Monday, March 10, 2014

Does stereo vision make that big of a difference?

I remember having a conversation with Ewa, a member of DIY Vision Therapy, a long time back about the visual difference between having stereopsis and having stereoblindness.  She asked the question 'Does it really make that big of a difference?' or 'I wonder if it really makes that big of a difference.'  I made some long response (heh), about how yes, it makes a giant difference.

The first hint that I got that they are very different ways of seeing is when I was a child and when my brothers and I were sharing a View-Master (one of those red things that shows you joint pictures with stereo disparity cues).  When they looked in, they were in awe, clearly impressed by what they saw.  When I looked in, I was like 'okay, there's a picture of a car... it looks fine, what's the big deal?'  Of course, my brothers and I were like three or four years old and so lacked the vocabulary to explain the difference.

Then when I was around five or six on vacation with my family I saw a row of people watching a 3d movie at Universal Theaters.  The entire row was freaking out as though things were coming at them within inches of their faces.  When six year old me watched the movie, everyone around me reacted similarly.  I just sat there.  What the hell?

Later in life when I learned about my condition, and had read Fixing My Gaze, I became aware of all of the different accounts of those who had been stereoblind and through one method or another recovered stereopsis.  These people describe the difference between the two ways of seeing as fundamentally different, with stereopsis being vastly superior.  A common response is 'This is how normal people see?!!'  Then they want to re-look at everything, go to art museums, and look at sculptures.  They become fixated on trees (people often talk about how amazing it is to look at trees after they've recovered stereopsis).

Here is a quote from a Reddit AMA from someone who recovered stereopsis via surgery.
woah.... you mean to tell me the doctor literally pulled and tugged your eye into place? damn thats nuts
whats the most beautiful thing / amazing thing you have seen now?
[–]souobixo[S] 164 points  ago
He literally did just that. It's hard to name just one thing that stood out. I would have to say trees have been the one thing that comes to mind. They have SOOOO much depth and detail. There is a national park not too far from where I live that has absolutely amazing colors in the fall. Even little things like chairs in the kitchen, or people walking around in a crowded room still make me stop and just look.
Someone that I know online named Heather explained the difference saying (paraphrased) 'Things are not the way that I thought they were.  Walls that I thought were straight, actually are not, and vice versa.  I feel like the world is more real now'.

Here is a long quote from Oliver Sacks's book The Mind's Eye, which I scraped from Michael Lievens's review of the book.  He has a great blog with a lot of good book reviews.  He's linked below.  
One has to lose the use of an eye for a substantial period to find how life is altered in its absence. Paul Romano, a sixty-eight-year-old retired pediatric ophthalmologist, recounted his own story in the Binocular Vision & Strabismus Quarterly. He had suffered a massive ocular hemorrhage, which caused him to lose nearly all sight in one eye. After a single day of monocular vision, he noted, “I see items but I often don’t recognize them: I have lost my physical localization memory.… My office is a mess.… Now that I have been reduced to a two-dimensional world I don’t know where anything is.”
The next day he wrote, “Things are not the same at all monocularly as they were binocularly.… Cutting meat on the plate—it is difficult to see fat and gristle that you want to cut away.… I just don’t recognize it as fat and gristle when it only has two dimensions.” After almost a month, though Dr. Romano was becoming less clumsy, he still had a sense of great loss:
Although driving at normal speed replaces the loss of depth perception with motion stereopsis, I have lost my spatial orientation. There is no longer the feeling I used to have of knowing exactly where I am in space and the world. North was over here before—now I don’t know where it is.… I am sure my dead reckoning is gone.
His conclusion, after thirty-five days, was that “even though I adapt better to monocularity every day, I can’t see spending the rest of my life in this way.… Binocular stereoscopic depth perception is not just a visual phenomenon. It is a way of life.… Life in a two-dimensional world is very different from that in a three-dimensional world and very inferior.” As the weeks passed, Dr. Romano became more at home in his monocular world, but it was with enormous relief that, after nine months, he finally recovered his stereo vision.

Susan herself describes the sensation of stereopsis as 'touching with the eyes'.  Objects are suddenly in your face.  Seeing is now intensely stimulating.

In her book, Susan talks about the experience, and how even though you've essentially gained another eye, it's not quite enough to say that your vision has improved by a factor of two. A person with stereopsis has vision that is much more than twice as good as a person who has stereoblindness.  It has to do with way that the two eyes are used together. This, I told Ewa, is because of evolution.  Evolution is all about optimization, and optimizing a function with what is currently available to work with. That is why the mammalian eye, even with all of its flaws, works pretty well.

If we dare anthropomorphize evolution, we might imagine evolution asking 'How can we improve situational awareness?  We've got two forward-facing eyes.  Each eye sees well.  Are there any inferences we could make by combining their input?  Would that enhance situational awareness?'

When we look at something, we direct our eyeballs, pointing at that object with our foveas.  The fovea is the dead center of our retina.  It is the area most densely packed with photoreceptors.  The area of our fields that we look at with our foveas is quite small.  If you extend your hand out in front of you looking at your thumb, the size of your thumbnail represents your foveal viewing field.  It's pretty small.  Everything else is seen with our peripheral vision.  We're not aware of how low-resolution everything else is because filling in gaps is one of the primary jobs of the brain.  When we switch our attention to something that was in our blurry periphery, we put our foveas and attention on the object, and then suddenly that object is in sharp focus.  Everything else is back in our blurry periphery and out of our attention.

But the periphery is a giant part of our visual field.  What if evolution could take advantage of the massive peripheral overlap between the two eyes to make some valuable inferences about the environment?  Well, that's precisely what it did by evolving specialized neurons to do that job (binocular depth neurons).  This is why building peripheral vision is an important part of stereopsis recovery.  It's because peripheral vision, while beneficial on its own, becomes vastly more beneficial when the giant overlap between the eyes can be combined to make inferences about the environment.

So even though the visual information coming in from our periphery is low-resolution and it's not providing a ton of information, the real benefit comes from the special sense of depth inferred by binocular depth neurons via processing both foveal and non-foveal visual input of each eye.  This, I suspect, is a big part of why a person who has stereopsis has vision much more than twice as good as a person with stereoblindness.  This is why people who have recovered stereopsis are stunned by how different and how wonderful things look.  The massive difference between the two ways of seeing is why I've put so much time and investment into fixing my vision.  That's why when Ewa innocently asked me 'Is it worth it?  Is the difference between stereopsis and stereoblindess really that big?'  I answered with conviction Yes! and Yes!

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