Category Archives: Illusions

Illusory drifting within a window with Sae Kaneko

Anstis, S. & Kaneko, S. (2014) Illusory drifting within a window that moves across a flickering background, i-Perception, 5, 585-588

What you see in the movie is two striped disks moving bodily across the screen. The stripes are painted, stuck to the disks. On a static background (upper field), what you see is what you get. But on a flickering background (lower field), the stripes appear to move much faster than the disk, overtaking it! Stripes are no longer seen as stuck to the disks but seem to drift within a circular window. The illusion is stronger in peripheral vision; but you can see some of it even when you track the disks with your eyes.

Patterns don’t matter – random dots and sinusoidal gratings also give the same effect.

You can judge the strength of the illusion in this movie. Within each window, the grating moves at 25%, 50%, 75%, 100% of the window speed (from top to bottom). Since the flickering background (on the right) makes the gratings appear to move much faster than their windows, they look quite different from their corresponding windows on the left (control conditions). For most observers, the gratings appear locked to the windows labelled 25% or 50%. So the gratings appear to move 2x or 4x faster than their window.

While a gray contour around the window enhances the illusion, you can still see it without the contour (at least in some cases).

We think this is due to reverse-phi illusion – windows lag behind because of reverse-phi, making gratings appear relatively faster.

To see reverse-phi motion, the target (in this case, window) has to change its polarity on each frame, i.e. being alternately lighter or darker than its background. So, if we replace the background flicker with color flicker with minimal luminance change, the illusion disappears.

For the same reason, if we replace the gray contour with black contour, the illusion disappears.

this only applies to WITHOUT-gray-ring stimuli; when background flicker has very low contrast, the illusion disappears, again. But lowering contrast of the grating does almost the opposite, enhances the illusion.

Moving Sawteeth

As the moving ramp waveforms move back and forth they appear to change in brightness.  As they move to the right, the upper field seems to brighten and the lower field seems to dim.  A fixed retinal receptor viewing the upper half will see continuous brightening ramps punctuated by sudden drops.  Probably, visual nonlinearities reduced the effectiveness of the sudden drops, so the ramping brightness predominates. (Cavanagh & Anstis, Vision Research 1986)

When the stripes move steadily to the right, the upper field looks apparently brighter.  Adapt for ~20s, then click the Pause button.  You will see a leftward motion aftereffect, plus a ‘ramp aftereffect’ of apparent dimming in the upper half and apparent brightening in the lower half.

Half Moon Illusion

Incomplete contour erasure can affect the perceived brightness of a uniformly filled light grey disk.  First adapt to the flicker of the semicircle, presented superimposed on the left edge of the disk. Following flicker adaptation, not only has the left edge of the disk disappeared, but also a brightness gradient is apparent, making the disk appear like a half moon. In the absence of edge information on the left half of the disk, the brain appears to interpolate the brightness levels from the unadapted right edge to the background level.


The blue and yellow squares move exactly in step.  But when the background is striped they seem to speed up and slow down in alternation.  Reason:  when the leading & trailing edges of the dark blue square lie on the black stripes, their contrast is low so the motion looks slower (Pete Thompson 1976).  On the white stripes the edges have high contrast so the motion seems to speed up.  The opposite is true for the light yellow squares.  Maybe this is like cars that appear to go slower in the fog.

Fixate on the red cross. The squares appear to wiggle in and out toward and way from each other.

Reason: As the dark square hits a white vertical line and its horizontal motion speeds up, it also hits a black horizontal line and its vertical motion slows down. For the dark square, the top and bottom edges have high contrast, the left and right edges have low contrast, and so its vertical motion is exaggerated. For the light square, the opposite is true.


Rotating Rings with Patrick Cavanagh

The texture filling the rings is either stationary (rings seem to move slowly) or moves in the opposite directions to the rings (rings slide over each other, each spinning CCW as the pair turn CW) or in the same direction as the rings (rings appear to move very fast as a rigid figure-8 .)
The rings appear to slide over one another when the intersections are dark so that they obey Metelli’s transparency rules.  They lock together into a rigid trefoil when the intersections are light and look opaque.


Left-hand rotating rings with painted-on spots were parsed as a solid figure of eight. Observers could easily track rigid intersection (lower right graph) (Anstis & Ito, Perception 2011).
Right-hand rotating rings with vertically aligned gaps or spots appeared to slide.  Observers could not track sliding intersection where rings cross.   Upper right-hand graph shows the noisy pattern of eye movements.


Eye-movement tracked while watching the stimuli.

Second Order Motion

Early Study of Second Order Motion

I made this movie  clip in the 1960’s or 1970’s, when I had more hair than brains.  Randomly- spotted vertical rods were held in a frame so that they could move up and down along their own length but not sideways.  When I slowly pulled the rug out from under them, they fell in sequence, so that a contour (defined by vertical motion) moved to the left.

I then turned the machine upside down and cranked the handle.  The rods, resting on the barley-sugar twist table leg, moved up and down sinusoidally, producing a travelling wave of second-order motion.  Since this ‘motion grating’ was defined by texture, not by luminance, a Reichardt motion detector would be blind to it.  Observers could see it quite clearly.  I looked for a motion aftereffect but found none.

Bicycle Spokes with Brian Rogers

The sectored grey disk steps around clockwise. The thin grey spokes never change their brightness or position, yet they appear to drift around counterclockwise.  Gaze at the centre for 20s, then stop the movie, and you will see a clockwise motion aftereffect in the spokes. A spoke appears to move only when a sector of the same luminance effectively jumps across it.

The disk jumps back and forth through one sector width (red line).  Look carefully and you will see the sector borders move OPPOSITE to the  overall sector movement.

Induced Movement

No flies on Rama: The flying bugs illusion
These two bugs fly clockwise along circular orbits of the same size, in all 3 movies. They are in counter phase; one is at 6 o’clock when the other is at 12 o’clock.

Now the right orbit looks twice as big as the left orbit, because the CW moving background is in phase with the LH orbit but in counter-phase with the RH orbit, which it enhances.

The orbits look elliptical, wide on the left and tall on the right.  The background moves CCW and is in counter-phase with the horizontal components of the left fly but the vertical components of the right fly.

El Greco

Was El Greco astigmatic?

Why did El Greco (1541-1614) paint such elongated figures?  Could he have suffered from a visual astigmatism that optically stretched his visual field?  Art historians doubt it, and logicians ague that this is a fallacy because any visual defect would affect sitter and painting equally and would cancel out.


I converted a volunteer into an ‘artificial El Greco’ with an experimental telescope that expanded the world horizontally.

When asked to copy a square, she drew an exact square copy, but when asked to draw a square from memory, she drew a tall, El Greco-style rectangle. This might suggest that El Greco’s portraits from life would be normal, but his portraits from memory would be elongated. However, the volunteer adapted over two days to the visual distortion; a series of her drawings of a square from memory gradually became perfectly square. So even an astigmatic El Greco could have painted in normal proportions if he chose. His elongations arose from his mannerist style, not from defective vision.

Motion-Based Position Shift with Patrick Cavanagh

    What do you see?

Motion undershoot. Bar rotates through 180°, from 12 to 12 o’clock.  But it appears to move only from 1 to 11 o’clock.

Red and green dots are in identical positions, vertically oriented. But when flashed at the motion endpoints of the rotating ring, they appear to be offset.

Same as the ring but for linear motion. Red and green bars are in the same position but appear to be offset. Try tracking them with your eyes; your eyes feel as if they move, but they really don’t!!

Same idea here! Right-angled cross looks wonky because moving sector edges shift the cross arms more than moving middles of sectors.