Rod influence on chromatic discrimination away from chromatic and achromatic backgrounds

摘要

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. Specifically, an unresolved issue is whether the rod influence is symmetrical between cone increment and decrement discrimination. The present study aims to investigate the conditions under which rods might influence chromatic discrimination by comparing discrimination threshold of 6 observers in bleached (minimal rod influence) and dark-adapted (maximal rod influence) conditions. Discrimination thresholds were set for L/M or S-cone isolating gratings, by incrementing or decrementing compared to a monochromatic or achromatic disk reference stimulus. Rod impairment of discrimination (consistent with previous literature) was found for most observers for extra-foveal stimuli, and for some observers for near-foveal stimuli. Moreover, the present study found mostly symmetrical rod influence on all axes of discrimination, as well as on all directions of discrimination. However, in the dark-adapted conditions at least 3 observers showed enhanced discrimination of extra-foveal chromatic stimuli on an achromatic background. These experiments show that the rod influence on chromatic discrimination is not consistent. Instead, different task procedure and stimulus a?? specifically discrimination away from achromatic a?? lead to variation in the influence of rods. The present study is in inconclusive on this apparent rod enhancement of chromatic discrimination: the same pattern of thresholds could result from impairment in the bleached condition. Further investigation of the rod effects on chromatic discrimination away from an achromatic background continues.