title: Contrast dependency of foveal spatial functions: orientation, vernier, separation, blur and displacement discrimination and the tilt and Poggendorff illusions.
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.
authors.affiliation: Division of Neurobiology, University of California, Berkeley 94720-3200, USA. gwest@socrates.berkeley.edu
authors.forename: G
authors.initials: G
authors.lastname: Westheimer
authors.forename: S
authors.initials: S
authors.lastname: Brincat
authors.forename: C
authors.initials: C
authors.lastname: Wehrhahn
mesh_terms: D000328:Adult; D000368:Aged; D015350:Contrast Sensitivity / Q000502:physiology*; D004056:Differential Threshold; D005584:Fovea Centralis / Q000502:physiology*; D006801:Humans; D008027:Light; D008297:Male; D008875:Middle Aged; D009903:Optical Illusions / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology; D011594:Psychometrics; D012399:Rotation