Long-term variability in the spectral loci of unique blue and unique yellow

The spectral loci of both unique blue and unique yellow were measured over a 16-month period. Using the method of constant stimuli, two neutrally adapted observers made forced-choice green or red responses to monochromatic test flashes. Results showed a consistent difference between observers (about 5 nm), considerable within-subject variability (standard deviation 3 nm), and significant long-term drifts (greater than 5 nm) in spectral loci. These results demonstrate the importance of frequently redetermining unique-hue loci when they are used as baselines in chromatic contrast and adaptation studies.