visual illusions
ddobbs | May 12, 2010Traveling. But here's what I'm reading during train, plane, and bus rides -- and over meals:
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Gravity-defying ramps take illusion prize. This contest always produces fascinating stuff. This time, the ball rolls up. Video here.

Vaughan Bell ponders cortisol, dopamine, neuroplasticity, and other things that set off his bullshit detector. Riff launched from a post from Neuroskeptic on cortisol and childcare scare stories, equally read-worthy.
Dan Vorhaus does a wonderful round-up of reactions and implications stemming from the news that genetic testing is coming to Walgreens. Best blog-…
mixingmemory | June 23, 2008UPDATE: I've messed with some of the images below the fold, which will hopefully make it easier for people to see the illusion without having to move all round the room.
Last year, Rob Jenkins published a seriously spooky-looking illusion (it freaks my son out) in the journal Perception (1). Take a look at this face (from Jenkins' paper, Figure 1, p. 1266):
Spooky, right? Hopefully you all see a spooky looking woman (it's actually a combination of two female faces, which is why it looks so creepy) who is looking to your left (her right). Now take a look at the face again, only this time,…
mixingmemory | June 16, 2008 One of my favorite optical illusions is the rotating face, an instance of depth inversion. I like it so much that I sometimes use the above image as my avatar around the web.
If you're not familiar with the illusion, what you're seeing is a 3D mask, one side of which is convex, while the other is concave. When viewing the convex side, you see what looks like a normal face. As the mask slowly rotates, however, you begin to see the "inside" of the mask, on the concave side. Suddenly, however, the mask switches back to looking convex, like a normal face. This is likely the result of a…
mixingmemory | June 9, 2008I love afterimages and aftereffects, so I was excited to see that the 2008 winner for Best Illusion of the Year is a new afterimage illusion. To see the illusion for your self, watch this sequence of images for about 30 seconds (it takes at least 30 seconds for it to really work for me):
The illusion isn't really strong, so you may need to know what you're looking for in order to see it. What you should see is, after 20 or 30 seconds, the blank shapes start to be filled with a "ghostly" color. That's the afterimage, and though the actual colors only fill part of the the shapes, the…
mixingmemory | November 19, 2007I frequently hear people imply, if they don't state directly, that two working eyes are required for depth perception. This is surprising because with a moment's reflection, it's easy to see that there are depth cues that don't require both eyes. In fact, out of the many, many cues to depth that our visual system uses, only a couple -- convergence, or the relative position of the two eyes, and disparity (though there are two or three different kinds of disparity, depending on who you ask) -- require both eyes. The rest are all monocular.
The most obvious monocular depth cues are size (…
mixingmemory | November 9, 2007Cool Stuff:
And right now, I'm doing some stuff with motion aftereffects, so I've been looking for them all over. This one rocks (it's even better if you watch it in full screen mode):
mixingmemory | October 20, 2007
From Anstis & Casco, 2006, Movie 1, p. 1088
OK, here's a really, really cool illusion published last year, and that I learned about only recently. To see it, go to Stuart Anstis' page here, watch the first movie only, and then come back here.
You should have seen two flies moving in circles with the same radius. The flies' rotations are offset so that one is at 6 o'clock when the other is at 12, but otherwise, the circles they're tracing are identical. Now go back and watch the second movie.
As the caption notes, the two flies are still moving on identical circles, except that they're…
mixingmemory | September 19, 2007Everybody knows the Margaret Thatcher Illusion. If you've forgotten about it, here's the best example I've found (from Schwaninger et al.1)
Both the top and bottom pairs are the same photos, but they look very different depending on whether they're upright or inverted. In the top pair (the inverted ones), the face on the left is normal, and the face on the right has inverted eyes and mouth. Since they're upside down, though, both look pretty normal. When you flip 'em over, though, the face with the inverted eyes and mouth looks, well, gross.
Aside from being just plain cool, this illusion…
mixingmemory | May 19, 2007Everybody's seen Kanizsa's triangle:
It's a simple illusory figure illusion, first reported by Kanizsa(1). The illusion is likely caused by the processes that the visual system uses to separate figures from their ground(2), but to date there doesn't appear to be any consensus about exactly how these processes cause the perception of illusory figures (here's a list of several competing explanations).
Recently, researchers in the Human Technology Laboratories have begun producing 3D versions of the Kanizsa triangle:
These figures are created by starting with the original Kanizsa triangle,…
mixingmemory | May 15, 2007In honor of the announcement of the Best Visual Illusion of the Year (via Steve), I thought I'd revive the old cool visual illusion series. I may post about this year's winner, the leaning tower illusion, in the future, but I just now read the paper, so I have some work to do first. Instead, I'm going to talk about an illusion discovered by one of the members of the team that came in third place this year, the winking effect. To see the effect, you'll have to head on over to the Journal of Vision website, which I'll link you to in a minute, 'cause I don't know how to put flash animation into…
mixingmemory | November 18, 2006One of my favorite perceptual illusions isn't actually visual. It's often called the "cutaneous rabbit" illusion1, for reasons that will be apparent in a moment. I stumbled across it when reading a paper by Dennett and Kinsbourne2. Here's their description of the illusion (p. 188):
The subject's arm rests cushioned on a table, and mechanical square-wave tappers are placed at two or three locations along the arm, up to a foot apart. A series of taps in rhythm are delivered by the tappers, e.g., 5 at the wrist followed by 2 near the elbow and then 3 more on the upper arm. The taps are delivered…
mixingmemory | November 11, 2006 In keeping with the theme of illusions that result from crossmodal interactions, this week's illusion is the ventriloquism effect, first reported by Howard and Tempelton in 1966. As you can probably tell from the name, the ventriloquism effect involves visual information influencing where we think sounds occur, like the moving mouth of a ventriloquist's dummy influencing where we perceive the voice to be coming from. In a typical setup for the ventriloquism effect, participants are exposed to a sound and asked to point to where they think the sound occurred. Participants are then exposed to…
mixingmemory | November 4, 2006Last week, I talked about the sound-induced flash illusion, in which presenting a single flash with two or more auditory beeps caused people to see two or more flashes. This week, a study showing that the same effect can be obtained by replacing the beeps with touches. Here's the setup (unfortunately, I can't show you this illusion)1. Participants were placed in front of a computer screen, on which they observed a small dot flash for 10 ms. At the same time, they received either no taps, one tap, or two taps (spaced 60 ms from each other) from all 8 pins of a Braille cell on their left index…
mixingmemory | October 28, 2006You may have seen this illusion in a post from earlier in the week over at Cognitive Daily, but I thought I'd say a little bit more about it, and talk about a related illusion. First, click play (from Sham's demo site)
If the illusion is working -- Dave at Cognitive Daily had a bit of trouble getting it to work for his readers -- you should see the dot flash on the screen twice. However, the dot only flashes once. If you don't believe me, go to Sham's page and watch the single beep movie. It's the same movie, but with one beep instead of two. The illusion is pretty strong for me, but as Dave…
mixingmemory | September 23, 2006I was reminded of this illusion by the Seed Daily Zeitgeist yesterday. In order to get the full effect, I'll show you one set of photos here, and the rest of the post will be below the fold. The first are from Schwanginer et al. (2003)1:
They look pretty normal, right? Now look at these:
Gross, right? Those are the same two photos (the one on the right is now on the left, but rightside up this time. The distortions to the one face, which didn't look so bad when it was upside down, now look... grotesque. What's going on here? Well, in the inverted photo on the right (on the left when the…
mixingmemory | July 8, 2006This week's visual illusion is related to Mach bands, and similar in some ways to the watercolor effect. It's called the Craik-O'Brien-Cornsweet effect (or just the Cornsweet effect)1. This is the best example I've ever seen (from here):
What you should see is a dark square over a light square (almost white). Now take your finger and place it over the boundary between the two squares. What do you see? Two squares of the same color!
What's going on here? Well, at the boundary between the two squares, the top one really is dark, and the bottom one really is white. The visual system seems to be…
mixingmemory | July 1, 2006Discovered in the 1860s by Ernst Mach (hence the name), Mach Bands are actually a set of interrelated phenomena. Take a look at this image:
From here
The individual bands should appear as gradients, and they may even appear to be curved. In fact, they are all solid colors. Now look at this one:
From here
If you look closely at the area above the center two arrows, you should see a thin bright line (left-middle arrow) and a thin dark line (right-middle arrow). Once again, this is despite the fact that each of the three areas (dark, light, and in between) are solid colors.
This figure (from…
mixingmemory | June 24, 2006Here is an illusion that was discovered relatively recently. Take a look at this (from here):
You should see two figures with a purple outter border and an orange inner border. What color is the interior of the figure? It probably looks like it's orange, though a lighter shade of orange than the inner border. It's actually white, though. Don't believe me? Take a look at these two figures:
These are the same two figures as the ones above, except that I've removed the orange inner border from the figure on the left. Now you can clearly see that the interior is white.
This is called the "…
mixingmemory | June 18, 2006In yesterday's post on afterimages and aftereffects, I mentioned that demonstrations of neural adaptation for a particular feature (in the post, I used the examples of color and motion) is generally taken as evidence of the existence of specific neurons or groups of neurons that detect/process that feature. With motion or color, which are very basic features of the visual environment, this isn't very surprising, but in this post, I'm going to talk about some recent research demonstrating neural adaptation for a much more complex and surprising feature. But first, a little background on…
mixingmemory | June 17, 2006Most of you have probably seen this before, but if you haven't, look at the flag for 30 seconds (if it doesn't work with 30, try 60), and then look at the white space underneath it.
You should see a red, white, and blue flag when you look at the white space. That is a color afterimage.
Again, most of you probably know how this works, but just in case you don't, I'll briefly explain it. As I'm sure you know, when light comes through the iris, it is projected onto the retina by the lens. The retina is covered with photoreceptors, which come in two types, rods and cones. Photoreceptors are…