Foveal contrast processing of increment and decrement targets is equivalently reduced in glaucoma

Background: Psychophysical measurement of the function of individual precortical visual pathways (magnocellular, parvocellular and koniocellular) has enabled the development of sensitive tests for glaucoma and has enhanced understanding of its pathophysiology. Suchpathways can be further subdivided into their ‘‘On’’ and ‘‘Off’’ components, which have anatomical and physiological asymmetries. This study investigated whether On and Off subdivisions of the magnocellular (M) pathway are differentially affected by glaucoma.Methods: 20 participants with glaucoma and 20 controls underwent two psychophysical procedures that have been shown to assess the M pathway (steady pedestal task) and its On and Off subdivisions (pedestal-deltapedestal task) respectively. Luminance discriminationthresholds were measured foveally, using both increment and decrement stimuli. Results: The steady pedestal (undifferentiated M-pathway)task separated the glaucoma and control groups (p=0.04) with equivalent outcomes for increment and decrement targets. The pedestal-delta-pedestal task (isolated On and Off M-pathway subdivisions) alsodifferentiated between groups (p=0.025), but the outcome was not dependent on which subdivision was isolated. Conclusions: This study found that increment and decrement targets can be used with equal effectiveness for detecting contrast processing deficits in earlyglaucoma. Outcomes further suggested that glaucoma affects On and Off subdivisions of the M-pathway equivalently.