Some profoundly blind people can “see” after all – although not in the way we traditionally think of vision. Cells at the back of their eyes monitor light levels and use them to set the body’s clock to either night or day.
The new insights come from experiments involving two blind people who lack the rods and cones needed for normal vision. Although neither person could see anything, they unconsciously sensed whether it was light or dark by detecting blue light.
While this had been expected from earlier work in animals, the experiments provide compelling evidence for the way in which the brain tells night from day. The cells responsible are called retinal ganglion cells and are located in the outermost layer of cells lining the retina at the back of the eye (see Diagram). Retinal ganglion cells usually help to transmit visual information to the brain. Now it turns out that about 3 per cent of the ganglion cells respond to light and make melanopsin, a light-sensitive pigment. Normally, when we see, light penetrates this layer to reach rod and cone cells in the zone of the retina where light is focused to create sharp images.
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“Retinal ganglion cells are fundamentally different to rods and cones,” says Russell Foster of the Nuffield Laboratory of Ophthalmology at the University of Oxford, co-leader of the team whose results appear in Current Biology (). “They’re gauging the gross amount of light in the system, a bit like a photographer’s light meter.”
The light-sensing properties of these cells were first discovered in 2002 in rats by David Berson of Brown University in Rhode Island (see 91av, 16 February 2002, p 17). Since then, experiments in mice engineered to have no rods and cones or melanopsin have proved that the ganglion cells are responsible for setting the animals’ circadian clocks and respond most strongly to blue light. The latest experiments provide the most convincing evidence yet that the same cells do this job in humans.
Foster teamed up with Steven Lockley of the Brigham and Women’s Hospital in Boston, and for 6.5 hours they shone light into the eyes of a 56-year-blind man who had no rods and cones. When they used blue light at night-time, they were able to delay his body clock cycle by 1.2 hours, proving that his ganglion cells were registering light. At the same time, blood levels of the “sleep hormone” melatonin fell by 60 per cent, the man’s alertness sharpened and his brain activity increased, demonstrating that his body clock had been fooled into thinking it was daytime.
Nothing happened to the circadian cycle when green light from the centre of the visual light spectrum was shone in the man’s eyes, showing they could only detect blue light. Further experiments involving an 87-year-old blind woman from England confirmed this.
The researchers say the results help to explain why blind people who have their eyes removed for cosmetic or health reasons suffer horrendous sleep disruption. Lockley recommends that doctors check to see whether blind people still have regular circadian clocks before doing such surgery. If their ganglion cells aren’t functioning, they can set their clocks by taking nightly doses of melatonin.
