To understand the generality and mechanisms of previously reported rod
hue biases, we examined whether they are present for small foveal stimuli
by comparing the wavelengths of the three spectral unique hues under
dark-adapted and flash-bleached conditions. Rod green bias (shift of
unique yellow) and rod blue bias (shift of unique green) were found for
some observers with 1°-diameter foveal stimuli, the size most likely
to stimulate rods. Smaller stimuli (0.2° and 0.6° diameter), which
were least likely to stimulate rods, produced no large or consistent
differences between dark-adapted and bleached conditions. This suggests
that rod hue biases result from the local stimulation of rods by light,
not from remote suppression by dark-adapted, unstimulated rods,
and not from bleaching light artifacts.