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 2% of all the cones in the retina, and they are scattered in a loose pattern².

At the exact centre of the fovea, the pattern has a hole. A circle about 0.35 millimetres across contains no blue cones at all¹. On the visual field that is about 1 degree, roughly the width of a thumbnail at arm’s length¹. Inside that circle the eye can tell red from green, but it cannot tell blue from grey. This is called foveal tritanopia, from the medical term tritanopia, which means blue-blindness³.

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 not an error. Blue light is focused slightly in front of the retina by the lens, so blue images are always a little blurred. Blue cones are also slower than the other two types. Fine detail comes from the red and green cones, so leaving the blue cones out of the sharpest spot costs almost no resolution¹.

History and culture

The effect is one of the items on the XKCD “Visual Field” map, which marks a small blue-blind circle at the very centre of the eye’s view⁴. It 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.