In animals with colour vision, the retina contains at least two types of photoreceptor cell. Rod cells function optimally at low light intensities, whereas daylight vision is mediated predominantly by cone cells. Light causes visual pigments in the photoreceptors to undergo a chemical reaction, and for the cell to maintain its photosensitivity, the pigment must be recycled continually. An enzymatic pathway called the visual cycle was thought to be responsible for this process, but it has become clear that this pathway is too slow to maintain photosensitivity during constant exposure to light. Now, however, Mata et al. have identified a new, faster regeneration mechanism that seems to be exclusive to cone pigments.

The photosensitive component of the visual pigment is the vitamin A derivative retinaldehyde. When a photoreceptor is exposed to light, the 11-cis form of retinaldehyde is isomerized to the all-trans form, and to restore the photoreceptor to its resting state, it must be converted back to the 11-cis form. Mata et al. analysed retinas from the chicken and the ground squirrel, both of which contain a high proportion of cones. They found three previously unidentified enzymatic activities — an all-trans-retinol isomerase, an 11-cis-retinyl ester synthase and an 11-cis-retinol dehydrogenase — which form a pathway that catalyses the conversion of all-trans-retinaldehyde to the 11-cis form. Interestingly, the level of enzymatic activity was proportional to the percentage of cones in the retina, indicating that this pathway might be important for cone pigment regeneration. The rate of pigment regeneration through the new pathway was 20 times faster than that of the classical visual cycle, and the authors calculate that this should be sufficient to sustain vision in all but the brightest sunlight.

It seems surprising that this alternative pathway has gone unrecognized for so long, but this might be explained by the fact that previous studies on visual pigment regeneration have focused on rod-dominant retinas, such as those of the mouse or the cow. As this pathway has already been identified in two quite distantly related species, it is expected to be a common mechanism for all animals that use cones for daylight vision.