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Cross-eyed since early infancy, neurobiologist Susan Barry saw the world in a very different way. Although she had three childhood operations, the pathways in her brain that allow for binocular vision did not develop normally; instead, she saw a flat, 2D world. Over 40 years later, she defied expectation by teaching herself to see in 3D. As Barry publishes her memoir, Fixing My Gaze, Helen Thomson found out how her world took shape

When did you realise that you saw the world differently?

My parents told me that my depth perception might be a little weak, but I didn鈥檛 know I was missing stereo vision until I was at college, age 20. I distinctly remember sitting in class, learning about some cats whose eyes were misaligned, and whose binocular neurons hadn鈥檛 developed in their brain so they didn鈥檛 see in 3D. After that I tried all sorts of 3D experiments and failed them all.

What was it like to see in 2D?

If I looked at a tree I didn鈥檛 have any idea of how round the canopy was. It was like a child鈥檚 drawing. The concept that the branches commanded a volume of space just wasn鈥檛 there.

How did things change?

When I failed the 3D tests, I went to see my doctor, who said, 鈥榠t鈥檚 no big deal鈥�. People don鈥檛 realise how important stereo vision is. But in my forties my sight became jittery so I went to a development optometrist. She taught me how to point two eyes at the same place at the same time, for example, by looking down a Brock string (right). After one session I sat in my car and the steering wheel floated in front of the dashboard. I closed one eye and it seemed to sink back again. I thought I was too old for my vision to change.

鈥淎fter one session I sat in my car and the steering wheel floated in front of the dashboard鈥�

Describe suddenly seeing in 3D

It was an incredibly joyful experience, a whole new world. I had the hardest time listening to my students because I was fascinated by the way their hands looked while gesturing. Leaves on trees, house plants, door knobs! Everything looked so beautiful. It was hard to describe to people: they looked at me like I was nuts.

How did you remodel your brain?

It鈥檚 conjecture, but to see comfortably as a child I had to use only one eye at a time, to suppress the input from one eye, or I would have had double vision, so there are probably inhibitory circuits in my brain. When one eye pathway is dominant, it inhibits the other. Once I could point two eyes at the same point I could reduce the effectiveness of those inhibitory synapses.

Do you consistently see in 3D now?

Almost always. But one time, I took an eye alignment test when I was tired and thinking about my father who was ill, and my eyes turned in like they used to. It was an indication to me that if I was very depressed I鈥檇 revert to my old way of seeing. It made me realise that the effectiveness of my therapy or any rehabilitation has a lot to do with mental state.

What can that teach us?

There are areas in the brain that are activated by anticipation of a reward or by a novel experience: those areas release neuro-modulators like dopamine and serotonin which help to facilitate and stimulate synaptic changes and make those changes last longer. In my opinion, it was the positive feedback from my experiences that snowballed and facilitated changes in my brain.

It was thought binocular vision could only be learned during a critical period at a young age. Have you proved this to be untrue?

Definitely. So often you鈥檙e told you鈥檙e too old to change 鈥� it鈥檚 not true. It鈥檚 about small improvements. It鈥檚 not about turning cross-eyed kids into professional baseball players, but allowing them to drive with confidence so that their day-to-day functioning is better.

Is there a downside to 3D vision?

Now, I鈥檓 not happy about being way up high or overlooking a cliff!

And the biggest gain?

Realising how much we can rehabilitate ourselves with the proper tools and encouragement from others.