Amazing illusions

From Quirky Mind Stuff, by Richard Wiseman (via Matthew Cobb), we have a bunch of illusions, most of them amazing.

There are two that you can try yourself, but you’ll need a friend. I tried them, and they both work for me, but for the “tightening thread” phenomenon, be sure that your fingers are relaxed and you’re not trying to keep them apart.  The “numb finger” trick also worked for me, but didn’t for two of my co-workers.

Anyway, have a look, and report what you see for the final illusion.


  1. Pak Liam
    Posted November 30, 2012 at 12:06 pm | Permalink

    nice illusions, but shame there was no explanations for any of them.

    • suwise3
      Posted November 30, 2012 at 1:03 pm | Permalink

      Yes, but you probably aren’t the intended audience. These are absolutely great for introducing illusions to people who neither know or care about the science behind them, but are amazed by the fact that illlusions work, and that they (everyone) can be easily fooled.

    • Ougaseon
      Posted December 1, 2012 at 11:00 am | Permalink

      Ask an ye shall receive, check out my post below!

      • Pak Liam
        Posted December 1, 2012 at 5:41 pm | Permalink


  2. Fergus Gallagher
    Posted November 30, 2012 at 12:12 pm | Permalink

    Better (?) version of the last illusion at

    • abrotherhoodofman
      Posted November 30, 2012 at 12:20 pm | Permalink

      Yes indeed, Mr. Gallagher!

      I HIGHLY recommend his link to everyone. The effect was an order of magnitude stronger than the “spiral” — very freaky!

    • Posted November 30, 2012 at 2:23 pm | Permalink

      Same puckering effect with the screen after I exited the site you linked to as I saw after the end of the ‘spiral’ segment in the video embedded in the main post here.

    • neil344
      Posted November 30, 2012 at 4:10 pm | Permalink

      Far out and groovy. I guess they didn’t need to legalize pot in my state after all.

  3. Chance
    Posted November 30, 2012 at 12:33 pm | Permalink

    The first one (pink straws) isn’t true! Measure them :o

  4. Lee
    Posted November 30, 2012 at 1:03 pm | Permalink

    Dammit!! I have enough trouble with reality as it is.

    • NewEnglandBob
      Posted November 30, 2012 at 3:24 pm | Permalink

      You assume there is a reality? :)

  5. MadScientist
    Posted November 30, 2012 at 1:15 pm | Permalink

    Gee … so many years and I don’t recall ever noticing the ‘threads’ effect. Yet one more mechanical reflex …

  6. suwise3
    Posted November 30, 2012 at 1:19 pm | Permalink

    At the CSICOP convention in Chicago (1988 I belive), skeptic, illusionist and magician, JERRY ANDRUS gave a presentation on illusions and brought with him his marvelous, oversize devices so everyone could see and play with them. (Videos are nice, but being able to walk inside illusions is powerful.)

    He (and Carl Sagan) begged the skeptical community not to be dismissive of people who seem not as smart as they are in some things, because EVERYONE can be fooled.

    THIS is what should be taught in schools. they can stand on their own. You don’t need science or religion to explain them.

  7. Gregory Kusnick
    Posted November 30, 2012 at 1:20 pm | Permalink

    I’m not sure it’s meaningful to call all of these “illusions”. In the ones that involve interpreting 3D scenes, your brain is correctly inferring the relative sizes and colors of (presumed) 3D objects based on their 2D projections. It’s only when those projections are lifted out of their proper 3D context and manipulated as pure 2D shapes that the “illusion” is revealed. So if these count as “illusions”, then so does any photograph, perspective drawing, or other 2D representation of a 3D scene.

    • Posted November 30, 2012 at 7:30 pm | Permalink

      “So if these count as “illusions”, then so does any photograph, perspective drawing, or other 2D representation of a 3D scene.”

      Yes, so it does. It is an illusion. When it’s done well without the aid of a lens, it’s called Trompe-l’œil – “fool the eye”. You forget how relatively new the art of pespective drawing is.

  8. Posted November 30, 2012 at 1:38 pm | Permalink

    On the last one, for the first time, the spiral appeared to repeatedly flash for about 5 seconds toward the end, the next time, the spiral appeared as though it was viewed through a fisheye lens-it began to become bloated. It had the same flashing effect as in the first time for over 10 seconds. When I took my eyes off it the second time, the screen appeared to have been partially sucked into the center, and, over the course of a few seconds (about 4 or 5, maybe more), it appeared to begin to expand back to its normal state.

  9. Leslie
    Posted November 30, 2012 at 1:44 pm | Permalink

    I didn’t see anything on the back of my hand (the last illusion). I am hesitant to try it again (or the one mentioned in the comments) as I am a migraineur. Don’t need to trigger anything!

    • NewEnglandBob
      Posted November 30, 2012 at 3:26 pm | Permalink

      I saw the middle of the back of my hand grow out towards my face.

      • Matt G
        Posted November 30, 2012 at 4:06 pm | Permalink

        So did I.

        • Posted November 30, 2012 at 7:11 pm | Permalink

          It was as if my hand was turning into a werewolf’s….

          • HaggisForBrains
            Posted December 1, 2012 at 7:21 am | Permalink

            The night before last was a full moon. Coincidence; I think not.

  10. will
    Posted November 30, 2012 at 2:06 pm | Permalink

    There has to be a connection between how easily our minds buy into optical illusions — and how susceptible our human minds are to superstitions.

    • Jeff Johnson
      Posted December 1, 2012 at 1:04 pm | Permalink

      I doubt that. These optical illusions are created by specific visual processing behaviors of our visual cortex. They are purely unconscious as our brain seeks to make sense of the pattern of photons hitting the retina.

      Superstitions are a combination of unconscious emotional responses, such as fear or desire, with faulty reasoning based on unfounded hypotheses.

      Two totally different aspects of the brain. Whether you believe in superstitions or not, you see these illusions as all humans do, because of how your brain works. You can’t decide not to believe in the illusions based on new information.

  11. Grania Devine
    Posted November 30, 2012 at 2:22 pm | Permalink

    In the last illusion, when I looked at the back of my hand, my skin appeared to be expanding from the center. When I looked at a page of print instead, the lines of text appeared to be moving away from each other.

  12. Marella
    Posted November 30, 2012 at 4:19 pm | Permalink

    I saw the skin on my hand spiraling outwards just I have in the past when I’ve done this one.

  13. Thanny
    Posted November 30, 2012 at 7:10 pm | Permalink

    I don’t buy the one about an imaginary thread tightening around the fingers, squeezing them together. I tried doing that by myself, and found that it takes a fair bit of effort just to get your fingers that far apart, and soon after you do so, they will begin moving closer together. It’s a physiological thing, and has nothing to do with some other person pretending to wrap the fingers with string.

    • Jeff Johnson
      Posted December 1, 2012 at 1:08 pm | Permalink

      I concur with this. I tried it without the “string” and there was a completely natural tendency for the fingers to flex toward one another.

      • Gregory Kusnick
        Posted December 1, 2012 at 1:37 pm | Permalink

        Agreed, with one minor nitpick: the spread position is the flexed position. So the tendency is for the fingers to relax toward each other, relieving the tension of the flexed (i.e. contracted) muscles.

        • John Scanlon, FCD
          Posted December 3, 2012 at 7:45 am | Permalink

          I’ll raise on that nitpick. Technically, flexed would be bent towards the palm, the spread position is extended or hyperextended. The forearm muscles are in opposing sets with tendons running on the palm (flexors) and back of the hand (extensors). I’ll agree that fatigue will tend to bring the fingers back together though.

  14. Posted November 30, 2012 at 7:13 pm | Permalink

    I doubt that it would convince any believers, but if our eyes and brains had been Intelligently Designed, we would not be subject to these illusions, but would always see things as they are.

    • Gregory Kusnick
      Posted November 30, 2012 at 7:26 pm | Permalink

      What do you mean by “see things as they are”? What we see is literally a two-dimensional array of pixels on the retina. Perceiving the three-dimensional world as it really is requires a leap of inference from two dimensions to three. Such inferences are necessarily vulnerable to errors in the form of visual illusions, where random two-dimensional patterns by chance resemble the projections of non-existent 3D objects.

      So even an intelligently designed visual system (such as computer face-recognition systems) must be subject to illusions if it is to work at all.

    • Ougaseon
      Posted December 1, 2012 at 10:59 am | Permalink

      Actually, these illusions reveal that our brain is in fact doing the ‘correct’ thing in that what we care about are causes in the world, not the raw data that lands on the retina. For example, in the shaded checker square illusion, there’s nothing interesting about the raw image intensities. In fact, those are improperly biased by the presence of a shadow, so you aren’t comparing apples to apples if you just accept them at face value. See my post below!

  15. Mike Lee
    Posted December 1, 2012 at 4:09 am | Permalink

    His book: “Paranormality” is a fascinating read!

  16. Posted December 1, 2012 at 6:17 am | Permalink

    Reblogged this on Wo sind die Tränen von gestern abend? Wo ist der Schnee vom vergangenen Jahr? and commented:
    That last illusion was too creepy

  17. Ougaseon
    Posted December 1, 2012 at 10:54 am | Permalink

    I see at least one person has asked about explanations for the illusions, and couple people have alluded to the fact that visual perception is actually a great metaphor for inference. The purpose of vision is not to relay physical properties to the brain, but to reconstruct behaviorally relevant causes of the light hitting the retina.

    Consider that all your brain has access is a pair of two-dimensional retinal images, each of which is basically an array of photoreceptors. Each photoreceptor gives information only about the direction light is coming from and its intensity. Some photoreceptors respond more strongly to long wavelengths, some to medium, and some to short. So from an array of ‘pixels’ that only have information about light direction and intensity, your brain has the rather gargantuan task of inferring the causes in the world that best explain the retinal data.

    So now, armed with that framework, we can see how many of these illusions work. The first one is an especially good demonstration of this. The vast majority of the time, an image on your retina is a perspective projection of a 3D world, not a 2D drawing. Your interpretation of a perspective drawing, then, is essentially its 3D reconstruction, not ‘two lines far apart on the left and near each other on the right’. Once we’ve decided that the drawing is actually nearer to us on the left and farther away on the right, we need to explain how it could be that a pink bar on the ‘nearer’ (left) side, and a pink bar on the ‘farther’ (right) side of the drawing could have the same angular size on the retina. Well a very good explanation is that the bar on the left is actually smaller than the bar on the right. It only has the same retinal size, because it’s nearer! So perceptually, you shrink it.

    Notice here that your brain is modifying the ‘actual’ retinal image when it generates your perception of the world. The fact that the two bars are the same retinal size is not really behaviorally relevant. What you really want to know is how large they are in the world.

    The same thing is true for the shaded checker square illusion. As he shows with the mask, the two squares actually have the same intensity, which is all your retina can detect. But how could it be that a lit square and a shadowed square have the same intensity on the retina? Well, the shadowed square must in fact have a lighter color than the lit square in order for them both to have the same intensity on the retina. So your brain is actually doing the ‘right’ thing by taking the context of the shadow into account. Again, what you care about is the ‘true’ color of the square, not the intensity of light landing on your retina.

    Wiseman has missed my favorite illusions, though!

    The Impossible Motion Illusion shows again that you ‘reconstruct’ a 2D video into a coherent 3D explanation. It also shows, that you have knowledge about gravity, so that balls rolling up-hill are very surprising indeed until the camera rotation disambiguates the structure!

    The Hollow Mask Illusion is an example of how your prior knowledge influences your interpretation of a scene. You always see the face in this video as convex, because you have a very strong prior that faces are never concave. Even if you know cognitively that its a trick, you can’t help but always see that face as convex, even if it means the nose ring has to move in a weird way for that explanation to work.

    And my favorite, the Color Afterimage Illusion. This one is really cool because it gives you a lot of information about how the brain computes color. Neurons adapt after a while to their current stimulus and kind of bias your perception. That’s why you tend to get used to smells after a few minutes. Its also why luke-warm water will feel very hot if you first stick your hand in ice water and adapt to that for a few minutes. Once you’ve adapted to cold the ‘neutral’ water feels the opposite: hot! The same thing is try for color. Here you adapt to certain colors such that when a ‘neutral’ or black and white image is shown, you see an afterimage of their opposites. Pretty cool that colors have perceptual opposites (blue-yellow and red-green mostly).

    This is also the explanation for the last illusion. Your motion sensitive neurons are adapting to the inward spiral motion of the image. When you look a still page, your perceptual experience is of the opposite, or outward motion!

  18. Richard
    Posted December 1, 2012 at 11:53 am | Permalink

    After looking at the spiral and then at the back of my hand, a small central region of the back of my hand seemed to expand.

    I don’t believe the paper arc illusion. I’m going to have to try that one myself.

  19. alexandra
    Posted December 1, 2012 at 1:04 pm | Permalink

    An anonymous reader writes
    “A story at the BBC explains how the UK government has put an extra clause into a funding bill to ensure that any new ‘free schools’ (independent schools run by groups of parents or organizations, but publicly-funded)must teach evolution rather than creationism or potentially lose their funding. ‘The new rules state that from 2013, all free schools in England must teach evolution as a ‘comprehensive and coherent scientific theory.’ The move follows scientists’s concerns that free schools run by creationists might avoid teaching evolution. Sir Paul Nurse, president of the Royal Society, said it was ‘delighted.’ Sir Paul told BBC News the previous rules on free schools and the teaching of evolution versus creationism had been ‘not tight enough.’”

  20. Don
    Posted December 2, 2012 at 6:46 am | Permalink

    the ilusion involving holding two fingers apart while some one wraps an imaginary string around it is a total hoaks. The way in witch you are holding your hands together makes it so you have to exzert quite a lot of strength to keep your fingers apart as your hands get tired of holding that position (witch happens quite fast) your fingers move together, has nothing to do with the imaginary string or power of sugestion.

  21. John Scanlon, FCD
    Posted December 3, 2012 at 7:59 am | Permalink

    My daughter showed me a good one the other day that I hadn’t seen before, kinaesthetic not visual.

    You hold your hands out in front of you, palms facing about 20 cm apart, and press out against the other person’s hands pushing inward. After a few tens of seconds to get used to this being stable, they take their hands away and you bring yours together slowly.

    Spoiler (rot13 cypher): Jung lbh srry vf fbzrguvat yvxr na vaivfvoyr onyybba orvat fdhrrmrq orgjrra lbhe unaqf.

    Very impressive! Even stronger than the ‘floating arms’ version that you do in a doorway.

    • John Scanlon, FCD
      Posted December 3, 2012 at 8:03 am | Permalink

      BTW, I don’t get much from the spiral one, probably because I don’t have normal stereoscopic vision (the cyclopean illusion) since my strabismus wasn’t corrected till I was five. Moving colour fringes, but no depth illusion.

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