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TERRY GROSS, host:

This is FRESH AIR. I'm Terry Gross.

It wasn't until my guest, Susan Barry, was in college that she realized she didn't see like other people did. She didn't see in 3-D; her world was only two-dimensional. She didn't have 3-D stereoscopic vision as the result of having developed severely crossed eyes when she was a baby.

Once she comprehended that she was missing something important, she found a developmental optometrist who taught her how to train her eyes to see in 3-D. Susan Barry's success challenged the popular theory that certain aspects of sensory development, including stereoscopic vision, had to be acquired in the first few years of life.

Neurologist and writer Oliver Sacks devotes a chapter to Susan Barry in his forthcoming book. Barry has written a memoir called "Fixing My Gaze: A Scientist's Journey Into Seeing in Three Dimensions." It's just come out in paperback.

She's a professor of neurobiology at Mount Holyoke College. Her husband is an astronaut who has dealt with sensory changes in space, as we'll hear a little later.

Susan Barry, welcome to FRESH AIR. Describe what you were missing when you didn't see in stereo.

Professor SUSAN BARRY (Neurobiology, Mount Holyoke College): When I didn't see in stereo, space was very contracted and compacted. So, for example, if I looked at a tree, the leaves or the branches would appear to overlap one in front of another. But I didn't actually see the pockets of space between the individual leaves or branches.

GROSS: Kind of like a movie or a moving painting.

Dr. BARRY: Yes, kind of like a movie or a moving painting or a collage, yes, something like that.

GROSS: So if you're not in 3-D as you're looking, and the world seems kind of flat, there's no space between things, where are you? Are you in front of everything that you're looking at?

Dr. BARRY: That actually was my impression, although I have to tell you that before my vision changed, I could not have told you that space was contracted and compacted. I didn't know any other way of seeing. And that was one of the great surprises of gaining stereo vision was seeing the world in a whole new way that I could not even imagine.

GROSS: So how do the two eyes produce stereoscopic vision, and what was wrong with your eyes that prevented you from seeing that way?

Dr. BARRY: So the key to stereoscopic vision is that the two eyes have to point or be aimed at the same place in space at the same time. And when that happens, the brain can integrate the information coming from the two eyes.

I was cross-eyed since the very first months of life, which meant that if I were to look at you, Terry, I would be looking at you with one eye, and the other eye was turned in. And so the two eyes were not aimed at the same place in space at the same time, and the brain then could not integrate the information from the two eyes, and therefore, I had no stereo vision.

GROSS: So did the brain have to learn how to deal with two different pictures from two different eyes?

Dr. BARRY: Exactly. Actually, you know, a person is meant to aim the two eyes at the same place in space at the same time and integrate that information. And if you don't do that, if one eye is looking one place, and the other eye is looking someplace else, then you're going to suffer from two major consequences.

One is double vision, and the second consequence is a phenomenon called visual confusion. And that means, for example, if I were looking at your face with my right eye, and my left eye, let's say, was turned and was aimed at the clock, then I would see your face and the clock in the same place in space, as if they were overlapping. That's visual confusion.

And so it should be obvious that if you either see a single thing as double, or you see two things in the same place that are actually located in different positions in space, then you are going to have a hard time moving around and manipulating objects.

GROSS: So your brain was tuning out one of your eyes.

Dr. BARRY: Exactly. You know, I needed to have a single view of the world in order to move through it. And so, the primary adaptation to having misaligned eyes, a condition also called strabismus, is to suppress the input from one eye, and then you don't have to worry about double vision or visual confusion. Of course, you don't have stereo vision, but you don't have to worry about a double view of the world.

GROSS: Is it always the same eye that's being suppressed?

Dr. BARRY: That depends on the person. In my case, no. I alternated rapidly. So I might look at you, for example, with my right eye and turn in the left and suppress the left eye's input and then in the next glance, look at you with the left eye and turn in the right.

GROSS: So you'd keep getting alternating views of me if you were doing that.

Dr. BARRY: Exactly, exactly.

GROSS: That must have been kind of confusing.

Dr. BARRY: It was very confusing, although I was not aware as to why my vision seemed unstable and confusing until I went to see a developmental optometrist at the age of 48. But I would often look in the distance and everything would appear to jitter.

GROSS: Now, so what caused your crosseyedness that led to this lack of stereoscopic vision? Was it a muscular problem?

Dr. BARRY: Unlikely. In fact, I have very good eye muscles. I can move them in all directions, and that is generally true for people who develop strabismus in infancy.

It's generally the case that their eye muscles and the innervation of the eye muscles are fine. The problem is in the way the brain coordinates the two eyes.

My guess is that if I was unable to fuse images as a baby, that means point the two eyes at the same place in space and therefore merge the input from the two eyes in my brain - if I was unable to do that, unable to fuse, then I'm going to suffer from visual confusion and double vision.

And so the brain needs then to suppress one eye's input. And it's actually easier to suppress an eye's input if you turn the eye. So I might have started out with a subtle eye misalignment and had trouble fusing input from the two eyes. And so that the brain then did was turn in one eye even more, leading to a very frank strabismus, a very obvious crosseyedness.

And often, parents notice with their cross-eyed children that they start out with a little misalignment, and it gets worse over time. And that may be because a child is adapting to this confusing view of the world by turning in and suppressing one of the eyes even more than it was initially turned in.

GROSS: Now, you had several surgeries as a child to try to correct your crossed eyes. So how many surgeries did you have, and what were they doing? What was surgery supposed to accomplish?

Dr. BARRY: So I had three surgeries, and the first one was when I was two years and four months old, and the second one was when I was three years and four months old, and the third surgery was when I was seven.

And the idea behind the surgeries was to realign the eyes in their orbits. So your eyes are actually suspended in the orbit by six, what are called external ocular eye muscles. And what my surgeon told my parents was you could sort of think of the eyes like being suspended in their orbits like a horse's head being held by the reins, and if you pulled one rein more, let's say if you pulled the rein on the right side more and loosened the one on the left side, then the horse's head would turn to the right.

And so what the surgery was supposed to do was to realign the eyes in their orbits by taking the eye muscles, shortening some of them, reattaching them to the eye orbit to, you know, kind of reorient where the eye sat.

Also by doing those operations, the idea was to sort of make it harder for me to turn in my eyes and easier for me to turn out my eyes in the hopes that that would correct the tendency I had to look with one eye and turn in the other one.

(Break)

GROSS: So the theory has been that if you don't train your eyes to see in stereo by a certain age, it's never going to happen. Your brain can't relearn how to process imaging after a certain age. And what was that age believed to be?

Dr. BARRY: So you're talking about a theory called the critical period, that there would be a critical period for the development of stereo vision, the ability to see in 3-D, and some people would say, oh, that critical period lasts through the first year of life or maybe the first two years of life or the first six to eight years of life but certainly not past early childhood. That was the mental mindset for the, oh, for the past half-century.

GROSS: But at the age of 48, you started therapy to retrain your eyes. What convinced you, knowing what you knew about this critical period theory, that you could maybe do it anyways, that you could maybe retrain your eyes anyways?

Dr. BARRY: So I actually went for vision therapy not to gain stereo vision. I am a neurobiology professor. I had taught for years in the classroom about the concept of the critical period, that stereo vision had to develop in early life or it was not possible to gain it in adulthood.

And when I went to see a developmental optometrist, what my major complaint was this jittery world view that I had, that when I looked in the distance, everything seemed to jitter.

I couldn't drive and read road signs at the same time. And, in fact, even in the classroom, I wouldn't look at students in the back of the class because to look, especially in a large classroom, to look into the distance took a great deal of effort even though the vision in my two eyes, with my glasses, was 20/20. My acuity was just fine but because of the conflict between the two eyes, it was difficult to look in the distance.

So when I went to see my developmental optometrist, a woman named Theresa Ruggiero, my major goal was to find a way to see more comfortably, to be able to drive more comfortably, to be able to read signs while driving, to be able to drive at night, to be able to play tennis or read for more than 20 minutes without feeling some eye fatigue and so on.

I did not expect that I would be able to gain stereo vision. In fact, I was quite convinced of the opposite, that stereo vision was well beyond me.

GROSS: So what are some of the things she taught you that enabled you to see in 3-D?

Dr. BARRY: The most important thing that Dr. Ruggiero taught me was that I had to break my old habits, my old ways of seeing. And the way that I saw was to look with one eye and turn in the other. When I turned in that eye, I could suppress its input, and that gave me a single view of the world.

What she taught me was how to point the two eyes at the same place in space at the same time. And when I could do that, I no longer had to suppress one eye's input because the brain was receiving correlated information from the two eyes.

And to my absolute astonishment, the result of learning that skill was that I began to see in 3-D.

GROSS: So you've done a series of exercises, a whole course of training, to get your eyes to look in the same place at the same time, to work as a team, as opposed to having each eye do something individual. And this has enabled you to see in 3-D. Give us an example of one exercise that's been very helpful to you.

Dr. BARRY: Well, I think people now are aware of various 3-D displays through 3-D movies like "Avatar" and "Up!" and "Despicable Me," and in these 3-D movies, perhaps people are aware of the fact that what's happening is that a different image is being presented to the two eyes through the 3-D glasses.

So I did a lot of work, as well, using 3-D glasses and stereograms or stereo displays. And basically, the way these stereograms worked is that when I wore the 3-D glasses, each eye only saw one image.

So the right eye saw a different image from the left eye, and then it was up to me to fuse the two images into one. And when I did, the image I was looking at would pop off of the stereogram sheet and float in the air, either in front of the sheet or behind the sheet.

And I continued to work with stereograms like that and see those stereo images floating in space, either in front or behind the sheet, depending upon the way that I'm looking, and that helps to keep reminding my brain to see in stereo.

GROSS: Interesting. What is the most thrilling thing that you've seen since learning to see in 3-D?

Dr. BARRY: I guess I would have to say trees. Over and over again, something that I see every day because I walk to work every day and so on and so forth, the way tree canopies are, where the branches are reaching out toward you, where the different branches enclose palpable pockets of space, I sometimes find myself just admiring the pockets of space between the different branches in a tree and actually walking and immersing myself into those pockets of space. It is just beautiful. It's just a beautiful sensation.

The sense of immersion in a 3-D world is very dramatic and very different from the way that I used to see. For example, in the past, when I would look at a snowfall before my vision changed, the snow would appear to fall in one flat sheet at a distance, just a little bit of a distance in front of me.

I did not feel like I was part of the snowfall but I was looking in on the snowfall. But after my vision changed, when I watch the snow coming down from the sky, each flake is in its own space, and there are palpable volumes of space between each snowflake. And all of the snowflakes produce this beautiful three-dimensional dance, and I feel myself immersed in the snowfall, surrounded by these snowflakes, enveloped by the snow, and this is just a wonderful experience.

And so it's not like I would tell you, oh, one day, I saw the Grand Canyon and that was spectacular. It's as if every day, over and over again, I get these moments of absolute joy when I look at a tree or a snowfall or the leaves coming down from a tree and so on and so forth, and I see all this in three dimensions. And it is, it's just a very joyful experience.

GROSS: But you write that sometimes, your new immersion in the world can be unnerving and even frightening.

Dr. BARRY: Yes, it can, and I've developed a fear of heights, which I really never had before. When I am looking, let's say, down from a window in a very tall building, I find that unnerving, or shortly after my vision changed, we took a trip to Hawaii, me and my husband and kids.

We all went to Hawaii, and we stopped at one of those scenic viewing points. And I went up to the fence at the viewing point and looked down over the fence at this spectacular canyon, and I had this sense of just how much volume I was looking into, how much space I was looking into, how deep that canyon was, how big it was. And I found it extremely nerve-wracking and backed away from the fence, put other people in front of me, other tourists who were also looking, you know, down at that beautiful canyon.

And later on that day, my children and husband and I went for a hike, and every time they came close to the cliff's edge, I really had to discipline myself not to scream out at them, don't get so close. Because now with this greater sense of the volumes of space around me, I did develop a sort of fear of heights. I mean, it's not a pathological fear of heights but certainly a greater sensitivity to that sense of being up high than I had before.

GROSS: And do you think that's because you perceive being high up more clearly and more three-dimensionally than you did before and therefore perceive the potential for horrific damage if you fell?

Dr. BARRY: I think it is because I perceive much more volume than I did before. But I don't think the fear is because I then sort of cognitively imagine, gee, if I fell, I would kill myself, let's say. I think it's a new sense for me still of where I am in space, how I am located in space, how far away I am from that cliff's edge and how, you know, how much space there is beyond the cliff, that I am still learning.

GROSS: So once you learned to see 3-D, do you think you were any more or less clumsy than you were before, not to imply that you were ever clumsy, but everybody's a little clumsy?

Dr. BARRY: Oh, I was clumsy. I was definitely clumsy, and once I learned to see in 3-D, I was indeed less clumsy. I found I could do things like my kids love to play Ultimate Frisbee. And sometimes we will go out and play a Frisbee game. And I could actually jump and turn and catch the Frisbee all in one motion. That would have been impossible before I learned how to use the two eyes together.

GROSS: Does driving seem any more or less risky with 3-D than it did with 2-D vision?

Dr. BARRY: Oh, it is much easier to drive now than it used to be, especially at night, when your sense of depth through stereopsis, through stereo vision, is really important.

And I had a funny incident because I drive very, very slowly. In fact, I had one neighbor who told me she didn't even have the nervous system to drive behind me when we were driving down our neighborhood streets, just that it would drive her crazy because I drove so slowly.

After my vision changed, driving became much smoother. I could come up to a corner, look both ways and turn the car without sort of having to stop, assess the situation and then move on.

So driving became much smoother, and I can remember one day driving along a country road between South Hadley and Northampton, Massachusetts, and my son said to me, mom? And I said, yes. He goes, do you know you're going 50 miles an hour?

And I didn't normally drive 50 miles an hour, but this was after my vision had changed. I was much more relaxed. I was actually enjoying the drive down the country road. And I had no idea that I could comfortably drive at that speed. So yes, in short, driving became easier after my vision changed.

(Break)

GROSS: You were told that if you didn't develop the ability to see in 3-D in your childhood that you'd never develop it, because the brain basically isn't plastic enough to relearn things like that, you know, to relearn how to process vision.

Dr. BARRY: Correct.

GROSS: But you disproved that...

(Soundbite of laughter)

GROSS: ...at the age of 48. But you had some inklings that the brain was more capable of relearning perception than scientists said. And one of the reasons why you thought that was because of your husband's experiences. He is or was an astronaut and his perceptions changed when he was in space in a gravity-free environment, and then he had to relearn how to be back on Earth in a gravity environment. Can you talk a little bit about his perceptual changes and what implications they had for you?

Dr. BARRY: So my husband, Dan, was an astronaut from the years 1992 to 2005, and he flew on the space shuttle three times during that period. And when an astronaut goes up into space and is orbiting the Earth on the space shuttle, everything is in freefall. And so what that means is that he basically appeared to float on the shuttle and everything that was not attached with Velcro to the walls of the shuttle floated along with him. Well, when that happens, your brain experiences sensory conflict.

So let me explain that. For example, if you're on Earth and you tilt your head downward, then your percept is it looks like the visual world goes up as your head goes down and there are sensory receptors in your neck muscles that say hey, I just flexed my neck and I just bent my head down, and there are receptors in the vestibular system in your inner ear that say gee, I just moved my head down toward the center of the Earth.

When you're up in space, and you move your head downward, your visual system says the head went down, the receptors in your neck muscles say the head went down, but your inner ear is now giving you not particularly truthful information because you're in this freefall condition. You're not in the normal gravity conditions of Earth. And so, you're experiencing conflict. You're experiencing the conflict between what your inner ear is saying and what your other sensory systems are saying.

And so, like with me, experiencing sensory conflict between my two eyes, Dan had to find a way to cope with this, and the way he coped and the way all astronauts cope, is you sort of turn off your vestibular system or you use it a lot less. You ignore that information from your inner ear because it doesn't correlate with the information you're getting from the other sensory systems, like your vision. And that is how he learned to move about in space. And then when he came back from space, he initially ignored his inner ear. So, for example, if you asked him to close his eyes and stand on one foot, he couldn't do it, because he was ignoring that inner ear information which is important for us on Earth to know what is up and what is down and it's important for our balance.

He couldn't walk in a straight line with his eyes closed when he first returned from space. But, he readapted to the gravity environment of space in about three days. But after his third flight it only took about an hour for him to become perfectly attuned to Earth's gravity again.

GROSS: Did you immediately see implications for your vision when your husband had to learn how to adapt to space and then readapt to Earth? Did that immediately make you think oh, well, maybe I can retrain my eyes, maybe I can retrain my perceptions?

Dr. BARRY: Yes and no. I was actually at the time working at the Space Center studying adaptations to, you know, changes in gravity and so on and so forth. And I was very aware of the fact that these changes were happening to astronauts who are adults who presumably have a nervous system that isn't supposed to be so plastic anymore. And so it did occur to me that if adults were - if their brains were that much more adaptable, then perhaps there was something that could happen that could allow me to see better. But I had no idea what kind of training I would need for that to happen, for me to be able to learn how to see in 3-D.

And so, when I ended up, in 2002, first visiting with a developmental optometrist, and she started to guide me through these exercises, then I thought to myself, well, if Dan could adapt to space and could change his habits and learn to adapt to a new way of living, then perhaps the adult brain really is plastic and with these procedures, these vision therapy procedures, perhaps I, too, could learn how to see in a new way.

GROSS: You say something in your book that I suppose is obvious but it wasn't obvious to me. You write: seeing isn't passive.

Dr. BARRY: Not at all. And it isn't obvious to a lot of people. Even people who study stereo vision, they just assume that the two eyes are pointing to the same place at the same time and therefore, what really matters is then how that input from the two eyes is getting to the brain and what the brain does with it. And they don't step back to yet a sort of more beginning point, which is that you have to move your two eyes into position so that the two eyes are aimed at the same place.

When you look at a close object, you have to turn in your two eyes so that the image that you're looking at falls on the central part of the retina, called the fovea, of the two eyes. When you look at a more distant object, your eyes will turn out. And those movements, they're called vergence movements, are the movements that I had to learn to make. A person with strabismus, a person with misaligned eyes doesn't make those vergence movements. I had to learn how to make them. And once I learned how to make them, then everything else followed.

So for me to learn to see in stereo was a very active process. I had to be very aware of how I used my two eyes and the procedures gave me the feedback to know that. And then I had to learn in a very conscious way, how to move my two eyes differently and it was only after that that I could begin to see in stereo. So yes, it's all a very active process.

GROSS: Since it was only after the age of 48 that you were able to see in 3-D with the help of eye-training exercises that you did, and when you saw in 2-D, you couldn't really imagine what 3-D was like therefore, you had no idea what other people were seeing. So does this make you think that maybe all perception really is relative, that maybe what we each think of as visual reality is relative?

Dr. BARRY: Yes, I do think that what we all think of as visual reality is relative. Take for example, somebody with red-green colorblindness, that's a pretty common condition found in about seven percent of men. Their perception of what red looks like may be very different from what your perception of what red looks like, so their visual reality is a little different from yours. And I think at various levels - some subtle, some not so subtle - that is true in general for visual perception.

GROSS: Do you spend a lot of time looking into people's eyes? And if so, do you think you're looking for something different than other people would be?

Dr. BARRY: I do now. When I was younger and my eyes were crossed, or even after my surgeries when my eyes didn't look so crossed but still looked a little odd, I didn't look into people's eyes for the obvious reason that I felt that I didn't look so normal. Now I do look into people's eyes and yes, I'm always looking to see if their eyes are straight as a result of my experience.

And I'd like to tell you one story about a student of mine who was walleyed, that is she had one eye that looked straight and the other eye that turned out. And she asked me once about a year ago what it was like to look into somebody's eyes. And I said to her it could be a very intense experience. And then she went through vision therapy. And one day I saw her about a year later and I was working very hard. I was - my head was down. I was reading a book and she came up to me and said hello, and I knew who it was. Let's call her Liz. And I said hello, Liz but I didn't look up because I really wanted to continue to do the work I was doing. But I did say to her, how are you, Liz? And she said, binocular.

(Soundbite of laughter)

Dr. BARRY: And I looked up and I looked into these two straight eyes, and a chill just went down my spine, it was like an electric moment, to see her looking at me and to return her gaze. It was spectacular.

GROSS: So part of the way that you taught yourself how to see in 3-D was through the use of 3-D glasses. I assume you went to see "Avatar"?

Dr. BARRY: Oh, yes. I loved "Avatar."

GROSS: Good experience for you? Uh-huh, right.

Dr. BARRY: Oh, yes. Yes. Especially the scenes of the forest that sort of receded way past the screen, the seeds - oh, I think there were seeds of a tree that sort of evoked that image of the snowfall that I was describing earlier in the program. Oh, yeah, I loved "Avatar," and all 3-D movies.

(Soundbite of laughter)

GROSS: Great. Sue Barry, thank you so much for talking with us.

Dr. BARRY: Thank you.

GROSS: Sue Barry is the author of the memoir "Fixing My Gaze: A Scientist's Journey into Seeing in Three Dimensions." You can read an excerpt on our website, freshair.npr.org.

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