Contrast dependency of foveal spatial functions: orientation, vernier, separation, blur and displacement discrimination and the tilt and Poggendorff illusions

To examine the effect of reducing luminance contrast in human foveal vision , discrimination thresholds were measured in four tasks and also a numerica l measure of two visual illusions were obtained by a nulling technique. The patterns used for all tasks were made very similar to facilitate compariso n between them-all featured luminance step edges whose contrast could be va ried from near unity down to the detection threshold. Orientation, vernier and blur discrimination thresholds rise on average 5-6-fold when the contra st is reduced from near unity to a Michelson value of 0.03. Jump displaceme nt thresholds are somewhat more robust to contrast reduction, and the curve of separation discrimination versus contrast is much shallower, rising by a factor of about 2. The magnitude of the Poggendorff and tilt illusions ch anges very little until the inducing contours are barely detectable. (C) 19 99 Elsevier Science Ltd. All rights reserved.