Generality of rod hue biases with smaller, brighter, and photopically specified stimuli.

This study tests the generality of previously demonstrated rod hue biases (red and blue biases at shorter wavelengths and a green bias at longer wavelengths) that cause the loci of the three spectral unique hues to shift to longer wavelengths. We found rod hue biases for 2-deg targets to be generally similar in magnitude and light-level dependence to those observed for 7.4-deg targets (the size most often studied) when measured at 7-deg eccentricity. The largest effects for both test sizes occurred at the lowest light levels tested, 1 log scotopic troland. All three rod hue biases were found with 0.6-deg targets, but were not reliably measurable at the lowest light levels and were reduced in magnitude and consistency across observers. The largest rod hue biases all occurred at the same scotopic light level, which corresponds to different photopic light levels for the three hue biases, because of differences in photopic and scotopic spectral sensitivity. This suggests that no single photopic light level will produce such large effects for all three rod hue biases. Finally, when the rod influence on a specific unique-hue locus was measured using photopically (rather than scotopically) constant stimuli, rod hue biases were still found but were more variable in magnitude and incidence across observers. We conclude that the rod hue biases we have previously described can be found with smaller stimuli, at somewhat higher light levels, and under photopically constant conditions, although our prior conditions tend to produce larger, more reliable rod hue biases.