Abstract

Colour vision starts in the retina where light is absorbed in three different cone classes, sensitive to long-, medium-, and short-wavelength light. The cone signals then feed into three different post-receptoral channels, a luminance channel and two chromatic channels. Interestingly, these two chromatic channels do not correspond to perceptually salient colour mechanisms (red, green, yellow, blue) and recent evidence suggests that the two sub-cortical chromatic channels are recombined in visual cortex into orderly hue maps. I will report fMRI studies consistent with a hue map in V1.

Secondly, I will report behavioural experiments with a large sample of adult colour-normal observers of a wide age range showing that the cortical hue mechanisms are almost invariant with age. In contrast, the sensitivity along the protan, deutan and tritan line declines with age; the latter measurements are likely to reflect the sensitivity of more peripheral mechanisms.

Our results suggest that the human visual system is able to compensate for retinal (peripheral) signal changes by adjusting the relative cone weightings of the hue mechanisms. Such an adaptive weighting is useful to maintain colour constancy throughout the life span in the presence of known changes in the ocular media (yellowing of the lens) and retinal sensitivity losses. It may also be responsible for the (relatively) small inter-observer variability compared to the large differences in the observers’ retinal make-up. The mechanism underlying this hue compensation is still poorly understood, but it is likely that it utilises invariant sources in our visual environment.