Dark versus bright equilibrium hues: rod and cone biases.

Equilibrium (unique) red, green, blue, and yellow stimuli look bright in a black surround, but they look dark in a bright white surround, and yellow changes to brown. We investigated differences in equilibrium-hue chromaticity between bright and dark hues to reveal changes in weighting of cone and rod signals. The largest, most consistent shifts were found between yellow and brown, with equilibrium-brown chromaticity shifted toward red compared to equilibrium yellow at both photopic and mesopic levels. Also, at mesopic levels, rod influence reversed for most observers from a green bias for yellow to a red bias for brown. Bright/dark differences for blue, green, and red were much smaller and/or less consistent. Thus, shifts of cone and rod hue biases between bright and dark hues are most prominent in L-M-cone pathways, especially those activated by yellow and brown stimuli.