When a person who can see is in a very dark place where there are no visible light photons--deep in a cave, for instance--they see nothing, of course, but they have a field of view that is devoid of content. What does a blind person "see"? Do they have a field of view, in their head, that is always black?

This is probably a very hard question to answer currently, since it's a process within the brain.

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Yeah...

What's the space or astronomy hook?

Questions and Answers subforum description: "Got a space/astronomy question? Get it answered here."

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Roll up your trouser leg, or lift up your skirt or sarong. (Nudists, ignore the previous instruction.) Look about the room using only your knee. That's what people who have been totally blind from birth see.

It probably depends on whether the person was blind by birth or whether they became blind. I would guess that a person who has been blind from birth does not have a field of view in the way the others do, but probably construct a spatial environment based on hearing. But this is just my speculation.

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A person blind from birth doesn't see anything... not even black.

They have nothing with which to compare against.

A person who goes blind has enough information to form pictures.

People who are blind from birth actually have more of an advantage than those that lose their sight.

Depends on the type of blindness, too. Cataract sufferers, for example, would probably see blackness, while those with damage to the optical centers of the brain would not be capable of seeing anything, even blackness.

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I don't believe there is a difference between seeing nothing and seeing blackness, other than that someone who's never seen anything wouldn't think of it as blackness or have the blackness take up much of his/her conscious awareness. Certainly, in the only cases I've heard of in which blind people were made able to see, they described their prior experience that way, as blackness that they just hadn't known was black... but then, if you're not aware of the blackness being "something" to see, then it is the same as seeing "nothing" anyway.

People who also happen to have synesthesia (a condition in which input from one sense is accidentally sometimes sent to the part of the brain that's meant for another sense) often "see" red sparks when they hear sudden, loud, high-pitched noises; it's one of the most common forms of synesthesia if not the most common. People who have this but also happen to be color blind to red report "seeing" sounds in colors that they never see in their "real" vision. This is an indication that the mind is set up to perceive inputs in a standard way whether such inputs are ever supplied or not.

A blind person who still has a functional visual cortex will probably see more than blackness--I imagine the phosphenes will dominate until the person's brain learns to ignore them the same way we ignore the nose on our face.

If the person could never see, and even if it had nothing to do with the visual cortex--the cortex does have to be "trained" (programmed?) to work, so never having been stimulated, the born-blind person probably sees nothing (not the same as "black"--it's like, you egrek nothing because you have never had an egrek sense--an alien who had one and lost it would egrek that there is nothing there to egrek, which is a different sensation).

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No. Cataracts produce blurring and opacity, along with some loss of colour vision. But the sufferer can still see light and shadows.
It's a common misconception that blind people see nothing at all. Blindness isn't an all-or-nothing disorder, but a spectrum-disorder; that means that most 'blind' people possess a small amount of vision that enables them to see colour or shadows, and only a few are totally and completely without sight.

Moved to General Science.

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I had a friend when I was growing up who's legally blind--she's completely lost the sight in one eye. In other words, what she sees depends on what's around her, but she's got depth perception problems.

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I had a girlfriend who went blind in her teens. (She was 20 when we met.) She could "see" the sun, but otherewise the colours and things that she perceived bore no relation to reality - they were merely the consequences of the condition of her retinas.

When she dreamed she dreamed visually. In one dream, she was in a church, and she could see all the details of the church interior. We had fallen out for some reason, and she didn't want anything to do with me, but she did need me to escort her home because she was blind. It did not strike her until after she woke up that her blindness was inconsistent with being able to see the church!

Her best friend, known as Rachel the Tart, had been blind since very early childhood - her eyes had been removed when she was about 3 weeks old because of a tumour. Not knowing this, I asked Rachel if she dreamed visually; she replied that in dreams, she knew things that she couldn't know in real life (e.g. she could know that someone's dress was yellow without being told) but had no idea of what "yellow" actually meant.

I do not agree with this at all. People blind from birth (or nearly so, as in Rachel's case) have the huge hurdle that they have no initial concept of the world beyond their own reach, and so are more prone to autism.

Here is an article about a blind person who paints pictures having proper perspective and color.

New Scientist: Seeing without Sight

The online article may not have images of his paintings. His colors are reasonably accurate. The viewer's perspective of objects is generally correct. Objects in the distance are depicted smaller than those in the foreground. There are shadows and reflections of objects in water.

I find it interesting that congenitally blind people will use hand gestures while they speak. Googling the terms blind gestures speech will return a number of papers and articles on the topic.

The gesturing used by blind people varies from that of sighted people according to circumstances. Sighted people typically use gesture to help convey a concept being described in speech, but use of gestures possibly reflects the underlying thought process or facilitates the process. I watched a filmclip of a congenitally blind person's hand motions as she spoke, and would never have guessed that she was blind based on that.

Dr. Paul Bach-y-Rita experimented with sensory substitution. One device he built transmitted optical information through the skin. It consisted of a camera mounted on a pair of eyeglasses connected to a plate containing a 20-by-20 grid of vibrating points. Think of them as tactile pixels.

Blind subjects would don the eyeglass-camera and place the grid on the stomach or back After a a few hours of training, the subjects could reliably identify simple geometrical shapes shown to them, read signs, and identify faces. After a while, the subjects no longer felt the tingling of the tactors, although itches continued to be felt as itches and were not "seen."

The subject's point of view was not the stomach, but the head. Bach-y-Rita zoomed the eyeglass-camera on one subject and the subject instinctively lurched backward and raised his arms to protect his head as if objects in the environment were coming at his head.

After the training the grid could be moved from the stomach to back, or vice-versa, and the subject would immediately adapt to the new location. Later experiments showed the fingertips and especially the tongue to be good locations to inject visual information into the body.

We tend to think of vision as "images" formed by the brain that are presented to some "mind." Some modern viewpoints suggest that the differences between the senses, between vision and sound, say, reside in sensorimotor contingencies, or how the information you detect in the world changes in response to your behavior. It is the patterns of information that make vision vision, in other words. Bach-y-Rita's experiments suggest that it doesn't matter how those patterns are delivered, the eyes, the stomach, or the tongue, we treat them as vision.

This is quite a coincidence, but this thread reminds me of something that's been happening with one of my senses lately. I've spent most of my life with nearly no sense of smell, but in the last few weeks, for no reason that I can imagine, I'm suddenly smelling a lot more odors in a lot more contexts than I could have before. At this moment, I smell nothing. At any given moment, I usually don't. But the reasons have changed. Until a few weeks ago, it was often because of my olfactory "blindness"/"deafness"; I knew that there was something right there which other people could smell, and I got nothing, even while inhaling heavily with the thing right in front of my face. Now, my ability to detect some odors has reached the point at which I can confidently say that if I'm not smelling it at any given moment, it's because it's not there: the equivalent of a person who's not blind or deaf, but is sitting in a silent/dark environment. Both experiences are the same: no smell is no smell, regardless of whether it's due to inability or lack of stimulus. There's no sensory "numeral zero" standing in for the concept of where odors should be when they aren't there.

Wow, this has come up yet again.

Well, I can answer this one. And Mr. Brak, as usual, has the right of it.

That epilepsy that I whine about? The injury that caused it also blinded me temporaly. Completely *expletive* *expletive* I might add. There was no "black" like when you close your eyes or are in a cave. My eyes were not hurt at all, it was my brain that was upset. There was just nothing. I can't think of a good analogy here. Mr. Brak's is correct, but somebody who hasn't been there still won't get it.

I also get severe migranes like a lot of epileptics and have suffered bouts of scintilating scotoma that progress to involve the whole field of vision in both eyes and leads to total blindness, as above, for a half hour or so. (Did I mention I don't drive OR ride a bicycle?) Every time that happens, I find it "exciting". And I always say a prayer of thanksgiving every time my vision comes back. But that could be just me, big ol' wuss that I am.

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Gimme a minute to read through Jay's latest observations...

I get scintilating scotoma before I get a migrane also. At max I lose half my field of view, and it isnt dark or anything like that. It does scintillate a bit, but the lost view is mostly just not there. A huge blind spot like what you can see when you do the trick to see your retinal blind spot.