h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I had a condition called Keratoconus (http://en.wikipedia.org/wiki/Keratoconus) in my left eye which basically involves a deformation of the cornea.
The treatment I received for it was a cornea graft (I realise things have moved on since then, but was the best option at the time).

The upshot of this is that my left eye has a scar on the cornea running 3/4 of the way round the outside and, still not being a perfect hemisphere, the focal length is about 3" in the vertical and 3' in the horizontal. The scar means that certain areas of my peripheral vision detect nothing. The mismatched focal lengths mean that main vision is very blurred. I can, for example, make out the window boundaries of my browser on the screen and a grey smear where this text is. But no sharp lines, just more concentrated areas of blurry colour.

My other eye is fine with corrected vision.

Theories of human binocular vision that I have encountered say that the brain takes the images from the two retinas and, using the differences between them, is able to determine 3 dimensional qualities of objects detected.

Why, then, given that one of my eyes provides a very bad picture with no real detail, can I still function in 3 dimensions? I would say I experience no problems now, and only had very occasional problems immediately after the surgery (which was back in 1992), such as being unable to determine where to hold my lighter to light my ciggie, a source of much amusement to those with me!

I realise the brain (and eyes) adapt and learn, but where is the extra information to turn the detailed right eye 2d image into a usable 3d image coming from? I know some things like occluding objects, paralax and so on are used, but my 3d vision even works when stationary. Unless very tired (when I get headaches and eyeaches) all I consciously perceive is the detailed version of the image, I can only 'see' the fuzzy version by closing my right eye. When tired they occasionally overlap.

Any ideas?

(p.s. my left eye is not lazy either, a common problem with this, since I made a conscious effort to make it track. Nowadays it tracks quite happily on its own)

I suspect it's your brain filling in for what your eyes aren't actually noting anyway...

When you close one eye you can still pick up a coffee cup without spilling stuff everywhere...


This is basically because your brain knows how big a coffee cup should be so can work out how far away it is without having to fall back on processing two images and working out how far away it is.

You can focus your good eye at different distances, and your brain uses this information to generate a 3d view of the world.

presumably that means it is relying on information obtained from before the surgery?
Or do you tactile feedback from actually picking one up?
Have to say, experimenting (and getting some odd looks in the office!), I do seem to still have some trouble with very close up distince information, and realise that I don't actually use vision to really work out where the lighter needs to go, just previous experience.

The corollary to the op question is this:
I've never tried any of the 3D imaging stuff - from the earliest ones through to modern 3D TV. I suspect it won;t work on me. Any thoughts?

If you are in the UK you can watch this Horizon documentary wnich covers some aspects of how we see the world and the part the brain plays in this: http://www.bbc.co.uk/iplayer/episode/b00vhw1d/Horizon_20102011_Is_Seeing_Believing/

"You can focus your good eye at different distances, and your brain uses this information to generate a 3d view of the world."
So it works, but probably not as well as full binocular vision?

Combing those two I guess I probably have 60% binocular vision rather than just the 50% I though, and that is good enough to provide 80%-90% accurate 3d world?

Seems reasonable.

Remarkable thing, the neural system

Binocular triangulation is only one of many ways the brain works out 3d information from human vision.

Focus is another, for nearby objects your brain can infer how close something is to your eye by how your eye focusses on it. Parallax is another, as you move your head about, near things move against the distant background. Perspective is another.

Lots of common optical illusions work by exploiting these.

Zube

Hi Atticus101 - yes, watched that! And watched the TED talk with the guy in the colourful shirt in it as well. I covered basic psychology of perception/attention and how we see but it all assumes a fully functioning binocular system. Very interesting proggies, cheers!

Many of the techniques used in art are about fooling our 3D vision.Both eyes see the same flat image, but perspective, blurring of stuff in the distance, greater contrast in nearby objects all combine to give our brain cues on depth.

Zube

I figured it might be a lot more complicated than that!
cheers Zube

"blurring of stuff in the distance"
Like the depth of vision and motion blur stuff used to create 3d visual effects in computers. Makes sense

Ictoan, you seem to think that because you basically have one eye, that gives you 50% binocular vision and the additional focussing effects build on that.

You should start at 0% and build on top of it. I'd say a normal person would get 75% of their 3d vision from the 2-eye effect, 10% from focussing 7% from parallax and the rest from perspective. So you have 25% normal 3d vision.

I'm guessing these figures from comparisons of real life which has all four effects, normal 2-d movies which have perspective and nothing else, and 3-d movies which have 2-eye effect and perspective but not focussing or head movement parallax.

*waves to Ictoan - you might have known I'd turn up*

Binocular vision & depth perception are quite a weighty topic!
In your case it's possible that there's enough vision from the weak eye for you to have some stereopsis based on the two images you see. But as has already been mentioned, it's still possible to judge distances monocularly although you're more likely to make mistakes. Don't forget proprioception too in the case of physical tasks.

Binocular vision is a function of two eyes - one eye gives uniocular vision, not 50% binocular...
Someone can have two good eyes & yet poor stereopsis.

"because you basically have one eye, that gives you 50% binocular vision and the additional focussing effects build on that."

"one eye gives uniocular vision, not 50% binocular..."

What I was trying to get at was that I do have *some* input from the bad eye, there is only 50% good enough for the binocular processing. Bad terminology and shortcuttting of what I meant on my part. Since the nerves and visual processing centres are not damaged and thus still functional, they'd only be getting half the input, ie from 1 eye, the input from the other being ignored or not being worth anything. Especially as the image I consciously see is (when not overly tired) fully focused and details. I.e. there's no difference in main field vision if I close my left eye, although since peripheral vision (outside scarring area) is still A1 in left eye that is affected when closedd.

*waves to Ictoan - you might have known I'd turn up*

Indeed I did, was written with you in mind

Arguably you have 100% binocular vision if you are not getting double vision - I think you indicated that you do sometimes. Although if your brain is suppressing the blurred image, then you are not getting binocular vision.

What would be interesting is to measure your stereoscopic acuity. Ask your optician if they can test this next time you go!

My daughter has 'biocular' vision, which means that she only uses one eye at a time. Her brain ignores the image from the other eye. She uses one for close up vision and the other for distance vision.

That's what I would call monovision!

Might sound like a daft question... but have you seen Avatar? Or any other recently released 3d movie? Have you tried a 3d TV in your local electrical retailer? Do they work, for you?

>>> Why, then, given that one of my eyes provides a very bad picture with no real detail, can I still function in 3 dimensions?

>>> I realise the brain (and eyes) adapt and learn, but where is the extra information to turn the detailed right eye 2d image into a usable 3d image coming from? I know some things like occluding objects, paralax and so on are used, but my 3d vision even works when stationary. Unless very tired (when I get headaches and eyeaches) all I consciously perceive is the detailed version of the image, I can only 'see' the fuzzy version by closing my right eye. When tired they occasionally overlap.

Basically, whenever there is a (relatively) small defect in your visual field, your brain tells you a nice, plausible story about what is actually or likely to be there. This is why no one is troubled overmuch about the blind spot caused by the ocular nerve exiting the eyeball.

The brain compensates for a relatively small visual defect by extrapolating information from the surrounding areas to fill in the blank. It does this amazingly smoothly.

Persons with scotomas (blind spots caused by defects in the visual field) often don't realize they have them until the scotomas become so large or so numerous that there simply isn't enough information for the brain to weave into a coherent picture.

This ability of the brain to extrapolate information to create the impression of a complete visual field might explain part of your 3D experience.