Peripheral colour vision: effects of rod intrusion at different eccentricities

Chromaticities of monochromatic lights from different parts of the spectrum were measured both during the cone-plateau period of the long-term dark-adaptation curve and in a completely dark-adapted state. The measurements were obtained at 3, 8, 30 and 65 deg in the temporal field of view and at 1, 2, 3 and 4 log units above the cone-plateau level. The results show that cone-mediated colours obtained during the cone-plateau period in general are desaturated when rod signals intrude during long-term dark adaptation. The desaturation effect of rods obtained at high mesopic illumination levels was found to increase when the test field was moved from 3 to 30 deg but to reduce markedly between 30 and 65 deg. Surprisingly, the desaturation was clearly observable even at a retinal illumination of 20,000 ph td. The desaturation effect of rods is explained by the suggestion that differences in ongoing activity rates of the different types of spectrally opponent cells become levelled out to some extent when light signals from rods intrude during dark adaptation.