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Abstract:
To examine the effect of reducing luminance contrast in human foveal vision, discrimination thresholds were measured in four tasks and also a
numerical measure of two visual illusions were obtained by a nulling technique. The patterns used for all tasks were made very similar to facilitate comparison
between them-all featured luminance step edges whose contrast could be varied from near unity down to the detection threshold. Orientation, vernier and blur
discrimination thresholds rise on average 5-6-fold when the contrast is reduced from near unity to a Michelson value of 0.03. Jump displacement 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 changes very little until the inducing contours are barely detectable. (C) 1999 Elsevier Science Ltd. All rights
reserved.
