For ECVP 2016
 The “witch ring” Illusion: experimental stimuli

This is a post to show animations to accompany a poster, which I and my colleague Priscilla Heard presented at ECVP 2016.  The poster reports experiments on a new, simplified version of the witch ring illusion.

This movie was shown to participants in experiments as an introduction, in a longer version to allow for discussion:

(For the movies shown to participants in the experiments, you may need to click on continue reading) Continue reading

For ECVP 2016
 The “witch ring” Illusion: supplementary videos

This is the second of two temporary posts to accompany a poster presentation at ECVP, August 2016

The poster reported work on the Witch Ring illusion, which I and Priscilla Heard published in the journal Perception in 2015.  The paper on the illusion and its related movies are online at:

1. Is the the witch ring illusion related to a “scrolling illusion”?


If this figure (above) is scrolled rapidly up and down on a computer screen, you may see an effect of sideways expansion and contraction reminiscent of the witch ring illusion.  (There may not be room on smaller tablet or phoone screens, but try moving a real world version, about three inches high on an A4 sheet, up and down within a second or so through about six inches).  This illusion however is surely quite distinct from the expansion effect in the witch ring illusion, and due to the aperture problem.  If the lines are replaced by dots, the effect vanishes.

(For the dot image, and more related material, you may need to click on continue reading)

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William Wollaston’s Gaze Illusion

The eyes in each pair of pictures don’t change at all, and yet in one picture in each pair they seem to look directly at us, whilst in the other they have rotated downwards. (featured: a Sportive Lemur; & a young man by German 16th century sculptor Michel Erhart in the Victoria and Albert Museum, London).

William Wollaston published this illusion nearly 200 years ago. He claimed that our brains seem set up to judge the direction in which the eyes are looking in relation to the face from the position of the pupils in relation to the whites of the eyes, but that direction of gaze depends on head orientation. You can read his original paper in the Philosophical Transactions of the Royal Society online. But here’s an animation of the original illustrations:

The original drawings were done for Wollaston by the leading portrait painter of his day, Sir Thomas Lawrence.  They are now in the Royal Society in London, and there’s a movie about them you can watch on Youtube.

A variant on the “Witch Ring” Illusion


Here’s another version of the illusion I’ve been working on with my colleague Priscilla Heard.  The bright V shaped zones imposed on the faces look like they are expanding sideways, especially near the sharp end of the Vs at the bottom.  But to the right an isolated bright zone shows that objectively the V tracks are quite static.  You may still see a bit of illusory lateral expansion even in this isolated set of tracks, but note that the top right corner of the track is quite static in relation to the corner of the movie.

It’s a version of the Witch Ring Illusion.   I posted about that back in 2011, noting a plan to take a look at it.  This year Priscilla and I published a paper about it in the journal Perception.

Best Illusion of the Year Competition

Here’s a copy with slight variations of a stunning new animation of the Ebbinghaus Illusion, by Christopher Blair, Gideon Caplovitz and Ryan E.B. Mruczek.  Their version won the Best Illusion of the Year Competition in 2014, a few weeks ago.  It’s a brilliant competition whose lead organiser is Susanna Martinez-Conde, and is accumulating a fascinating illusion resource as the ten finalists are added each year.

In the movie, as the figure moves up and down the screen, all the circles seem to change size.  Yet objectively only the outer ring of circles do so:  the central circle remains exactly the same size throughout.  It’s so vivid it’s hard to believe, but I’ve just added some yellow rails as a track for the central circle.  You can see that the circle always just fits the rails – and they don’t change size.

For more info and links on the Ebbinghaus illusion (aka Titchener Circles) see our earlier post on the traditional, static version.

Digital Kaleidoscopes – post no. 1

Everyone loves a kaleidoscope, particularly the ones with a lens at the end, so that as you look through them whilst sweeping the kaleidoscope around, the view becomes a dazzling starburst pattern.  (I find Nova Magic Marble kaleidoscopes are inexpensive ones for kids that work pretty well).   However, real-world kaleidoscopes can only tile the visual field with a limited repertoire of geometric shapes – typically triangles. Digitally we can tile with any shape that will tessellate – that is, fill the plane by repetition without gaps or overlaps. As with real-world kaleidoscopes with a lens at the end, each tile can enclose a streaming segment of a visual scene, if you are handy with graphics and 2D animation packages.  If that all sounds a bit puzzling, I think the movie will make it clearer.

But then there’s a surprise!  Illusions of movement may appear, dependent on figure/ground effects.

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Roger Shepard’s Arc de Triomphe revisited

Arc de Triomphe illusion

In 1990 the psychologist and artist Roger Shepard published a cartoon version of this effect, captioned “I stand corrected”, in his book Mindsights (page 91).  I wanted to try a photo processed version of it and here’s my second attempt.  When I tried before, back in 2008, I somehow couldn’t get my mind round what Shepard had done, and produced an even more twisted version.

M.C.Escher’s lithograph Belvedere from 1958 is famous variant on the theme.  Subsequent investigators have presented animated 3D versions of it that help explain the effect.

My new version is based on a late nineteenth century Photochrom postcard.  They were made by a beautiful process that added colour lithographically to black and white photos.  You can see the original in a collection of gorgeous period cards of views from all over the world in the USA Library of Congress.

Mamluk Interlace


I’m fascinated by the way that spectacular aesthetic effects often seem to involve bamboozling our everyday strategies for making visual sense of the world.  This is a beautiful example, a detail of interlace decoration on a 14th century (Western dates) Mamluk Period door in the Louvre from the Al-Maridani mosque in Cairo.  (I’ve shown other examples of a role for bamboozled perception in aesthetics in an earlier post, and in the Illusions and Aesthetics category to the right).

As you can begin to see in the image, where I’ve combined the interlace pattern on the door with a schematic analysis of its reflection, the interlace we see in the door is a segment of a rosette pattern that repeats across a wider field.  But that’s not obvious at all when you just see the door.  The artist has not emphasised the lines of the design, but rather the infills – stars and other little geometric tiles.  We’re distracted from grasping the overall geometry by all the assertive, enclosed shapes, with their heavy outlines.  And the overall shapes that do jump out for me are the beautiful curves that run from top to bottom of the image, which also distract attention from the hexagonal geometry of the pattern.  For more analysis of the pattern and the fabrication of the doors, see below, but first, here’s the whole door.

Mamluk Door

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Bars and Bands – illusory expansion

When the chevron pattern in the movie is in perspective, so that the bars get thinner and closer together with distance, the bars and the fan of bright bands on them appear to stream past us, as if we were travelling along a tunnel. When the bars are all the same size and equally spaced, so that they don’t show perspective depth cues, the fan of bright bands appears to be expanding. The outer bright bands even look as if they are sliding along the bars.

Thanks to Priscilla Heard for the suggestion that the key to the expansion effect is in the absence of perspective cues. If you’d like more on that ….

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Barbers’ Poles and the Aperture Problem

Go back a couple of centuries and there were no chains of shops or malls. In the high street in the UK you would have found the type of shop you were after by looking out for a sign hanging out.  There were signs for pharmacists, tobacconists, pawnbrokers, whatever.  Nowadays there’s just one traditional sign still sometimes to be seen – the barber’s pole, as left in the animation.

The barber’s sign shows a famous illusion.  The cylinder is rotating horizontally around a vertical axis, but the stripes look as if they are rising – which would be impossible, unless you had some long pole sliding through the cylinder.

You can begin to see why in the demo on the right:  focus on the vertical slot and the grating seems to be moving vertically (as in the barber’s pole).  But focus on the horizontal slot and in a moment the grating may seem to move horizontally.  Behind the round hole, for me it tends to look as if moving obliquely.

Want to know more about what’s going on?
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Illusions and visual special effects – explanations and tutorials