This is a psychophysical study of visual pattern perception in adult human observers with normal vision. It examines how performance on several discrimination tasks improves as the contrast of the stimuli increases. The tasks include discrimination between stimuli which differ in contrast, orientation, spatial location. The functions relating improving performance, as measured by d', to increasing contrast will be related to underlying properties of the visual system. One hypothesis tested is that each function is separable into two muliplicative components: one which with respect to the differences to be resolved; and one which increases with contrast and represents as underlying signal-to-noise ratio. Additional experiments seek to analyze the latter component into separate functions representing growth of the strength of the visual response and changes in the variability of the response. The stimuli will be Gabor packets (sinusoidal gratings multiplied by Gaussians). Each is spatially localized and contains a limited band of spatial frequencies. Stimuli will be drawn from different parts of the spatial frequency spectrum and from different regions of the visual field to determine how ability to resolve differences in contrast, orientation, spatial frequency, and location depends on the spatial frequency content and visual eccentricity of the stimuli. Additional experiments ask how the presence of noise and pattern masks alter the functions relating discrimination performance to contrast. Many conditions, including typical aging, involve changes in the lens and occular media which scatter light and reduce the contrast in the retinal image. The proposed research provides a basis for understanding what changes in visual performance must result from the reduced contrast and for differentiating them from changes due to other causes.
