A stint in the dark may be just what the doctor ordered—at least if you have "lazy eye." Researchers report that kittens with the disorder, a visual impairment medically known as amblyopia that leads to poor sight or blindness in one eye, can completely recover their vision by simply spending 10 days in total darkness.
"It's a remarkable study, with real potential to change how we think about recovery from amblyopia," says neuroscientist Frank Sengpiel of Cardiff University in the United Kingdom who was not involved in the work.
Amblyopia affects about 4% of the human population. It's thought to start with an imbalance in vision early in life: If one eye doesn't see as well as the other—because, for example, of a cataract or astigmatism—the brain reroutes most of the connections needed for visual processing to the "good" eye. Doctors often treat the condition by patching the good eye and forcing the brain to rely on the other eye, but the treatment risks damaging vision in the good eye, and if it doesn't succeed or occur early enough in a child's visual development, the vision loss in the impaired eye can be permanent.
Earlier studies with cats, whose complex visual systems are good stand-ins for human vision, showed that neurons in the brain's visual centers shrink when the brain decides to disconnect from the bad eye, but that they grow again when the cats are placed in darkness. So neuroscientists Kevin Duffy and Donald Mitchell of Dalhousie University in Halifax, Canada, set out to test darkness itself as a treatment.
They first induced amblyopia in 27 kittens by surgically closing one of each animal's eyes 30 days after birth, when a feline's visual system plasticity, its ability to change and grow, is at its peak. The eye was kept closed for 7 days, after which each kitten had amblyopia. Then the kittens were divided into two groups: one group that was placed in darkness immediately upon opening the deprived eye, and a second group that waited 3 months before its stint in the darkness. The darkness that both groups experienced was total—"a darkroom inside a darkroom inside a darkroom" that kept out even faint or transient sources of light, Duffy says.
When the first group emerged from its 10 days of lights-out, all the kittens were blind in both eyes. But over a 7-week period, each cat's eyes improved in lockstep, ultimately achieving normal vision in both eyes.
The second group, which during the delay had developed stable and presumably permanent amblyopia, also spent 10 days in the pitch dark. When those kittens emerged, their good eyes could still see and their bad eyes were nearly blind. But within 7 days, each kitten's bad eye had recovered to the point that it matched the good eye in visual acuity.
"This vision impairment that would have lasted a lifetime was completely obliterated by 10 days of darkness," Duffy says.
The results, published today in Current Biology, suggest that darkness restored some of the kittens' brain plasticity and thus enabled their vision's recovery. To understand the mechanism behind this, the researchers measured how darkness affects levels of a protein, called NF-L, that helps stabilize the shape and structure of neurons in the brain. These so-called neurofilaments accumulate with age and are thought to be molecular "brakes" that reduce the brain's plasticity over time. They put a different group of 30-day-old kittens—a group in which the researchers did not induce amblyopia, to make sure they weren't measuring the effects of the deprivation on neurofilament levels—into the darkness. After 10 days in the dark, these kittens showed 50% lower NF-L levels in their brain tissue than kittens of the same age that had never been kept in the dark. "It was like you were looking at an animal that was much younger than it was," Duffy says.
Sengpiel cautions that it's too soon to start suggesting doses of dark for humans with amblyopia, in part because the researchers haven't yet determined the limits of their treatment's effectiveness. They don't know, for example, just how dark the room has to be, or whether short breaks from the darkness would destroy its benefits. Duffy and Mitchell hope to answer some of those questions in future studies.