SUSAN STAMBERG, host:
Have you noticed there are more squinters in this world lately or people wearing contact lenses, glasses? Seems that we do not see as well as we used to. In the 1970s, a quarter of Americans were nearsighted - had trouble seeing far-away things. Today, that number has nearly doubled. So why is our vision getting worse?
For answers, we turn to neuroscientist Sandra Aamodt, who joins us from member station KQED San Francisco. Welcome to you.
Dr. SANDRA AAMODT (Neuroscientist; Author): Good to be here.
STAMBERG: You wrote about this recently in an op-ed in the New York Times. Why is our vision getting worse?
Ms. AAMODT: Because we aren't spending as much time outside - our kids aren't spending as much time outside as they used to, for most of our evolutionary history.
STAMBERG: What good does the brightness do to our vision?
Ms. AAMODT: There are a couple of ideas. One of the simplest ones is that because your pupils get smaller when you're outside, you can focus on more things at once in just exactly the same way that it happens with your camera, when you've got a small aperture, that you get better depth of the field. And that that might give a stronger signal to your retina about when the image is in focus.
STAMBERG: Does it have, then, something to do with the way the muscles - a very crude term; I'm sure it's incorrect - in the eye function?
Ms. AAMODT: No, it's actually about the growth of the eyeball itself, and as kids grow their eyes grow. And the distance to the retina needs to be maintained correctly for the strength of the lens. That process is what goes wrong in nearsightedness. The eye grows too much. It doesn't get the stop signal that it should be getting from bright sunlight.
STAMBERG: Well, what you're saying is, it's really an issue for our children and the way they're eyes develop. What about us adults?
Ms. AAMODT: Usually, nearsightedness starts in the school years, and rarely has any onset in adults. So unfortunately, if you're nearsighted now, you're stuck with it. But there might still be a way to keep your children from inheriting your nearsightedness.
STAMBERG: What about genes versus all the other bad things we're doing -environmental things?
Ms. AAMODT: It's analogous to height. You know that individual differences between people who are growing up in the same environment are typically genetic. But if you improve the nutrition, you can get children who are taller than their parents. In much the same way, which children are vulnerable to nearsightedness is genetic, but you can vary the incidence enormously by changing the environment.
STAMBERG: How much time should children spend out in the sun to avoid nearsightedness?
Ms. AAMODT: Looks like about two hours a day is the threshold.
STAMBERG: Sandra Aamodt. She writes about the sun and vision in her upcoming book, "Welcome to Your Child's Brain: How the Mind Grows from Conception to College." Thank you.
Ms. AAMODT: Thank you.
STAMBERG: This is NPR News. Transcript provided by NPR, Copyright NPR.