Illusion Sciences: why are we surprised by only some of the things that we see?

The FedEx Arrow

2008-07-16T07:29:00.000-04:00

This post concerns what is probably the most frequently encountered visual illusion in the United States--and possibly the world--yet most people don’t know that it is there.

If you have not yet seen this illusion, then be prepared, because once it is pointed out to you, you will see it all the time and, perhaps, wonder how you could possibly have missed it.

What to notice: Look at the FedEx logo and see if you can find the image of the arrow embedded in the letters. Give up? Click on the button and the arrow will appear as yellow outlined by blue. Click the button again to make the yellow arrow go away.

(Credits: The image of the back of the Fed Ex truck was taken by my brother, Chuck Shapiro; the Flash movie was made with the assistance of my nephew, Sam Shapiro. I do not know who created the FedEx logo. I think the logo is absolutely brilliant, and I would love to know its history.)

Comments: The FedEx logo is a reversible image, like Edgar Rubin’s famous Face/Vase image, and the M.C. Escher-like bird image shown below. As in these images, the logo can be interpreted either as an “EX” or as an arrow, depending on what we see as “figure” and what we see as the background.

Rendering of M.C. Escher-like image by my sister-in-law Beth Shapiro. The birds either fly to the left or to the right.

Reversible images are tremendously fun because even though the image doesn’t change, our perceptions of it do. Our brains, therefore, must somehow or other switch the interpretation, even though nothing physically has changed. Vision scientists have asked many questions about reversible images: What causes them to “switch”? Why is it that we typically don’t see both interpretations at the same time? To what extent does the image depend on where we look? What can the perceptual switch tells us about consciousness? etc.

To me, the most striking aspect of the FedEx illusion is that even though people see the image thousands of times, few will notice the arrow until it is pointed out to them. Personally, I did not perceive the arrow until the Society for Neuroscience Annual meeting in Atlanta in 2006 (it was pointed out to me by Eric Altschuler’s son, who must have been about eight years old).

Why should the arrow be so difficult to see? First, there is a question of how we segment the scene. The arrow is made from the same color as the background, which extends behind the letters. It is therefore easy to see the letters as the figure, and the arrow as part of the background. To notice the arrow, we must do the double task of ignoring the letters, and treating the parts between the letters as if they were separate from the background field.

The effect of the background can be seen in the image below. The arrow and the background have different colors, and as a result, the arrow stands out. Notice that the color of the arrow and the color of the area immediately surrounding the arrow have not changed from the original image.

Second, visual images contain a lot of information. In order to sort through that information, the brain seems to form expectations concerning what we are about to encounter in the world. These expectations can greatly influence how we perceive ambiguous images. In this case, we see letters and a logo; therefore, why should the visual system “work” to create an interpretation of an arrow? To see some classic examples of the effects of perceptual expectations, visit this site on “perceptual sets,” which was posted by Saul McLeod as part of Simply Psychology.

Take-home message: our visual system has a number of properties that make it quite good at finding the things we expect to see in the environment. However, if we do not work to find other interpretations, we miss the arrow.
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I am still on the road. My guess is that I will not be able to post again until early August.

Four Squares

2008-06-30T13:28:00.001-04:00

What to notice: The patterns in the four squares are stationary, yet they appear to move.

Brief Comment: This week’s illusion is a motionless image that creates the perception of motion. In this respect, the illusion is similar to the Rainbow Boom/Rainbow Bust illusion that I posted a few weeks ago. People differ in how they see peripheral drift illusions: for some people, the effect is very strong, and others do not see it at all. The effect seems to become less strong as you age.

This week’s illusion is based on a design principle made famous in Akiyoshi Kitaoka’s “rotating snakes.” To make this type of illusion, you start with an individual element made of segments that follow the pattern “dark, bright, less bright, darkest” (or, to put it into numbers, the luminance levels of the segments follow a pattern something like 0.3, 1.0, 0.7, 0.0—there are nuances concerning the luminance ratio and the size of segments). You then repeat the element enough times to form a ring. The effect appears to become stronger if you include multiple rings near the same location.

These patterns are fun to manipulate in programs like Adobe Illustrator and Adobe Photoshop. Here, I have created a pattern and placed the resulting rings into a four-square configuration with bright colors (Kitaoka-like elements in an Andy Warhol-like display with Keith Haring-like colors).

Why does this effect occur? Most explanations of peripheral drift illusions suggest that the illusory patterns fool the eye into sending to the brain information that mimics a real motion signal.

The eye sends information to the brain through different types of neurons. Some of these neurons transmit information faster than others, and some of these neurons respond more quickly to high-contrast parts of an image than to low-contrast parts of the image. Because of these differences, the response of the eye to one part of the illusory pattern reaches the brain at a slightly different time than the response of the eye to another part of the illusory pattern. The difference in the arrival time is exactly the same type of event that would occur with “real” motion, and so motion detectors in the brain signal that motion has occurred.

The details of these events for the peripheral drift illusions have not yet been settled. One explanation is based on the idea that the early visual system responds faster to high-contrast information than it does to low-contrast information. Variations on this idea were put forward by Ben Backus and Ipek Oruc (here is a link to their paper) and by Bevil Conway, Akiyoshi Kitaoka, Arash Yazdanbakhshm, Christopher Pack, and Margaret Livingstone (here is a link to their paper). Another explanation suggests that the motion occurs because the neural systems that respond to positive contrast changes (ON channels) respond faster than the neural systems that respond to negative changes in contrast (OFF channels). This idea was put forward by Maria Del Viva, Monica Gori and David Burr to explain a slightly different illusion (here is a link to their paper).

I will be on the road in California most of July. I will try my best to keep up with the blog while traveling.

Swimmers: a gradient-gradient illusion

2008-06-22T09:30:00.004-04:00

You are looking at a collection of “eggs” that are placed in front of a striped pattern. As the pattern moves from left to right across the screen, the eggs appear to swim up and down, and to change from light to dark.

Press the “Click to toggle background” button to replace the striped pattern with a stationary background. As you can see, the eggs are completely stationary.

The lever on the bottom left of the display allows you to rotate the eggs. Try rotating the eggs by about 45 degrees, and notice how the orientation of the eggs changes the pattern of the motion.

Brief comments: The motion of the eggs is surprising, but it really shouldn’t be. In the real world, we frequently encounter conditions where relative information makes stationary objects appear to move. For instance, when clouds drift by the moon, the moon may look as if it is shifting in the opposite direction (this is called “induced motion”). Also, there are motion aftereffects: if you view motion in one direction and then view a still object, the object will appear to drift in the opposite direction (here is another motion aftereffect).

Motion information from stationary objects can also be created whenever you move your eyes or your head, or whenever you move closer or further away from the stationary object. (Do stationary objects appear stationary when you move your camcorder?)

There are so many sources of motion information in our environment that I often find it surprising that anything ever appears stationary.

The motion in the swimmers illusion seems to arise from the contrast between the eggs and the background. The eggs are shaded light on top and dark on the bottom. When a light portion of the background crosses the egg, the egg appears to move upward; when a dark portion of the background crosses the egg, the egg appears to move downward. Does this type of motion sound familiar? The shift toward the point of low contrast is the same in some illusions in previous posts: the window shade illusion; Lucy in the sky; and the grouping by contrast illusion.

So, why should the contrast motion in the swimmers illusion surprise us? Motion from contrast seems to contradict an object-centered representation of the world. We have many words for objects, but not so many ways to describe the relationship between the objects and their surrounds—I am stuck with “high contrast” and “low contrast.” Perhaps the motion seems surprising because we have an impoverished ability to talk about (and consciously represent) relative stimulus information, compared to our ability to talk about objects.

The swimmers illusion was created when I was investigating effects of combining different sources of gradients in the environment (shadows on shadows). In 2007, this illusion was among the top ten in the third annual Best Illusion of the Year contest and was presented by Emily Knight, then an undergraduate in my lab.

All effects have precedence. The swimmers illusion is, in many respects, a two-dimensional version of Stuart Anstis’s “footstep illusion” (2001). Click here for a .pdf of the original footsteps article, and click here to see a demonstration of the footstep illusion.

Stuart Anstis gives extraordinarily engaging lectures that are both humorous and informative. Here is a link to his lecture entitled “Colours, Faces and Mrs. Thatcher's Bikini,” which was given December 2007 and posted by Cambridge Research Systems. You really should see the lecture. It is an excellent opportunity to watch a maestro at work and to see some absolutely fantastic new illusions.

Curtains: spotlights and shadows

2008-06-15T12:12:00.003-04:00

You are looking at two curtains. At the center of the curtain on the left is a circular region that looks like a shadow; at the center of the curtain on the right is a circular region that looks like a spotlight. As would be expected from our understanding of the real world, the region in the shadow looks darker than the region in the spotlight.

But this is so only when the disks are vertically oriented. Actually, the two disks are physically the same.

Move the slider to rotate the disks. When the disks are turned, they no longer “line up” with the background, and as a result, they no longer appear to be a shadow and spotlight. In this view, the disks appear to be filled with the same gradient pattern (and actually are).

The disks can be moved. You can get a similar effect by clicking on and dragging a disk to place it next to the other, on the same background.

Brief Comments: One of my favorite places on the web is The Situationist, a blog that explores how the “situation” (or context) affects interpretation. The site has numerous examples of how objects, people, and events in one context are interpreted differently from the same objects, people, and events placed in another context.

The visual display above presents an example of the effects of the visual “situation.” In one situation (vertical orientation for the disks), the viewer interprets the disks with reference to the background context (i.e., the two curtains). One disk looks like a shadow on the curtain, and the other looks like a spotlight. The disks are therefore interpreted as a dark spot and a lighter spot on the curtains. In another situation (horizontal orientation), the viewer is able to separate the disks from the context of the curtains and therefore will identify the disks as having the same shading.

From my perspective, the most surprising aspect of this display is that the spots look the same when oriented horizontally. Why should it be surprising that the disks look the same? Consider the relation of this visual display to an old and well known situational effect: “simultaneous contrast.” In a simultaneous contrast display, a gray patch appears brighter when placed against a dark background than when placed against a bright background. In the visual display above, the left curtain is brighter than the right curtain, and so it makes sense that the left disk appears darker than the right disk when the disks are oriented vertically. But when the disks are oriented horizontally, it is as if one situational effect – simultaneous contrast – disappears in the presence of a different situational effect (orientation).

Why does the effect of orientation apparently supersede the effect of simultaneous contrast when we interpret the appearance of the disks? The illusion above is a response to research conducted by Bart Anderson and Jonathan Winawer (here is a link to their 2005 Nature article, and to Bart Anderson’s demonstration webpage). Please read their article to learn the details of their interpretations.

My interpretation differs from theirs. I will write more about interpretations of simultaneous contrast and orientation in future posts.

Visual scientists have argued for more than a century about the causes of simultaneous contrast. If you are curious about this topic, here is a link to a recent book by Alan Gilchrist that gives a history of the research into lightness illusions.

Rainbow Boom/Rainbow Bust

2008-06-07T19:20:00.001-04:00

You are looking at a pattern of rectangles sitting in front of a multicolored background.

For most people, the pattern appears to expand or, if you press the “click to reverse direction” button, to contract. The perception of expansion or contraction (of “rainbow boom” or “rainbow bust”--an economic metaphor) occurs even though the image is completely still.

Comments:

The key to the motion is the contrast between the colored edges on the rectangles and the gradient background. If the yellow edges are on the outside of each rectangle, the pattern appears to expand; if the yellow edges are on the inside, the pattern appears to contract.

The effect is reminiscent of the Pinna-Brelstaff illusion, which consists of diamonds with two white edges and two black edges, in ring formation:

Stare at the blue star, and move your head closer to and further from the screen. As you do so, the rings should appear to rotate.

Unlike motion in the Pinna-Brelstaff illusion, the perceived motion in Rainbow Boom/Rainbow Bust does not depend on the observer moving back and forth. I achieved this effect by placing the rectangular pattern on a gradient background. In this way, even though the rectangles are the same throughout the display, the contrast between the rectangles and the background changes from the center outward. It is the contrast that seems to be driving the perception of expansion and contraction.

What causes the motion? One possibility is that small eye movements lead to changes in the contrast response; the visual system creates motion from the differences in contrast across edges. Another possibility, put forward by Ben Backus and Ipek Oruc, is that the brain’s motion detectors fail to compensate for the dynamics of local adaptation. The Backus-Oruc theory was created to account for the perception of motion in a different static illusion (here is a link to their paper). If it is correct, then it is quite likely that their theory applies to Rainbow Boom/Rainbow Bust as well.

One last note: Akiyoshi Kitaoka has created many wonderful and novel variations on the Pinna and Brelstaff theme. Take a look!

The peripheral escalator illusion

2008-05-31T16:50:00.003-04:00

Here is an illusion that I developed as part of a collaboration with Zhong-Lin Lu (USC) and Emily Knight (then, an undergraduate research assistant in my laboratory at Bucknell). Emily Knight presented this illusion at the 2008 Best illusion of the year contest and as a talk at the Vision Sciences Society 2008 annual meeting (abstract).

You are looking at columns of zebra-like ovals that swing back and forth in front of a diagonally striped background.

1. Look directly at the ovals. The ovals appear to swing horizontally.

2. Focus your gaze several inches above the screen, but pay attention to the ovals as you do so. The ovals should now appear to swing diagonally.

The difference between the two conditions is dramatic. If you don’t see it, try fixing your gaze a few inches higher above the screen.

Comments: If you’ve seen a diagram of a human eye, you know that in the back of the eye is a layer of cells called the retina, and in the center of the retina is an area called the fovea. The retinal area outside the fovea is referred to as the periphery. When you look directly at the ovals in the above display, the image of those ovals falls on the fovea. When you focus your gaze a few inches above your computer monitor, the image of the ovals falls in the visual periphery.

The machinery for foveal vision differs from the machinery for peripheral vision in numerous ways—too many ways, in fact, to list them all in a blog entry. As a result of these differences, we are able to see objects in much more detail when we view them directly (foveally) than when we view them indirectly (peripherally). Our representation of the world, therefore, is much different from the representation in a typical photograph, in which all objects in the same visual plane have the same amount of detail.

The different perceptions of the “peripheral escalator” illusion are partly due to the inability of peripheral vision to see fine detail. You can simulate this aspect of peripheral vision by removing fine detail from the display and then viewing the display foveally. Here are three simple ways to remove fine detail from the display: 1) view the display from about 6 feet away from the monitor; 2) scrunch up your eyes; or 3) if you can't (or can barely) see the words on the computer screen without corrective lenses, take them off and then view the display.

When you remove the fine detail, you will see a weird pattern of stripes that move up and down. This pattern is similar to what people see when they view the display indirectly, but it does not capture the peripheral perception of a diagonal sweeping motion that observers frequently report.

It seems, then, that relative to foveal vision, peripheral vision is missing not only the ability to represent fine detail but also “something else.” The nature of the “something else” is an important question for vision research. One possibility is that peripheral vision is not very good at alignment (in the trade, we say that the visual periphery is poor at representing “phase” information). Indeed, I created the peripheral escalator illusion as part of a search for the perceptual ramifications of the “poor-phase” hypothesis. While the poor-phase hypothesis may be correct, my colleagues and I suspect that central and peripheral vision may also differ in their capacity to segregate individual features. We have designated this possible capacity limitation “feature blur”… stay tuned.

As a final thought, primates differ from other mammals in the way cell types are distributed in the retina (see Masland, 2001, third paragraph). The primate fovea is dominated numerically by a particular group of post-receptoral cells, whereas in the retina of other mammals and in the primate periphery, cell types are distributed more evenly. It may be that most non-primate mammals see the world in a way that is closer to our peripheral vision than to our foveal vision. If this is so, then perhaps lions looking at a pack of moving zebras see something not unlike what we see when we look indirectly at the peripheral escalator illusion.

I will be on the road next week, so I may not be able to post a new illusion until after I return.

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Note 6/3/08 8:25: For your reference, here is a link to a pdf of an excellent earlier study (1992) on “Misdirected visual motion in the peripheral visual field,” by R. Cormack, R. Blake, and E. Hiris (Vision Research, 32, 73-80).

As you will see, the principle is the same, but I think there may be interesting differences.

Here is a link to Randolph Blake’s website at Vanderbilt University. I encourage you to view the visual phenomena on his demonstration page.

Squaring the Diamond

2008-05-24T08:53:00.005-04:00

Look at this image at your usual distance from your computer, and then from across the room. Does the image look different when you see it close up vs. far away? (If you don’t feel like getting out of your chair, you can get the same effect by scrunching up your eyes or, for those who wear glasses, by removing them.)

You should get two different impressions of the image. When you are close and the image is in focus, you should see horizontally striped diamonds on a field of blurry vertically striped diamonds. When you are far away (or if you have scrunched up your eyes or removed your glasses), you should see a field of squares.

Why does this happen? We often take it for granted that we see the world at different scales. Look at the page that you are reading: you can probably perceive fine details (say, letters and words), broader features (like the shape of the paragraph), and large features (the outline of the page).

The amount of information at each of these scales changes as we move about the world. When you are far away from your computer monitor, you can’t make out letters or words because they are too small to see. But as you move closer, the letters and words become bigger, and (voila!) this new information becomes part of your perceptual world.

Our ability to adjust to these changes in size is helped considerably because, in the real world, objects tend to scale together: zoom out, you see the page; zoom in and you see words on the page. As you zoom, a word on the page gets bigger or smaller at the same rate as the other words.

“Squaring the Diamond” is compelling (at least to me) because it seems to violate this basic assumption about how objects scale in the physical world.

The display is composed of blurry diamonds and regular (a/k/a non-blurry) diamonds. When you zoom out, the diamonds combine to form a field of squares; but when you zoom in, you can see detail only in the diamonds that have not been blurred. Unlike the non-blurry diamonds and the words on this page, the blurry diamonds do not gain more detail as you move toward them because the blurry diamonds do not contain fine detail (in vision science circles, “fine detail” is known as high spatial frequency information). The result is that your perception of the field of squares breaks apart when you are near enough to see the fine detail in the non-blurry diamonds.

The buttons in the display allow you to blur and unblur the sets of diamonds. Click on the buttons, and see what you can find out about the image. Notice that you see squares when the horizontal and vertical diamonds are both blurred, and when both are unblurred.

The principle is similar to Schyns and Oliva’s hybrid images (Aude Oliva’s gallery of hybrid images is really worth seeing; here is a link to an article about one of their hybrid images: Dr. Angry and Mr. Smile).

Grouping by contrast

2008-05-19T08:33:00.002-04:00

Here is an illusion from a brief article that I wrote with Kai Hamburger (“Grouping by contrast—figure-ground segregation is not necessarily fundamental,” Perception, 2007).

The five disks in the top row of the display all change gradually from black to gray to white and then abruptly back to black; in the bottom row, the five disks all change gradually from white to gray to black and then abruptly back to white.

Click on the “add/remove gradient” button to place gradient rectangles behind the disks. The switch in background from solid to gradient creates a dramatic change in the way the disks are perceived. The disks themselves have not changed, but now motion appears to sweep from right to left across each row. The perceived motion is similar – in some respects – to the perceived motion in the window shade illusion, since it follows the location of minimum contrast between the disks and the background.

There are many features about this shift in perception that we could talk about, but I would like to draw attention to what I think the display says about how we organize visual information. Notice that when the solid background is present, you tend to group the disks into two horizontal rows of five, but when the gradient backgrounds are present, you tend to track the motion and group the disks into five vertical columns of two disks each (so far, all viewers have automatically grouped the disks in these ways).

This shift in perceptual grouping is really unusual. When you group the disks into columns, you pair a white disk in one row with a black disk in the other row even though each of these disks has the same luminance level as the four other disks in its row. A horizontal grouping would seem like a reasonable way to organize the disks because – just like in the contrast asynchrony – the disks in each row become light and dark at the same time. However, it seems that the visual system prefers to group vertically because the disks in each column have similar contrast levels relative to their respective backgrounds.

In the article that I link to above, Kai Hamburger and I suggest that contrast-based grouping poses a bit of a puzzle. The Gestalt approach to visual perception proposes that the visual system organizes the world into simple perceptual units in accordance with well-known Gestalt laws (for example, similarity, symmetry, proximity, closure, common fate). Central to the Gestalt approach is the idea that the visual system organizes the world in terms of “Figure” and “Ground” (for an example, see Rubin’s face/vase illusion). Contrast information, however, cannot really be considered part of the Figure perceptual unit or the Ground perceptual unit because the contrast information cuts across the figure/ground border (the contrast information represents the luminance levels of the disks relative to the luminance levels of the background).

The illusion in this post illustrates a condition in which the visual system privileges contrast information over object similarity in order to organize the visual scene. At some level, this should not be surprising; a visual scene can be described in terms of a variety of stimulus dimensions (spatial scale, luminance, contrast, temporal changes, chromaticity, etc.). The visual system contains parallel neural channels, each of which responds to only a small range in a few of these dimensions. Presumably, the neural processes that organize the visual scene must do so by selecting a sub-population of the neural channels. However, since we generally think about the world in terms of objects that remain relatively stable regardless of context, it can be surprising to see the effect that contrast information can have on how we organize the visual scene.

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I was on the road last week, attending the Vision Sciences Society conference (sorry for the gap in the blog). In my last post, I mentioned the then upcoming, now past, best illusion of the year contest. Incredibly interesting new illusions were presented, and the event was a great success. My lab had two entries in the top ten, but (alas) our entries did not place 1st, 2nd, or 3rd this year. You can see the winning entries at this link. I also encourage you to visit the website of the major sponsor for the event: the Mind Science Foundation. They have many resources regarding consciousness that may be of interest to readers of this blog.

Next week: an illusion of spatial scale.

Perpetual collision illusion

2008-05-05T01:05:00.003-04:00

Here is a sneak preview of one the illusions that my laboratory will be presenting at the Neural Correlate Society’s “Best illusion of the year” contest that will be held next week (May 11th ) in Naples, Florida. The illusion was devised to investigate questions related to contrast and visual grouping (see article).

Warning: the illusion involves rotating, high-contrast diamonds. If you are prone to migraines or epilepsy, or get motion sickness, please do not stare at this illusion.

[The illusion is a flash file and will not appear in an RSS feed]

Description: You are looking at columns of pink and yellow diamonds separated by columns of spinning black/white/gray diamonds. The pink diamonds appear to move to the right; the yellow diamonds appear to move to the left.

There are two main things to notice about the display:

1. The pink and yellow columns are not really moving. Don’t believe me? Click and drag the spinning black/white/gray diamonds to move them out the way. When you do, you will see that the spinning diamonds are placed on top of a completely stationary colored background.

2. The motion is perpetual. The pink and yellow fields seem always to be headed towards (or away from) each other, but they never meet (and they never grow farther apart). This aspect of the effect can be quite mesmerizing, so be careful.

The motion originates from the edges between the spinning diamonds and the colored fields. The edges of the diamonds are tilted at -45 or 45 deg; the motion, therefore, should always shift in an oblique direction. To get a better handle on this, click the “add/remove diagonal bars” button. The diagonal bars cover up opposite sides of the rotating diamonds so that only every other edge is shown. When the diagonal bars are present, the pink and yellow fields move diagonally.

Why, then, should the pink and yellow fields appear to move horizontally when the diagonal bars are not present? Not to be too technical, but it seems to me that either the visual system is computing motion for the colored diamonds from a vector sum of the motion at the edges; or the visual system is using the information at the edges to define an object (in this case, a diamond), and motion for the object takes precedence over the motion that originates at the edge.

I have also included a button that allows you to “add/remove horizontal bars.” The horizontal bars stretch across the image so that the colored diamonds turn into colored triangles. Nonetheless, instead of seeing individual triangular segments, you perceive the image as a series of colored diamonds that appear to move behind a bar. It is as if the visual system joins the triangles to form the diamonds, so that you perceive a "whole" object.

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I will be on the road next week (traveling to the conference). I will try to post a new illusion on Tuesday the 13th or Wednesday the 14th.

Lucy in the Sky

2008-04-28T07:00:00.006-04:00

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.

Window Shade/Rocking Disk Illusion

2008-04-16T08:45:00.001-04:00

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.

Why a blog about illusions?

2008-02-23T08:32:00.001-05:00

Welcome to the illusionsciences blog. I decided to start this blog after seeing the response to Dave Munger's Cognitive Daily story about a recent article of mine in the Journal of Vision. Dave's post generated a remarkable amount of interest (well, at least it was remarkable to me) because he included the following "visual illusion" from my article:

The popularity of his story made me think that the time might be right for blog in which I discuss some of my own illusions and some of the illusions that others have developed in the vision sciences community. I hope that the blog will serve as a forum where people both inside and outside the field of visual science can discuss visual phenomena.

Stay tuned. I will start posting regularly in May--once the semester at Bucknell comes to a close. I hope to post an illusion and commentary once a week.