| Human visual information processing is controlled by two largely separate parallel pathways: the magnocellular and the parvocellular. One approach to ascertaining pathway function is through the study of color-deficient individuals, one of several populations in which one of the pathways is naturally altered. Based on reduced ability to make certain chromatic discriminations and on different color perceptions compared to color-normals, color-deficients are presumed to have an altered parvocellular pathway. In order to determine which visual pathway serves which visual function, color-normal and color-deficient observers were run on an extensive battery of visual tests including color detection, color discrimination, hue- and saturation-scaling, spatio-temporal contrast sensitivity, binocular vision, stereopsis, monocular hyperacuity, and motion sensitivity. Results show two principal findings. First, genetic color-anomalous observers, as a group, have equal or greater contrast sensitivity to nearly all spatio-temporal frequency combinations, with the greatest effects at higher spatial frequency stimuli bordering on “supersensitivity.” These individuals also displayed better stereoacuity and stereopsis, superior vernier acuity thresholds, and smaller minimum motion threshold values compared to the color-normals. The enhanced sensitivities found among these visual functions are discussed in terms of a common mechanism resulting from a reduction of chromatic antagonistic signals in the retinas of this subpopulation of observers, and the resulting gain in detection of luminance differences at high spatial frequency. Several possible versions of neural rewiring are considered and tracked from the level of the cone mosaic to higher cortical levels and advanced stages of information processing. Second, separate standard color-screening tests giving different diagnoses indicates a greater degree of overlap in characteristic traits exhibited by each subtype, indicating that color deficiencies are a more complex phenomenon than has been generally thought, with dimensions not clearly conveyed by existing designations. Additional experiments and directions for future research in the delineation of pathway function and control using a more varied sample of observers are suggested. Some of the potential clinical implications of the findings from the current investigation are also explored. |
