By studying groups of blind migraine sufferers, US scientists have identified a group of neurons that cause painful light sensitivity.

Almost 85% of people who have migraines also have light sensitivity, and even people who are almost completely blind experience light sensitivity while they are having a migraine.

Some people who are blind as a result of glaucoma or diseases like retinal cancer are unable to see images or light in any way, while people who are classified legally blind are sensitive to certain wavelengths of light.

For the purposes of the study, the research team studied the way migraines affect people with both types of blindness.

In the process, they uncovered the visual pathway in the brain that relates to light-sensitive migraines.

Signals transmitted from the retina seem to trigger migraine worsening in patients whose eyes could still perceive some light.

Lead researcher Rami Burstein of the Beth Israel Deaconess Medical Centre in Boston, US said that, while the patients whose eyes were totally blind did not experience increased migraine severity when exposed to light, people whose optic nerves were still partially intact did.

He said that when his team realised the mechanism of light sensitivity involved the optic nerve, they began to suspect that a group of recently discovered retinal cells was responsible for migraine-related light sensitivity.

The optic nerve cells in question involve melanopsin photoreceptors.

Melanopsin is a biochemical involved in the regulation of circadian rhythms and other non-visual responses to light.

A mysterious molecule only discovered about ten years ago, it resembles the pigments that make invertebrate lifeforms sensitive to light.

Melanopsin pathways are the only functioning light receptors in people who are legally blind due to retinal degenerative diseases.

Scientists believe migraine pain develops when the meninges become irritated and nearby pain receptors become stimulated.

Meninges are membranes that surround the brain and central nervous system.

In further studies, the researchers injected dyes into the eyes of rats to trace the path of signals through the optic nerve.