Science Question From a Toddler: What do blind people see?

This question—which comes to us from an Anonymous reader, asking for his or her formerly toddler own self—may sound like a zen koan, but I assure you, it has an answer. And probably not the one you're expecting. It's really a prime example of why I love doing the Science Question from a Toddler series—I get these questions that, on the surface, sound very "Duh", but end up leading to complex places.

So, what do the blind see?

It depends on the blind person. But the stereotypical assumption—that blind people live in the sort of black nothingness the sighted see when we close our eyes—is actually the most rare of all the possibilities.

What the blind really see varies quite a bit, depending on the cause of blindness and its severity, said Dean Bok, Ph.D., Professor of Ophthalmology and Neurobiology at UCLA's Jules Stein Eye Institute, and a researcher who's worked with the organizations Research to Prevent Blindness and the Foundation Fighting Blindness. Many people who are described as blind can perceive light. Even those who must use a cane or guide dog to get around are more likely to see the world around them as an indistinct fuzzy blur, rather than a formless void.

And that blur is likely to be in black and white.

You probably remember from grade school that there are two structures in our eyes—cones and rods—that enable us to see and perceive light. Rods are related to night vision. Cones to color vision and what we see during the day. But there's a bit more to it than that.

"Cones are extremely important, not only for color, but also for acuity," Bok said. "The ability to read fine print is from cones, not rods."

Basically, the worse your vision is, the less color you can usually perceive.

But what's really astounding is how new, high-tech treatments are changing what some blind people see. Cory Haas is a 9-year-old boy who lives in New York state. A couple of years ago, Bok told me, this boy was one of the people whose sight was limited to fuzzy, mostly colorless blobs. He couldn't read. He needed help walking. Today, Cory Haas can ride a bike and read books with large print. His sight is nowhere near perfect, but he's gone from being legally blind, to being just another kid who has to wear glasses.

The secret is gene therapy. Haas was born with a kind of degenerative blindness called Leber's congenital amaurosis. There are at least12 different kinds of LCA, but Haas' is caused by a defect in a single gene—rpe65.

"That gene produces an enzyme that bends Vitamin A into the form that we need for vision. Then it's ready to be attached to a group of proteins called opsins, and used by color and black and white photo receptors. That's what triggers the visual response," Bok said. "If you don't have the enzyme, you'll never see. Kids who have a totally non-functional rpe65 gene can't see at all. Some with a crippled gene have a semblance of vision but are really legally blind."

Because this type of blindness is based on a single, small gene in the retina, it's relatively easy for scientists to fix. They take an adeno-associated virus—a virus which is usually present in humans but not known to cause disease—remove most of its genes and patch a shiny, new, properly functioning version of the rpe65 gene into it.

Once injected into the eye, the virus goes to work doing what viruses do, i.e. invading cells and using their machinery to replicate its genetic information. But, in this case, that information is the rpe65 gene. Within a few weeks or months, the person has a supply of working rpe65 genes, churning out the enzyme they need to see.

Image courtesy Flickr user moriza, via CC