Monday, April 28, 2008

Lucy in the Sky

Here is a variation of an illusion that I presented at the Neural Correlate Society's annual illusion contest in 2005. The effect was published in the Journal of Vision in 2005.

[The Demo will not be seen in the RSS feed.]

You are looking at rings made up of diamonds with black and white edges; the diamonds change from bright yellow to dark blue and back again. The button makes the black and white edges appear or disappear. If the edges are present, the diamonds dance back and forth, and the rings appear to undulate. If the edges are removed, the diamonds no longer dance, but the bright-yellow to dark-blue modulation appears to circulate around the rings. (Justin Charles and I titled the effect "Lucy in the Sky" because of the diamonds, and because of the illusion of motion).

Brief Comments:

The diamonds appear to be moving because of the contrast with the edges and the background. As the diamonds shift from bright yellow to dark blue, the minimum level of contrast shifts location. This shift in contrast can be described by simple algebraic models.

The Lucy in the Sky demonstration is directly related to the "phenomenal phenomena" of Gregory and Heard (1983). Here is an interactive demonstration of figure 1 from their paper:

[The Demo will not be seen in the RSS feed.]

A compelling demonstration based on the same principles was presented by Michael Pickard in the 2007 illusion contest.

Notice that when a square becomes white, the motion moves away from the white edge, and when a square becomes black, the motion moves away from the black edge (this applies to the diamonds and edges as well). The motion is therefore not produced by a newly created thick edge; my supposition is that the motion is produced by shifting the minimum contrast between the edge, the square or diamond, and the background (not unlike the motion produced in the illusion presented in the previous post).

Although the contrast that drives this type of illusion may be physically present in the stimulus, the resulting appearance of motion may seem surprising (how can there be an appearance of movement if the diamonds and edges are stationary?). Perhaps the motion seems counterintuitive because we tend to represent static objects (in this case, diamonds and edges) much better than we represent shifting relationships between static objects (the changes in contrast between the diamonds, edges, and background).

Next week: a preview of one of my entries from this year's illusion contest, which will be held in Naples, Florida, on May 11.

Wednesday, April 16, 2008

Window Shade/Rocking Disk Illusion

Here is the "window shade/rocking disk" illusion from the Journal of Vision article I wrote with Justin Charles and Mallory Shear-Heyman in 2005:

In this demonstration, you see a ring that is half black and half white, and a center disk that changes gradually from white to black, and back again. [note: the demonstration does not show up in an RSS feed. ]

What to notice:
1. A veil of brightness appears to drift across the center disk (like a window shade that is pulled up and down). The effect vanishes when you remove the white/black ring (click on the "add/remove surround" button).

2. Click on the "inner ring" button to add a thin gray circle between the disk and the black/white ring. Now the disk appears to rock up and down.

3. Click on the "rotate" button to rotate the ring. The direction of the shading and the rocking shifts in response to the orientation of the ring. The shading appearance can be considered an illusion because the center disk is always physically uniform. That is, the disk is never a combination of colors; the color pixels that make up its surface are all white, or all black, or all an intermediate shade.

Why does the window shade, shade?

Many illusions surprise us (and attract our attention) because they violate our basic assumptions about how objects in the world behave. In the real world, objects appear to be relatively stable: if you roll a white ball from a concrete sidewalk to a green lawn, the ball doesn't suddenly appear red the moment it hits the grass. But many illusions show that context can create dramatic changes in the way something looks (take a look at Adelson’s checker shadow illusion for an example).

In the window shade illusion, the context is provided by a black/white ring. I started using the black/white ring to study the effects of contrast. Contrast refers to the relative difference between lights. Neuroscientists know that most of the information that the eye sends to the brain corresponds more closely to the relative difference between lights in an image than it does to absolute light level.

The window shade illusion is set up so that when the disk is white, the contrast between the disk and the white part of the ring is low, and the contrast between the disk and black part of the ring is high. When the disk is black, the contrast shifts in the opposite direction. The high contrast edge therefore jumps back and forth across the disk.

The window shade illusion is very similar to the "contrast asynchrony illusion" (see the previous post), in which two disks appear to modulate out of phase with each other, but get light and dark at the same time. Both the window shade illusion and contrast asynchrony illusion contain alternating contrast information. The difference between the two is that in the window shade illusion, the contrast alternation occurs within a single disk, whereas in the contrast asynchrony illusion the contrast alternation occurs across two disks.

As a general rule, contrast alternation across a single object creates the appearance of motion. Curiously, the motion tends to shift towards the side of the ring with the lowest contrast (when the disk becomes white, the shade moves towards the white part of the ring, and when the disk becomes black, the shade moves towards the black part of the ring). The motion seems to track the minimum contrast in the scene (for those who like calculus, this is analogous to a change in the sign of the second derivative).

The above explanation for the motion in the window shade illusion leads to a range of questions: Why does the motion shift toward the half of the ring with minimum contrast instead of the half with maximum contrast? Why does the disk appear to rock when an inner ring is added? Why does the shading effect spread across the whole disk instead of just staying at the edges? The answers to these questions help us understand how the brain works.

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