“These lines are actually the exact same length!”
“This is the same shape in these two images!”
You see these sorts of things all the time, but is it possible that some people don’t ‘get’ the illusion? i.e. they immediately see the lines as being the same length and don’t understand why anyone would be confused?
Good question!
I hope someone knows the answer.
A concave face shape is mistaken for a convex one (because humans are used to encountering people with convex faces). People with schizophrenia and, if I remember correctly, on psychedelics are less likely to falsely see it as convex.
Optical illusions come from top-down pattern-matching perception that creates a prediction based on ambiguous sensory data. If that function is perturbed, it can lead to not getting an optical illusion or getting optical illusions which are different from other people.
People can also vary in their ability to perceive the different possible meanings of works like All is vanity Charles Allan Gilbert - Wikipedia
Yes, there exists a significant number of people who are completely immune to optical illusions.
However, if you exclude blind people, I’m no longer certain what the answer to your question is.
Some rely on e.g. binocular vision, which many people don’t have. But “illusion” is a category of very disparate phenomena which affect different people in different ways.
That’s a spatial frequency thing and can be affected by visual acuity, but also how far away you’re standing from the screen. You can affect it by squinting.
To support thelurkinghorror’s point about binocular vision, there is a 3D version of the inverting staircase illusion that you can make by folding a piece of paper into an M shape. If you stare at it long enough, the M inverts to a W. Then when you move your head, the W seems to be twisting and glowing in an unnatural way. It is one of the most striking illusions I’ve ever seen. Everyone I’ve shown it to was able to see the illusion except one. It turned out that he had only one functional eye.
Obviously, there are also illusions that won’t work for some color blind people.
Anecdotal but I can not discern the 3D pictures in those stereoscopic things like this and this. (I spent an evening looking at one and eventually got it but it was not easy and afterwards my eyes seemed all cross-eyed and messed up.)
My sister, on the other hand, walks up and at a glance can see what they are.
Take that FWIW.
In some cases it’s a physiological limitation, but for these (autostereograms), it’s strongly learned. They became popular when I was young, and all the standard viewing recommendations never worked. Eventually it just “clicked” and now I can converge/diverge my eyes quickly to see them (“AT” and a shark. The animated shark on Wikipedia is very cool).
Autostereograms? I have no idea what you people are talking about. 
They just explain it badly. You’re staring at a picture for ages waiting for some mystical thing to happen, when all that’s required is to consciously control the swivelling of your eyes. Here’s my best stab at it:
Can you cross your eyes (like the guy in this photo is doing)?
Do that now while looking at your screen. Notice that you can now see “two” screens (because the screen is in a different position in each eye’s vision and so your brain is not resolving them into one object).
Now, here’s the thing: you need to practice crossing your eyes a little. So the “two” screens are mostly overlapping.
Now take a look at a stereogram. Notice there’s always some kind of repeating tiling, like wallpaper. All you need to do is cross your eyes slightly, making the images overlap such that one “tile” overlaps its neighbour. At that point, the full 3D image pops out.
Crossing your eyes is an uncomfortable thing to do, but its just to learn the trick.
Once you’ve got it, crossing your eyes to view a regular stereogram will eventually feel just like the normal refocusing you do hundreds of times a day (because it is).
Note for autostereograms you do not cross your eyes!
You do the exact opposite. You converge your eyes past the screen.
Many autostereograms would give a reversed front to back effect if you cross your eyes. E.g., for the shark one (2nd in Whack-a-Mole’s link) you’d get the far side of a hollowed out shark.
Reversing the polarity, on the other hand, always works.
I’m reliving my frustration from around 30 years ago when the Magic Eye books came out. All I see for the supposedly cool animated shark linked by thelurkinghorror is a bunch of moving dots. For Whack-a-Mole’s links I see jumbled colors.
Yes, the illusion I’m talking about is three dimensional, but it involves the same inversion as the 2D Schroeder Stairs illusion that you cited. A key difference is that, in three dimensions, one of the two ways of perceiving the shape is correct, while the other is not. It is relatively easy to “see” the folded paper in either view. When your eye/brain locks into the incorrect view, you can move your head and the paper appears to move. You can also balance a pencil on the top of the “M”, which looks normal until your brain locks into the wrong view, in which case the pencil seems to be suspended in space.
It is the same effect as the inverted heads on Disneyland’s haunted house ride. As you move, the heads seem to be turning. Cleverly, the heads are lighted from below, opposite to normal, so your eye/brain is strongly locked into the “wrong” view.
Right, but crossing your eyes is necessary for those who don’t “get” stereograms yet. The key point is that it’s just controlling the eye muscles, nothing more complex than that. I think if people experiment with trying to “cross their eyes” a small distance they’ll generally get it, even if the advice is not technically correct.
Back when those autostereograms were popular I read a hint on how to see them. It worked for me then and it works now though it sometimes takes a bit of patience.
Take the picture up close so it is almost touching your nose and then slowly draw it away from you trying not to refocus your eyes. Eventually you should see the 3D object in the picture.
I’ve never been able to see the Magic Eye pictures. I was told it’s because I have an astigmatism, so it’s impossible for me to see it.
We had a ViewMaster when I was a kid. I had no idea that the images were supposed to appear 3-D. I never saw that, and I couldn’t understand the purpose.
Magic Eye pictures are worthless to me. I wish someone would publish pictures that show what I am supposed to be seeing.
I’ve generated RDS’s over the years myself. I notice some of my students had problems with seeing them so a took one and copied it onto a transparency and put it on a glass door. I’d tell the non-see-ers to look thru the door a bit. This helped a few people but not many.
Astigmatism is a matter of degree, maybe a severe enough case makes a difference, but other people with the same diagnosis can see them fine.
All the Magic Eye branded books had a cheat page in the back.
Magic Eye image of the week with answer. http://www.magiceye.com/3dfun/stwkdisp.shtml
I can see all the Magic Eye pictures (although half of the time I can’t make out exactly what the figure is supposed to be). I can’t see the shark normally in the gif movie. In my browser, I think the frame rate is too high. But if I cross my eyes I can see the hollowed-out version. When I first saw the shark gif years ago, I had no trouble seeing it in 3-d.
ISTR an article in Scientific American decades ago that said if you observe at an amputee in an Ames room walking back and forth, your perception of the illusion is reduced. The thought was that the slight anxiety you experience was somehow the cause.
As for Magic Eye, this is from one of my posts from 12 years ago! :eek: Maybe it’ll help.
Here’s a simplified pattern that illustrates the Magic Eye principle. I wonder if those who can’t see Magic Eye can see the depth in this:
N N N N N N N
F F F F F F
E E E E E E E
A A A A A A
A A A A A A A
R R R R R R
R R R R R R R
Merge the repeated letters with slightly crossed eyes (try looking at the tip of your nose), and you will see the word “FAR” an inch or two in front of the screen and the word “NEAR” farther away. You can verify this by moving your finger to a position where you see it as one finger. It’s a rather striking effect.
The Magic Eye technique is to un-cross your eyes by using techniques described earlier in this thread. Personally I find it helps doing that by bowing your head while keeping your eyes on the image until it’s at the top edge of your field of vision. If you succeed, you’ll see that the above pattern now has the word “FAR” farther away than the word “NEAR”.