Scientists found that darkness worked far better than they expected as a treatment for kittens with lazy eye.
Amblyopia — found in 4 percent of humans — is a vision problem, usually only affecting one eye, in which the eye itself works but the connections between the eye and the brain are disrupted. These connections fail to develop properly when the eye is deprived of normal visual input during early life.
For this study, amblyopia was induced in seven kittens by stitching one eyelid shut during a critical period for visual development – around 30 days after birth. After one week, the kittens' eyes were re-opened and, as expected, the kittens were unable to see out of one eye.
(This technique, called monocular deprivation, has been used for over 50 years, including for Nobel Prize-winning research, and does no lasting harm to the kittens' eyes or eyelids.)
Following monocular deprivation, the half-blind kittens were then put in complete darkness for 10 days, either immediately after having their eyes re-opened or after waiting for five to eight weeks.
An eye deprived of normal visual experience (blue dots) can attain visual acuity comparable to a normal eye (red dots) after 10 days of darkness.
Courtesy of Kevin Duffy
Remarkably, after emerging from the darkness, all seven kittens gradually regained their eyesight. For the kittens that had been immediately plunged into the dark, this took seven weeks. But for the kittens whose darkness treatment was delayed, complete sight was regained in just five to seven days.
"We were just amazed by that. It's quite remarkable," says Kevin Duffy, associate professor of psychology at Dalhousie University and co-author of the study.
Duffy and his colleague, Donald Mitchell, think that complete darkness somehow resets the brain's visual system to an earlier, more adaptable stage of development. The researchers aren't exactly sure why recovery was so much more rapid in the delayed-darkness group.
But why does it work at all?
Some of Duffy and Mitchell's earlier research, also published in the present paper, may hold the answer to that question.
Long before these scientists started sticking blind kittens into dark rooms, they were looking at a type of protein called neurofilaments, which provide structural support for cells in the brain.
For neurons, "structure is function," explains Duffy, "so [neurofilaments] are essentially what we need in order to have a functioning brain."
Duffy and Mitchell's hypothesis was that, since neurofilaments are known to act as rigid scaffolding in the brain, these proteins might play a role in reducing neural plasticity with age.
And that's exactly what they found.
Looking at cat brains, the researchers found that levels of neurofilaments in the visual cortex were related to age. Young kittens, whose brains were still in the critical stages of development, had very low levels of neurofilaments. Older cats had a lot more.
The difference suggests that neurofilament proteins play a role in reducing the ability of the brain to adapt and change as we get older.
Duffy and Mitchell then decided to look at the brains of 30-day-old kittens that were kept in darkness for 10 to 15 days. These kittens emerged from the dark with about half as many neurofilaments in their visual cortex as when they first went into the darkness.
Based on these findings, the researchers reasoned that if exposure to darkness can release the visual system from the rigid structure imposed by neurofilaments, then it might help animals with impaired vision recover.
And so they set out to test this hypothesis with their amblyopic kitten experiment, which confirmed that total darkness really can restore vision in as little as a week. "We've never seen anything like this," says Duffy. "[We're] really optimistic that this might eventually end up translating to some human benefit."
Right now, the most common way to treat amblyopia in human children is to patch the dominant eye in order to force the lazy eye to get stronger. However, patching can have side effects, including losing depth perception and causing the previously dominant eye to become weaker. Immersion in darkness could prove to be a less complicated alternative to patching.
Duffy cautions that we shouldn't start throwing children into the dark just yet. There are still many issues to iron out before an approach like this could be tried in humans. How much darkness is necessary? How many days are required for it to be beneficial? And how long would it take to recover?
"Clinicians will want to know the answers to these questions before they consider it," acknowledges Duffy. "At this stage, I think it's really premature."