To investigate how rod signals influence hue perception and how this influence can be incorporated into opponent-color models, we measured the shift of unique-hue loci under dark-adapted conditions compared with cone-plateau conditions. Rod signals produced shifts of all spectral unique hues (blue, green, yellow) but in a pattern that was inconsistent with simple additive combinations of rod and cone inputs in opponent-color models. The shifts are consistent with non-linear models in which rod influence requires non-zero cone signals. Cone-signal strength may modulate or gate rod influence, or rod signals may change the gain of cone pathways.

Rod influence on hue appearance of spectral lights was characterized by comparing the scaling of red, green, yellow, and blue hue sensations for an 8 degrees-diameter, 7 degrees-eccentric test spot under conditions that minimized (cone plateau) and maximized (dark adapted) rod influence at two mesopic light levels (1.5 and 3.0 log scoptic trolands). At the lower light level, the hue-scaling functions showed that rod signals influenced the spectral range and magnitude of all four primary hues. The rod influence could not be characterized as a ubiquitous augmentation or diminution of any hue over the entire spectrum. This constrains models of rod influence on color vision.

Hue-naming was used in conjunction with a probe-flash procedure to determine the time-course of rod-mediated effects on hue appearance across the spectrum. Two types of rod influence on hue are distinguishable on the basis of differences in both spectral specificity and time course of effect: (1) a "faster" rod influence enhances green relative to red and (2) a "slower" rod influence enhances short-wavelength red relative to green and blue relative to yellow. The results show that there are separable rod hue biases that operate over different time courses and that the overall rod influence on hue appearance depends importantly on the temporal properties of the stimuli, presumably because rods interact in different ways with different portions of the neural pathways that mediate human color vision.

Rod influence on hue discrimination was assessed by the Farnsworth-Munsell 100-hue test. Rod influence was taken as the difference in error scores obtained after complete dark adaptation and during the cone plateau at three mesopic (23, 9, 3 td) and one standard (158 td) light level. On the FM 100, rods produced a differential discrimination loss along a tritan axis as compared with a red-green axis without any bias toward a rod confusion axis. Rods appear to impair discrimination mediated by S-cone pathways, which at moderate levels of illumination can differentially elevate tritan errors on the FM 100.

Rod signals interact with red-green and blueyellow perceptual hue dimensions to influence color appearance across the spectrum. Hue-naming studies from our lab have identified at least two rod-mediated mechanisms with different spectral and temporal properties: (1) a "faster" rod influence enhances green relative to red; and (2) a "slower" rod influence enhances short-wavelength red and blue. These two types of rod influence are revealed by a probe-flash paradigm . In the present study, we examine the effects of background light level on "faster" and "slower" rod mechanisms. We find both rod influences at higher light levels, but only the "faster" influence at lower light levels. These data provide additional support for the existence of two types of rod influence on hue that differ from each other in spectral specificity, temporal properties, and light-level dependence.