Category Archives: Illusions

Reverse Phi

 Do you see what I see?

The four spots move back and forth in exact synchrony, in the direction shown by the arrow.  The two upper spots are correctly seen as moving in the direction of the arrow.  However, the two lower spots change their polarity between black and white as they shift.  These are perceived as moving backwards, toward the earlier stimulus and opposite to the true displacement.  This is reverse phi.  It is consistent with Ted Adelson’s motion energy model (JOSA 1985).

Both movies are identical and both rotate clockwise.  But in the right movie the dots are black and white on alternate frames, and appear to rotate counterclockwise.  This is reverse phi (Anstis 1970: Anstis & Rogers 1975), in which the motion energy does go counterclockwise.
Gaze at the centre of each movie for 20s, then stop the movement.  Which way does the movement aftereffect go?  CCW in the left-hand movie of course.  But CW in the right-hand movie, appropriate to the perceived motion direction, not to the physical dot displacements.
This shows that reverse phi does adapt neural motion detectors; possibly in brain area MT (V5).

In this reverse phi movie, made by PATRICK CAVANAGH, the spokes reverse their polarity on every movie frame.  Thus the inner ring actually steps counterclockwise (track a spoke with your eyes to check this) but it seems to rotate clockwise.  The opposite is true for the outer ring.  Adapt to the motion for 20s, then stop the motion.  In the motion aftereffect, the outer ring appears to move CW and the inner ring CCW — appropriate to the illusory reverse phi, not to the physical displacement.

Reverse Phi: Four-Stroke Cycle

Scroll down and click on each four-stroke cycle movie. The objects appear to move continuously without changing their average position.


Expansion/contraction

Vertical movement

Horizontal movement

Rotation

Each movie is four frames long, in the sequence positive-positive-negative-negatives.

All Kinds of Motion

When the black and white bars switch places, on a dark surround (left) the white bar appears to jump, but on a light surround (right) the black bar appears to jump. The bar with the higher contrast wins out. The mid-grey at which the motions balance is the arithmetic (not geometric) mean of the black & white, suggesting linear, not logarithmic processing of luminance. (Anstis & Mather, Perception 1986).


Ambiguous apparent motion. The two spots move either vertically or horizontally. Can you control the direction by willpower?


Proximity: Motion is seen between nearest neighbors, horizontally on the left, vertically on the right. Shorter motion paths win out.

The motion path changes gradually from a tall, skinny rectangle to a wide, flat rectangle. Perceived motion is always along the shorter side of the rectangle. Proximity wins.

Visual inertia drives ambiguous apparent motion. Each spot appears to follow a horizontal path, not jumping up or down halfway across. Straight motion paths are preferred to going round corners.

Do these all move together or do they move individually?

The center dot simply flashes on and off but it gets entrained by the other dots and seems to disappear and reappear from behind the green square. [V.S. Ramachandran]

Kinetic Edges

Although the three windows are actually aligned vertically, the central window appears shifted to the left or right, in the direction of the drifting dots that it contains.

Eight circular windows, arranged in a circle, contain random dot textures that move counterclockwise. Although the windows themselves are not moving, they appear to rotate together like a ferris wheel. This is a stronger version of the illusion demonstrated above — it introduces continuous illusory movement, not just a static illusory shift. Also, after fixating for a while, you may perceive the windows fade out and disappear.

Binocular Brightness

When two eyes see different grays:

Free fuse the two columns of gray squares in (a), so that the left eye sees column L and the right eye sees column R (or vice versa: it doesnt matter), to give a single vertical column of gray squares. Note the perceived brightness of the squares. On a light background (a), the middle square (arrowed) is the same to both eyes and probably looks lighter than the squares above and below. However, on a black background (b) the middle square probably looks darker than those above and below. Squares in (a) and (b) are actually identical, only the backgrounds differ.

What s going on? The stimulus presents a light square to one eye and a dark square to the other eye, so that the two luminances always sum to a constant. So if your eyes (and your web browser) were linear, all fused squares would look the same brightness. However, the visual system is non-linear, and systematically overweights the square with the higher contrast (not luminance), favouring dark squares on a light background in (a), and favouring light squares on a dark background in (b). Careful measurements have shown that the weighting function is quadratic for light squares (spatial increments) but square, or winner take all, for dark squares (spatial decrements), as shown in the graphs.

Adaptation to Flicker with Debbie Giaschi

Adaptation to Flicker and Spatial Frequency

Run the movie & fixate the red cross. Both gratings are the same, but following adaptation to spatially uniform flicker, the upper grating looks apparently finer. Reason: Adaptation of transient pathways that are tuned to high temporal and low spatial frequency.

Local and Global Motion with Juno Kim

At first, this ambiguous motion stimulus looks like four pairs of dots, each rotating about their common center, but after a while it perceptually reorganizes into two large squares (with a dot at each corner) floating over each other. These local and global forms of “common fate” often alternate; on a 30s trial, local motion is usually seen first, followed by global motion. And across a series of trials, global motion is gradually seen more often. This suggests two adaptation (or learning) processes with different time constants.

We usually see these upright cubes as two large globally moving squares, not as local pairs of cubes.

Conversely, the lovers gazing into each other’s eyes are seen not as a large female square and large male square but as locally moving pairs.

Each pair of spots is phase shifted by 45 degrees from its neighbors. The blue circles tend to constrain the pairs to remain local.

Without the circles, one perceives two intertwined global octagons.

Color Afterimages with Rob Van Lier and Martin Vergeer

Gaze steadily at the cross, ALWAYS! Without moving your eyes, note the colors of the squares (red, green, blue, yellow). Every so often, different colors will flash up briefly. These colors are afterimages–not on the screen, but in your head!

The adapting plaid consists of a blue/yellow vertical grating, superimposed on a red/cyan horizontal grating. After adapting to this plaid, vertical black test lines make the afterimage look yellow/blue, while horizontal test lines make the after image look cyan/pink. Thus one and the same adapting pattern gives differently colored afterimages.

Conclusion: the visual system averages after image colors within but not across luminance test contours.

Similar results using checkerboards instead of stripes. Test squares look blue/yellow or cyan/pink depending on the position of the test contours.

Three-way pattern gives diminishing returns.