Measurements of the sensitivity of chromatic discrimination are widely used in clinical assessments of color-vision deficiencies. Of both theoretical and diagnostic importance, the activity of rod photoreceptors has been shown to influence these discriminations, most often impairing discriminations mediated by L, M, or S cones. However, some studies show conflicting results, and offer limited understanding of the conditions that lead to specific rod influences on chromatic discrimination. The present study aims to investigate the conditions under which rods might influence the widely used Cambridge Color Vision test (CCT), by comparing bleached (minimal rod influence) and dark-adapted (maximal rod influence) conditions. No rod impairment of chromatic discrimination on the Cambridge Color Test was found. Instead, the bleached condition unexpectedly impaired discriminations mediated by M-cones at normal light levels and by each cone type under dimmer, mesopic conditions. To understand the basis for these findings, several follow-up experiments were carried out, testing rod influence on L/M or S-cone isolating stimuli. Consistent rod impairment was found for most observers for extra-foveal stimuli. However, in the dark-adapted conditions at least 2 observers showed enhanced discrimination between chromatic and achromatic elements (similar to the CCT). Further investigation of the differential rod effects on chromatic discrimination continues.
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