normal mechanisms governing these lateral interactions is critical for our understanding of normal visual processing. For example, excitatory contextual modulation is thought to reflect the operation of contour mechanisms that mediate reading, whereas inhibitory interactions over space may be involved in segregating parts of a scene. It is also of import to provide normal behavioral data as a baseline in clinical applications. Deficits shown in amblyopia and strabismus include both hyperacuity tasks and lateral interactions. Here, we use psychophysical discrimination tasks to ask how these mechanisms integrate or segregate information from different regions of space, and how the spatial arrangement of stimuli in one part of the visual field affects performance in another. Specifically, we test several hypotheses: (1) that the spatial extent of regions of psychophysically-determined summation and inhibition in discrimination depends upon the contrast; (2) that there are spatial asymmetries in contextual modulation of mechanisms mediating fine spatial discriminations; (3) that the modulation process can be characterized by at least one of three quantitative models of gain control; (4) that apparently contradictory results in the psychophysical literature can be resolved within the context of a relatively simple neural model of contextual modulation; and (5) that contextual influences are both qualitatively and quantitatively altered by changes in apparent figure-ground relationships. We propose to begin to characterize these properties quantitatively.