Colour vision deficiency primer
Colour vision
Full colour vision in humans is based on the responses from three types of cone cells in the retina. These cells are sensitive to light in three overlapping areas of the visible spectrum. Because the cells overlap in their sensitivity, their firing rates
By comparing the firing rate from the three cone classes, the brain can discriminate between about 3 million colours.
In colour vision deficiency (CVD) (aka “colour blindness”), the signal is limited by either two of the cone classes being too similar in their sensitivity, or by a cone class being absent entirely.
The cone cell activities are compared along two chromatic pathways which evolved at very different times.
The oldest pathway compares blue and yellow light
Strong colour vision deficiency
When a cone class is absent entirely This results in a stronger type of CVD, dichromacy, where colour vision is based on the comparison between two cone classes instead of three. Though dichromat colour vision is technically limited to a scale from blue to yellow, people’s lived experience is more nuanced, with dichromat’s describing colour experience across the spectrum.
Mild colour vision deficiency
When the sensitivity of two cone classes overlap too much This results in colour vision signals that are more noisy than usual, and therefore the experience of the colour is less intense. This can make it more difficult to discriminate colours in the red to green area of the spectrum. This milder form of CVD is called anomalous trichromacy, which can be so mild it goes completely unnoticed in daily life.
Colour vision naming conventions!
It can be tricky understanding scientific texts about colour vision because there are a lot of different naming conventions used!
Cone cells are named after the region of the visual spectrum they are sensitive to:
S-cone = short wavelengths (blue region)
M-cone = medium wavelengths (green region)
L-cone = long wavelengths (red region)
When reading about colour vision deficiencies, however, the naming conventions refer to them according to the Greek name for the affected cone cell class!
Here the L-cone is called "protan" from the Ancient Greek "protos" meaning first. Colour vision deficiency resulting from an anomalous L-cone class is called "protanomaly" or a missing L cone class is "protanopia".
The M-cone is called "deutan" from "deuteros" meaning second. Colour vision deficiency resulting from an anomalous M-cone class is called "deuteranomaly" or a missing M cone class is "deuteranopia". The S-cone is called "tritan" from "tritos" meaning third. Colour vision deficiency resulting from an anomalous S-cone class is called "tritanomaly" or a missing S cone class is "tritanopia".
Spectral region
Cone class name
CVD class name
Anomalous trichromacy class
Dichromacy class