Blue arcs of the retina
Also called: Blue arc phenomenon · Purkinje's blue arcs
Look at a small red light in a dark room and two faint blue arcs curve away from it towards your blind spot. The arcs follow the paths of the nerve fibres in your own retina.
How common
Does this happen to you?
How it works
The retina has a layer of nerve fibres on its inner surface. They carry signals from the light-sensing cells to the optic disc, where they bundle into the optic nerve2. Fibres that start on the far side of the centre from the disc cannot run straight there4. They curve above and below the centre in two arcs and reach the disc from the top and bottom3.
When a small bright light hits the retina near the centre, the fibres carrying its signal become active. You see a pair of faint glowing arcs that trace the fibre paths towards the blind spot1. How this activity makes the arcs is not known3.
The arcs look blue, in varying shades, whatever the colour of the light3. The reason for the colour is not settled3. They appear at once and last only about half a second3.
Eyes with damage to that layer see the arcs poorly, which is why researchers have tested them in glaucoma3.
How common it is
One study has counted this. In 2013 researchers tested 202 eye clinic patients with a red light in a darkened room. Of the 34 people with healthy eyes, 32 saw the arcs in both eyes, or 94%3. Only 30 of 74 patients with open-angle glaucoma managed it, or 41%3. That is a clinic sample, not the public. The test used only one minute of dark adaptation3. No population survey exists, so the share of all people stays unknown.
Try it
- Sit in a completely dark room. Wait 30 to 60 seconds for your eyes to adapt13.
- Use a small red light. A standby LED on a television or a phone charger works well.
- Look just to the side of the light. Keep it within a few degrees of your centre of vision.
- Two faint blue arcs curve away from the light towards the outer side of that eye.
- Blink, look away, then look back. They appear again for a moment.
The arcs are dim. Do not expect them to be obvious at first.
Is it a problem?
No. The blue arcs are a normal effect of healthy retinal nerve fibres, not a symptom3. Failing to see them says nothing on its own. Researchers have tested them as a possible glaucoma screen, but they are not a stand-alone test3. See an eye doctor about any real change in your vision.
History and culture
Jan Evangelista Purkyně first described the arcs in the early nineteenth century. He noticed them while looking at the embers of a fire3.
Sources and further material
- Wikipedia Entoptic phenomenon Lists the blue arcs among the entoptic effects and links them to nerve signals near the fovea.
- NCBI The blue arc entoptic phenomenon in glaucoma Tested 202 eye clinic patients. The only study that counts who sees the arcs.
- eyerounds.org Visual Field Testing: From One Medical Student to Another University of Iowa EyeRounds tutorial. Describes the papillomacular bundle and the arcuate fibres that curve around it.
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Related oddities
The blind spot
Each eye has a hole in its field of view where the optic nerve leaves the retina. You never notice it, because the brain paints over the gap with whatever surrounds it.
Purkinje tree
A branching shadow of your own retinal blood vessels. It is always on your retina, but you only see it when light enters the eye from an unusual angle and the shadow moves.
Phosphenes
The flashes, rings, and patterns of light you see when you rub or press your closed eyes, or after a knock on the head. Pressure on the eye makes the retina fire without any light.
Foveal tritanopia
The very centre of your vision cannot see blue. The middle of the fovea has no blue-sensitive cones at all. You never notice, because the brain fills the colour in from the surrounding area.