Foveal tritanopia

How it works

The fovea is a small pit at the centre of the retina. It is the part you use to read and to look straight at things. It is packed with cones and has no rods¹. The cones come in three types, tuned to long, medium, and short wavelengths. The short-wavelength type, often called the blue cone, is the rarest. Blue cones make up only about 7% of the cones near the centre of the retina⁵. They are scattered in a loose pattern².

At the exact centre of the fovea, the pattern has a hole. A circle about 0.35 degrees of visual angle across contains no blue cones at all⁵. One degree covers about 0.29 millimetres of retina, so the hole is roughly 0.1 millimetres wide on the retina itself⁵. Inside that circle the eye can tell red from green, but it cannot tell blue from grey. This is called foveal tritanopia, from tritanopia, the medical term for missing blue cones³.

You never see a grey dot in the middle of blue things. The blue cones just outside the hole report what colour the region is, and the brain paints that colour across the centre¹. A very small blue spot, viewed against a matching grey and held perfectly still on the fovea, would be invisible. In practice, your eyes are always moving, so the spot keeps landing on cones that can see it.

The layout is probably not an error. When red and green images are in focus on the retina, blue images are slightly out of focus⁶. Fine detail comes from the red and green cones, so leaving the blue cones out of the sharpest spot costs almost no resolution⁶.

Is it a problem?

No. Foveal tritanopia is a normal feature of every healthy eye, not a disorder¹². Despite the name, it has nothing to do with tritanopia, the inherited form of colour blindness that affects the whole visual field. Nobody notices a grey patch in blue objects, because the brain fills the gap using blue cones just outside the central hole¹. There is nothing to check with a doctor because of it. A genuine blue-blind patch that does not fill in, or any real change in colour vision, is a different matter and worth mentioning.

History and culture

The XKCD “Visual Field” map notes that blue cones are few, but that they are found out to the edge of vision⁴. Foveal tritanopia is one of several structural gaps in vision, along with the blind spot, that the brain hides so well that most people never learn they exist.