“Spine drift” illusion
What to see
In the image to the above you see an array of spindly thingies (“spines”), with
a gap demarkating a square in the middle. Does the central square appear to float,
to move relative to the background?
If so, you are perceiving seeming movement, namely Kitaoka’s “Spine Drift” illusion.
It is strongly affected by eye movements, thus perceived differently by different
people.
What to do
To enhance the illusion: shake your display a little (easy, of course, with a notebook
or iPad). Or scroll a bit, or shake you head, or glance around the display, or use
the “shake” button.
If you press the “rotate 90°” button you will find:
Whenever the central and peripheral spines are parallel or at 180°, there is no
illusion of differential movement; when they're at 90° –whichever way– the illusion
occurs. Whith the wheel you can try out any angle.
Comments
With slight modifications this is Akiyoshi Kitaoka’s Spine Drift Illusion, used
here with kind permission. I propose the following explanation, which is similar
to the one for the Ouchi illusion: Eye
movements cause motion blur on the retina. When applying motion blur to the original
spine object (below, left), the resultant retinal image differs strongly in contrast
between blur directions:
original
|
after applying motion blur in various directions
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0° →
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22.5°
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45° ↗
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67.5°
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90° ↑
|
112.5°
|
135° ↖
|
157.5°
|
180° ←
|
|
|
|
|
|
|
|
|
|
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Thus when the retinal motion is at 45° (or –45°), the retinal image is very poor
in contrast. It is well known that reduced contrast reduces perceived speed (Thompson 1982), so when the movement is
in the 45°-direction the image seems to lag. When the central square and the surround
differ in their spine directions by 45°, eye-movement induced retinal motion blur
(in one of the two), causing differential contrast reduction, and thus a seeming
shift between central square and surround.
To me this is effect is fleeting, and rather beautiful. In his 2010 congress poster,
Dr. Kitaoka makes a good point in drawing connections to the
Fraser and the Café Wall illusions:
there is a second illusion here, all horizontal/vertical arrangements seem a little
tilted.
Sources
Kitaoka
A (2010) The Fraser illusion family and the corresponding motion illusions.
Perception 39 ECVP Abstract Supplement, p178
Kitaoka
A (2010) Spine Drift Illusion (scroll down, a little below half down)
Thompson P
(1982) Perceived rate of movement depends on contrast. Vision Res, 22:377–380
Created: 2011-11-13
Last update: 2013-10-04