Fig. 13. Predictions of the duplicity theory (separate rod and cone retinae) that explain the relationship between spectral sensitivity and dark-adapted thresholds. The panel on the right shows the spectral sensitivity function for rods (solid line) and cones (dashed line). Left panel shows cone sensitivity changes for test lights of 440 and 620 nm. Threshold for both wavelengths drops initially. These 440 nm cones are less sensitive than the 620 nm cones, which is reflected in their initial thresholds and the final asymptotic threshold. The cone-rod break would therefore be significantly prolonged because rod sensitivity at 620 nm is poor. However, rods are very sensitive to 440 nm wavelength, which results in a higher cone plateau, a shorted cone-rod break time, and a vastly lower final threshold than for the 620 nm stimulus. (Hood DC, Finkelstein MA: Sensitivity to light. In Boff KR, Kaufman L, Thomas JP [eds]: Handbook of Perception and Human Performance. New York: Wiley, 1986).