Date of Award

Winter 2014

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

William W Stine

Second Advisor

Robert C Drugan

Third Advisor

Robert S Ross


Why humans are less sensitive to increment than decrement contrast remains unknown. One hypothesis is that the psychophysical asymmetry in increment and decrement thresholds results from an asymmetry in respective ON and OFF cell contrast gains. Contrast gain also differs, however, by the visual pathway within which the ON and OFF cells travel. Cells show greater contrast gain in the magnocellular (M) than parvocellular (P) pathway. Therefore, the asymmetry in increment and decrement thresholds may also differ by visual pathway. My first three experiments established that observers were more sensitive to decrement than increment contrast in both pathways, in monocular as well as dichoptic conditions. Contrast gain model analyses revealed greater asymmetry in increment and decrement contrast gains only in the P pathway. Lower decrement relative to increment contrast gains resulted in progressively lower decrement than increment contrast discrimination thresholds. Also found was that only decrement binocular summation increased as stimulus contrast increased. Therefore, the two remaining experiments assessed P pathway binocular summation of disparate increment or decrement contrast. An interocular contrast disparity in stimulus contrast generated a strong nonlinear weighting towards the eye with greater delta pedestal contrast during decrement binocular summation and reduced contrast discrimination threshold. Manipulation of two mediating contrast gain normalization processes reduced the disparity and increased the threshold. Combined, the five experiments provide psychophysical evidence that differences in increment and decrement contrast gain normalization underlie the asymmetries in increment and decrement contrast discrimination threshold and binocular summation.