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Comment by baddox | Hacker News Reader

"The most significant factor which contributes to depth perception is bionocular disparity"

There's no way this is true. How impaired are people with only one functional eye? Not very--their diminished peripheral vision is much more impairing than their lack of binocular disparity. I don't have any evidence to back it up, but I believe that motion parallax is the single most important source of depth information. Notice that one-eyed people and non-3D films don't suffer from any lack of motion parallax.

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Listen carefully to your argument.

You say that peripheral vision is more impairing than lack of binocular disparity. But peripheral vision has zero impact on depth perception, so the existence of the peripheral vision problem does not mean that binocular disparity can't be the biggest impact to depth perception, which was your initial assertion.

Also, motion parallax is useless for stationary objects seen by a stationary observer. You make it sound like lacking one eye is no trouble at all, I don't buy it.

> You say that peripheral vision is more impairing than lack of binocular disparity. But peripheral vision has zero impact on depth perception

Ah, but he didn't say "more impairing with respect to depth perception." He was talking about impairment in general. As in, the loss of peripheral vision that comes with having only one eye is probably a bigger deal than the loss of binocular depth perception that comes with having only one eye, because binocular depth perception doesn't confer that much of a perceptual advantage over the ability to process motion parallax.

That last part is certainly debatable, and really depends on the context. Your tennis example brings to mind the fact that binocular depth perception is really helpful for hunters (of many species), presumably because it gives them more precise depth information with less orthogonal movement. Wider peripheral vision is probably better for avoiding predators and other dangers. (I was just reading about animals with binocular 360 degree vision, after wondering why 360 degree vision wasn't more prevalent.)

For a modern human, which sort of visual impairment is more impairing likely depends a lot on lifestyle and environment.

I'm sorry I wasn't clear. Like dieterrams said, I just meant that when you lose sight in one eye, loss of peripheral vision is probably more impairing overall than the loss of binocular parallax.

If your depth perception was truly devastated by losing one eye (as the article implied by calling binocular disparity the most important facility for depth perception), losing eye would be a lot more handicapping than it is in my experience (I know several people with only one eye, one of whom is a pretty good casual volleyball player).

Lacking one eye really doesn't matter that much. You move your head a bit, you instantly know where everything is in 3D.

Simple trick: Take a pen with a cap. Close one eye. Hold it at some distance - as if you were holding it just above the desk while sitting at it. Without making the hands touch (just hold the end of the cap and the end of the pen), try to put them together again. It's actually not that easy and I miss almost every time. Moving your head a bit doesn't help that much in this situation.

So yeah - for big objects, scenery, etc. it works. For some manual actions 2 points of vision are much more useful than paralax when moving.

I have dubious depth perception; two modestly nearsighted eyes. I just took your challenge. I did it perfectly, first time. That's probably because my brain's depth perception mechanisms are used to adding in more cues than brains that can lean on perfect binocular vision do.

People tend to get really romantic about how brains work, so I should probably make something clear. When I say that my brain is more used to integrating multiple other environmental clues to get depth perception than brains with perfect stereoscopic vision, that's actually a bad thing. The resulting depth perception is clearly worse than good stereoscopic vision. I am lucky in that if I wear glasses, which I do not routinely, I can get true stereoscopic vision back; I do not lack the brain mechanisms entirely as some people with bad vision do. And that's clearly better. My brain is better adapted to degraded input, but it remains degraded input. Brains prefer stereoscopic vision when available because they naturally gravitate to the best sources of data.

Rolling back around to my main point, depth perception is a great deal more complicated than most people give it credit for. There's really two distinct depth perceptions, one using stereoscopic vision that only works out to a few tens of feet regardless of how good your vision, and one that works on the rest of the world using motion and occlusion cues and a variety of other things. These flashing pictures manage to fire some of my depth perception based on motion cues, but completely lack the precise depth perception I get from stereoscopic vision. There's really no such thing as a monolithic "depth perception", it's a lot of things that get integrated in the brain.

You ever tried to play tennis with one eye covered?

I'm not saying your life is ruined with only one eye, but don't downplay their plight. You can still see in 3D, it's true, but (in my experience) you loose a lot of accuracy.

I wear glasses and my left eye is pretty useless - my brain pretty much ignores any signal from my left eye unless I cover my right. So unfortunately I can't really tell how much importance there is in things like this. You're probably right for people who have really good/equal eyes.

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