Human trichromacy and refractive development

Abstract

In the post-natal developing eye, there is an active process of refractive development in which the neural retina uses optical cues to evaluate focus, and adjusts the rate of axial elongation to first achieve, and then actively maintain as the optics continue to mature, sharp focus: the process of emmetropization. Increasingly it looks as if chromatic cues are essential (if not exclusive) for this process.

Nearly all non-primate mammals are dichromats, with short- and relatively long-wavelength sensitive cones. However, most humans are trichromats, with short-, medium-, and long-wavelength sensitive cones—although many humans are dichromats like non-primate mammals (“red-green color blind”). This leads to two related questions: is trichromacy important for human emmetropization, and do experimental results from dichromatic mammals apply to humans?

The issue is far from settled, but the available evidence indicates that emmetropization in humans is likely functionally dichromatic similar to that of other mammals, with the medium- and long-wavelength sensitive cones effectively pooled into a single functional “long” cone. In support of this, human dichromats generally emmetropize as well as human trichromats, and they also become myopic to a roughly similar proportion. Trichromacy does not appear to be of fundamental importance for refractive development in humans. While there is some evidence that dichromats might be slightly less susceptible to becoming myopic than human trichromats, the data are inconclusive. Further studies on this topic may lead to an improved understanding of why emmetropization increasingly fails leading to myopia development in humans.