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Visual Illusions

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Illusions are a very useful hook into normal visual processing because they are a mistake made by the processing that goes into the construction of visual experience. We can use this mistake to try to work out the steps and computations involved in the construction of vision in general. Here are some of the visual effects that we have been studying in my lab over the years. The relevant papers are indicated as 'ref. x' corresponding to the numbering on the publications tab.


Form Illusions

What is the 3D shape of this? And of this? This reveals a bias to interpret objects as being viewed from above.

How do we recover shape from contours? 1 2 3 ref. 11

Examples of volume and amodal completion: cat posts bauble Nessie octopus blob wineglass refs. 1, 2, 4, 5, 6

Examples of stereo volume completion:
curved Kanizsas crossed disparity
curved Kanizsas uncrossed disparity
hats and pegs crossed
hats and pegs uncrossed
peanut-shell crossed
peanut-shell uncrossed
chicken legs crossed
chicken legs uncrossed
bucket crossed
bucket uncrossed
chili pepper crossed
chili pepper uncrossed
caterpillars crossed
caterpillars uncrossed
transparent crossed
transparent uncrossed refs. 79, 80

Amodal completion is incomplete refs. 1, 2, 4, 5, 6

Bouncing Ball Animation Crossed
Bouncing Ball Animation Uncrossed
Bump Animation Crossed
Bump Animation Uncrossed
Bump Crossed
Bump Uncrossed
Bumpy Slide Crossed
Bumpy Slide Uncrossed
Eggs Animation Crossed
Eggs Animation Uncrossed
Eggs Crossed
Eggs Uncrossed
Twisted Crossed
Twisted Uncrossed
Vortex Crossed
Vortex Uncrossed
Warping Crossed
Warping Uncrossed

Ref. 88

Motion Illusions

Fixate the fixation spot and notice how radically different your perceived motion is from the actual motion of this object. Play the infinite regress illusion in loop mode. Notice how it seems to move away from your point of visual fixation forever. Here is another version. This works even if there is only one element. Notice how it seems to move away from your point of visual fixation forever. Now notice how this one element version seems to move toward your point of visual fixation forever. In this version the internal motion switches just as the motion path switches direction; Patrick Cavanagh calls this the double drift illusion. Notice how the orientation of the motion path is very different from the real motion path. Here is another version and here is a a very slow version . Patrick has made a lovely fast circular version, as well as a slow circular version. The illusion happens even if the stimuli remains fixed on the retina! Also on youtube. The object appears to move away from the fixation point, even though it is just moving up and down. ref. 32

Visual pop-out happens at a late stage, after illusory motion has been computed. ref. 87. Perceived rather than real motion oddball pops out. Perceived motion oddball pops out. Real motion oddball does not pop out. Real motion oddball pops out.

Motion-induced color change demo 1 demo2 Equiluminant version 1 Equiluminant version 2 Illusory Rebound Motion example2 refs. 21, 25, 29

Gradient onset and offset induced motion example2 ref. 24

Examples of motion fading. The cross takes longer to motion fade, though made of the same dots The cross appears to rotate faster than the square rotating at the same angular velocity The solid line cross takes longer to fade than the squareref. 34. The second demo above is closely related to the findings of papers 36 and 28.

Examples of the drifting edge illusion. Look at a point not directly on the drifting grating. Notice how the barberpole appears to move when it in fact is completely stationary. Only the grating is moving, yet the edge appears to move up and down. If the movies do not play when you click them, right click to save as, then play the download in loop mode. If the gratings flicker, try making the size of the window the movie is playing in smaller. Drifting edge illusion avi file Drifting edge illusion version 2 avi file Drifting edge illusion mov file DEI vertical DEI horizontal No DEI Single edge DEI horizontal barberpole with gap without gap ref. 40.


Form-Motion Interaction Illusions

Dynamic Volume Completion example1a example1b example1c example1d example2a example2b example3a example3b example4a example4b example5a example5b example5c example5d example6a example6b example7a example7b example7c example7d example7e example7f example7g example7h example8a example8b see refs. 79, 80

Dynamic Stereo Volume Completion crossed example1 crossed example 2 see refs. 79, 80

Transformational apparent motion example1 example 2 example 3 example 4 see refs. 12, 27

3D transformational apparent motion ref. 12 2nd-order transformational apparent motion ref. 27

The thinner ellipse seems to rotate faster, though the two ellipses in fact have the same angular velocity (Quicktime movie) ref. 28, 36 read more.

The Bar-Cross-Ellipse Illusion. Is it a deforming cross, bars alternating in depth, a rigidly rotating ellipse behind four white square occluders, or a rigid cross viewed through a rotating elliptical window, complete with illusory contours? ref. 31.

Examples of the dancing bar illusion. If they do not play when you click them, right click to save as, then play the download in loop mode Dancing Bars Dancing Squares Dancing Gabors Expanding V Rubber Tree ref. 37

First consider Stuart Anstis' original dots version. Motion appears to slow down under a global perceptual organization. In the next example, note that Ls on left seem to move more than Ls on right. When the global pattern is seen, there appears to be less rotational motion. This slowdown occurs even in the absence of rotational motion. ref. 42. See non-rotational case here.


Temporal Illusions

The subjective expansion of time Clock example refs. 18 and 22


Color Illusions

Feature mixing rather than filling-in. Fixate very carefully. The color of the top field will merge into a uniform color approximately like that of the bottom field. Example1. Example2. refs. 26, 41, and 47


Eye-movement-based Illusions

Smooth pursuit motion suppression. See how motion slows down during smooth pursuit.


Attentional Illusions

Attention-induced brightness changes 1 2 3 4 ref. 20

Attention-biased after-image rivalry. What you attend to influences what afterimage you experience. ref. 57.

Voluntary attention modulates motion-induced mislocalization. View movies in loop mode. Fixate the fixation spot in the center. The colored disks are in fact vertically aligned, but if you attend carefully to the white layer, the colored spot pair will appear slanted one way, and if you attend to the black layer, the colored spot pair will appear slanted the other way. Single version avi file. Single version mov file Double version avi file Double version mov file This version rules out cyclotorsion as the cause, avi file This version rules out cyclotorsion as the cause, mov file Translation version, avi file Translation version, mov file ref. 51


Unexplained Illusions

Wakes You can right click to save this executable and play it on your computer: Wakes with 3 balls refs. 3 and 7

Gauguin painting