Canadian researchers have found out how to restore normal vision to kittens with a lazy eye without using an eye patch.

The cure was relatively simple — putting the kittens in complete darkness for 10 days. Once the kittens were returned to daylight, they regained normal vision in the lazy eye within a week, reported researchers at Dalhousie University in Halifax in the journal Current Biology this month.

Lazy eye is a condition where the brain effectively turns off one eye. It affects about four per cent of the population in humans, and the most common treatment is fix the vision problem (for example, by using glasses) and then patch the good eye, forcing the person to use their bad eye.

Treatment is most effective in younger children.

Kevin Duffy, a neuroscientist who co-authored the new study, told CBC's Quirks & Quarks that the condition is typically the result of a vision problem such as a cataract, a misalignment of the eyes, or poor focus in one eye, which then causes the brain to develop abnormally.

"If the eye is providing abnormal vision, then the circuits that connect to that eye are going to develop abnormally," he said. The brain "becomes effectively disconnected."

While patching is a common treatment, there are two main problems with it, Duffy said. One is that getting the child to wear a patch isn't easy. The other is that the patch prevents both eyes from learning to work together, which is needed for tasks such as judging distances.

In Duffy's study, kittens with normal vision developed lazy eye after having one eye sewn shut for seven days. The eye would be reopened, and the kittens would be allowed to see with both eyes for five weeks.

Tests showed they could see only one-third as well with the lazy eye as they could with their normal eye.

'Zero photons of light'

The kittens were then put in a special laboratory facility for 10 days.

"It's a darkroom inside of a darkroom inside of another darkroom," Duffy said. "So the central room is the proper darkroom and it has zero photons of light."

When the kittens were brought back to normal light conditions, the lazy eye regained normal vision within five to seven days.

The researchers think the darkness may put the brain into a state that allows it to change more easily.

"It may be that the pattern of activity produced by the eye in darkness is reminiscent of an earlier time of development," Duffy said, "and it may be that pattern of activity that's instructing the brain to revert back to the earlier stage."

Despite the promising results of the study, Duffy said he wouldn't yet recommend that the treatment be tested on humans.

Currently, he said, the researchers are trying to figure out whether they can get the same positive effect if:

• The period of complete darkness is shorter than 10 days.
• The darkness is not complete.
• The kittens are exposed to light for 30 minutes each day.

"All of these questions, I think, have to be answered," Duffy said, "and perhaps many more, before clinicians would be willing to implement this treatment in humans."