optical illusion

Factual Questions
1

I would like Cecil to address this interesting illusion, which anyone may easily demonstrate. Hold your hands in front of your eyes about 2-4 inches away, with your two extended index fingers touching, tip to tip, and focus on the distance.

You will perceive a “sausage” finger interposed between the right and left index fingers. How is this ghost finger produced? There is another illusion that I suspect has a similar basis. Get an empty toilet paper role and hold it in front of the ipsilateral eye and gaze through it at the distance while you hold your other hand, palm towards the other eye. You will see (if you focus right and hold the tube and open palm at the correct distances) a hand with a “hole” in it.

The quick answer is that the brain does all this magic. I suspect that there are more intellectually satisfying explanations that involve detailed analyses of neural circuitry and function. Like why is it that the left half of your brain controls the right side of your body?

2

Maybe parallax has a little something to do with it?

3

OK.
Why does one eye see in faint shades or red while the other sees in faint shades of blue?

3-D glasses use this to allow the user to see objects on a screen, recorded slightly out of sync, doubled, with one containing shades of red and the other shades of blue, in 3 dimensions.

Yet we use binocular vision to see things in 3-D under normal light, even those not red or blue.

If you look fixedly at a white wall for several seconds, then cover one eye at a time and alternate them, the wall will appear faintly shaded pink or blue.

So, why is this also?

BTW both eyes are slightly out of sync with each other, so each views a slightly different scene. It you look at a fixed object, then ‘wink’ one eye closed, alternating with the other, and do this somewhat rapidly, the object will jump, or shift position slightly as vision goes from eye to eye.

There is a condition known as hysterical blindness, which is temporary. A person looses the sight in 1 eye due to injury, then might temporarily loose the sight in the good eye as the side of the brain viewing the blackness of the bad eye over rides the view of the good eye. The person has to learn to concentrate on the view from the good eye. Sometimes, both views can overlap very rapidly.

Cover one eye and walk around for a while this way and you might see what I mean.

In an injured person, the condition is often caused by anxiety.

BTW, that would also explain the optical illusion of the tube. Both sides of the brains vision centers are trying to process two different views. If it fades in and out, then it is due to the concentration sliding back and forth between the centers.

4

I believe that the answer to this question is that they don’t. 3D glasses work slightly differently.

5

It’s a parallax phenonema.

Move your hands about six inches farther away, and the situation becomes clearer (or at least more obvious :slight_smile: ) - when your focus is in a distant object, your left eye “sees” your two fingers as being a bit farther to the right than your right eye does, with the net result that you “see” four slightly-opaque fingers. Move your hands slowly closer to you and eventually the left eye’s “left” finger occupies the same place as your right eye’s “right” finger and you only see three fingers.

6

RE: Faint shades of red and blue.

They do. I noticed this phenomenon in school, ages back, and later it dawned on me that the 3-D glasses used the very same colors.

It’s best to try by looking at a sheet of white paper in normal light, then close one eye and don’t try to focus on anything. Then switch eyes. Do it several times and you’ll notice a faint blue or pink cast to the paper, depending on which eye you use.

If you wear glasses, take them off to do this. Often, even clear lenses have a faint tint to them which might interfere.

7

"You will see (if you focus right and hold the tube and open palm at the correct distances) a hand with a “hole” in it. "

I couldn’t really figure out how to replicate this on the instructions provided, but it sounds to me like the optic nerve, which extends from the back of the retina back through the eye socket to the brain. Where it hits the retina there’s a dead spot, usually a non-player because the other eye makes makes up for it (i.e., light hitting the other retina’s dead spot is shifted (parallaxed) and not the same as what is hitting the opposite dead spot). The dead spot, if isolated, will look like a “hole”. Basic Psych 101 stuff.

8

I agree, although I don’t have the time or the inclination to pick apart/debunk that exceedingly long post. A few points:

3D movies work because when you put the glasses on, they block out the red image in one eye and the blue in the other. Here’s a more coherent explanation, but basically, the blue and red images in the movie are just slightly out of phase (not aligned), so with the colored filters in the glasses, it gives an approximation of stereo vision, which is related to my next point…

The “jumping” effect when you close one eye is not because they’re out of sync, but because of the distance between them. You’re looking at the object from a slightly different angle.

9

The illusions mentioned in the OP have nothing to do with dead spots in the retina (though this exists), neural circuitry or function, or the “brains vision centers.”

It’s the simple physics of parallax.

10

I recommend Martin Gardner’s Whys and Wherefores for an interesting treatise on optical illusions, including the efforts of the motion picture industry to replicate three dimensions in film. Of course, this is just one small chapter of a totally fascinating book for the generally curious.

11

Sorry, but I have to note that I have noticed the same thing as Moongazer, although I don’t think 3-D glasses are at all related but work under a different technology (remember the “Password” board game?).
If I look at a white wall, everything seems lent a bluish cast by one eye, and pinkish by the other-- very subtle, though. I wonder if this should be a new thread?

12

In my experience–and apparently about 300 other people that I’ve tested this with–the dead spot, or blind spot, looks as if the brain has interpolated the surroundings and “filled” it in. It doesn’t look like a hole.

If you close one eye, part of you see is actually obliterated, by that blind spot, but I haven’t heard anyone say that there appears to be a hole in their field of vision. If you look, with one eye, at a small spot on the wall, and slowly shift your gaze away from the spot (“towards your other eye”), the spot will suddenly disappear from view, then reappear.