Labeled lines for image blur and contrast.

It has been suggested that blur and contrast discrimination thresholds are limited by a common stage of contrast energy transduction, and that this explains the characteristic "dipper" functions found for contrast and blur discrimination. To test this conjecture, thresholds for discriminating increments from decrements in sharpness/blur, and similarly for contrast, were measured using the same chessboard stimuli with the Method of Single Stimuli (Experiment 1). Using a generic human contrast sensitivity function (HCSF) to calculate energy, thresholds were significantly lower for blur than for contrast. They could be made more similar only by using an implausibly narrow band-pass version of the HCSF. In separate sessions (Experiment 2), observers also attempted to discriminate between blur and contrast changes when they were randomly interleaved (channel discrimination). Channel discrimination thresholds were similar to those predicted from noisy independent channels, consistent with separate labeled lines for the two channels. Experiment 3 measured subthreshold summation of contrast and blur signals, in either energy-add or energy-subtract modes with a two-alternative forced choice task. Both add and subtract modes lowered thresholds. Experiment 4 measured standard T versus C ("dipper") functions for blur, and compared these with T versus C functions when a contrast cue was added to keep energy constant. The finding of a "dipper" function in the latter case suggests that it does not arise from a common energy transduction stage.