## Tuesday, December 16, 2008

### Rotating Reversals

What to notice: You are looking at two spinning rings. When you look at the yellow dot in the center of the spinning ring on the right, the rings spin toward each other; when you look at the red dot in the center of the spinning ring on the left, the rings spin away from each other.

What is happening? The rings are made up of two components:

1) Six ovals that rotate in one direction

2) Lines inside the ovals that rotate in the opposite direction

When you look directly at the display, you perceive the rotation of the ovals.
When you look toward the red dot or the yellow dot, you perceive the rotation of the internal lines in the ring that is further away from the dot.

Comments: Many illusions “work” because they pit two sources of information against each other (look at the first illusion in this blog for an example). In the rotating reversals demonstration above, the global motion of the ovals is pitted against the internal motion of the lines. To see what I mean, let’s take a look at one ring by itself in the demonstration below.

There are two sources of information.

The global motion rotates counter-clockwise; the internal motion rotates clockwise.

Your visual system has to “choose” how to perceive these conflicting sources of information. In other words, will perception be guided by the motion of the ovals? Or by the motion of the internal lines? Or by a combination of these two? Or will you be able to see both types of motion at the same time, while keeping their signals separate?

When you look directly at the one-ring display, you can discern both sources of information (the ring will spin one way, and the motion caused by the internal lines goes the other way). But when you look at this display peripherally, it becomes difficult to separate the two sources of information, and the internal motion drives the perceived direction of the ring.

(To look at the display with your peripheral vision, focus your eyes on a spot a few inches above the display.)

In the two-ring display, I simply flipped one of the rings so that there would be a conflict in perceived direction of motion when you focus on one dot or the other.

Why is there a difference when you view the display foveally (i.e., directly) and when you view the display peripherally?
The foveal visual system is quite different from the peripheral visual system. We know, for instance, that the world looks blurrier to the peripheral visual system than to the foveal visual system (vision scientists would phrase this as, “The peripheral visual system has poorer spatial resolution than the foveal visual system”). But a blurry peripheral perception alone does not seem to explain why the disks appear to reverse direction. If you blur the display and then look at it in the fovea, the rings do not seem to reverse motion. Well, at least to me and others who have done this experiment in my laboratory, they don’t. I’ll be interested to hear your comments on this topic.

In the Shapiro, Knight, and Lu talk at the recent Society for Neuroscience conference (Nov. 2008), we hypothesized that the machinery of the foveal visual system allows us to represent multiple features simultaneously, but this machinery is absent in the periphery. The peripheral visual system seems to mix up the features that are available in the scene. We called this “feature blur,” and we showed a number of illusions that are consistent with this hypothesis.

At first, the “feature blur” hypothesis may seem counter-intuitive: when you focus on one point, the features in the periphery don’t often appear to jumble together. I think that the reason that some, but not all, displays show strong feature blur is that the effect depends greatly on the contrast with the background. To see this, move the lever in the single-ring display to adjust the background luminance. When the background is brighter or darker than the luminance inside the ovals, the ring no longer reverses when you focus on a spot a few inches above the dot.

Peter Meilstrup, at the University of Washington, points out that the spinning rings also juxtapose motion over different scales. The brain can register “short-range” motion (i.e., motion over a small region) and “long-range” motion (i.e., motion over a large region). Changing the luminance of the background also changes the relative responses to short-range and long-range motion. When the background is gray, the short-range motion signal is strong, but when the background is black or white, the short-range motion signal is weak. As a result, processes that respond to long-range motion energy may predominate against a white or black background, but not against a gray background.

Here are two references (courtesy of Peter) that examine the juxtaposition of long-range and short-range motion processes:

G. Mather, P. Cavanagh, and S. M. Anstis (1985). A moving display which opposes short-range and long-range signals. Perception, 14(2): 163–166.

C. Chubb and G. Sperling (1989). Two motion perception mechanisms revealed through distance-driven reversal of apparent motion. Proc Natl Acad Sci U S A, 86(8): 2985–2989.

* * * * * *

History of the illusion: As indicated on the displays, the illusion has been developed independently in two laboratories, and was presented by both laboratories at the Society for Neuroscience conference in Washington, D.C., in November 2008.

I developed the effect as an extension of the illusions that Emily Knight, Zhong Lin Lu, and I presented at the May 2008 Best illusion of the year contest (here is a link to the pdf of the entry) and as an extension of our work on “feature blur” in the visual periphery.

Peter Meilstrup and Mike Shadlen presented their version of the illusion as part of a continuation of Shadlen and Movshon’s work on motion signals in the brain (specifically, in area MT of the visual cortex). Here is a link to Professor Shadlen’s webpage.

* * * * * *

Here are code snippets for you to post the illusion on your blog or website, if you would like.

Two rings demonstration

One ring demonstration

* * * * * *
Once again, I feel a need to apologize for the long delay between posts. I have changed the tag line on my blog to read “An illusion whenever I can get around to it.” I find it quite difficult to manage the blog while the semester is in progress.

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Thursday, Dec. 18, 12:15
The site was running up against a google-pages bandwidth limitation and so people who visited were not able to see the illusion. I moved the illusion files to a different host site--that should fix the problem for now.

## Thursday, September 4, 2008

### Four Bars: wiggle wiggle

What to notice:
The four center bars are always vertical and straight, and they do not physically change, but the bars appear to wiggle as the surround rotates.

You can slide the lever to change the spacing between light and dark in the background circle. The different-sized bars make the bars wiggle differently. You can also press the button to see what the four bars look like when the background circle is not present.

Brief Comments: As I have said before, many illusions capture our attention because they violate our expectations about how objects behave in the world. In the real world, straight bars don’t typically wiggle when the background changes, but here they do. (Well, I suppose you could argue that a stick appears to bend when you put it into water, but that has an entirely different cause than this illusion).

In many respects, the effect is similar to the Fraser illusions that I wrote about in my last post. The local information (the contrast between the bars and background) indicates that the bars wiggle; the global information (the bars themselves) tells you that the bars are straight. Here, though, the effect seems to be due not only to contrast but also to brightness changes induced into the bars from the surrounding field. (This is a type of grating induction—you can find some excellent research on grating induction at the webpages of Barbara Blakeslee and Mark McCourt at North Dakota State University.)

We seem to expect the global information (i.e., the information about the bars) to be correct and invariant even though the local contrast is equally real. To explore the effect of the contrast, I have included another version of the illusion that allows you to spin the background at your own speed. I find it most compelling to move the background between -45 deg and 45 deg.

I have been a little neglectful of the blog for the past few weeks—sorry. I was out of town, and classes have started.

## Wednesday, August 20, 2008

### 100th anniversary of “A New Visual Illusion of Direction”

What to notice: The letters in the word “LIFE” appear to tilt left and right. The letters are actually vertical, even though they are made up of little tilted line segments.

Press the button to put red vertical lines on the display. This way you can convince yourself that the letters are indeed aligned.

Brief Comment: The image is my reconstruction of Figure 1 from “A New Visual Illusion of Direction,” written by James Fraser in 1908.

The 100th anniversary of Fraser’s paper is worth commemorating. Many of the illustrations in the paper—like the one above—are a staple in books on illusions.

Fraser worked with two strands of fiber, one black and one white. When the two fibers are twisted together, the resulting cord looks like a series of black and white line segments, all inclined at a similar angle. Fraser referred to the line segments as “units of direction” that could make lines appear to tilt one way or the other (see the image above, for example).

The “units of direction” can also make a collection of circles appear as a spiral. In the example below (from the original paper), the image looks like a spiral, but if you click on the button to place red lines on top of the twisted cords, you can see that the image is composed of individual twisted cords that form circles.

The important point is that the twisted cords can be thought of as a global object (i.e., lines and letters) composed of local features (the line segments). The illusion occurs because the visual system receives different stories from these two sources of information: in the word “LIFE,” the global object (a series of lines) says straight, while the local features (line segments) say tilt. The visual system must create a reasonable percept from the conflicting stories.

There have been many studies that have examined how a global percept is influenced by local features. One of the most revealing is Michael Morgan and Bernard Moulden’s 1986 paper, “The Munsterburg Illusion and twisted cords,” published in Vision Research. Morgan and Moulden digitally filtered a twisted cord image to produce a new image in which the tilt of the line is physically present. That is, if you remove some of the information from the original image, you can measure the tilt in the new image with a ruler.

In some ways, then, twisted cord displays disagree with “reality” only if we are tied to the idea that the line (or the circles) are what is important for vision. If you want to read more about this, I strongly recommend Michael Morgan’s chapter on visual illusions in the book Unsolved Mysteries of the Mind, edited by Vicki Bruce (here is a Google sample from the chapter).

A complete pdf of Fraser’s original paper can be downloaded at this link (the pdf file is 3.5 Mb).

## Thursday, August 7, 2008

### Thin lines can stop the perception of "winking"

Here is an illusion from Shapiro, Charles, and Shear-Heyman, “Visual illusions based on single-field contrast asynchronies”

What to notice: You are looking at two rectangles that change from dark blue to bright yellow and back again. The colors of the rectangles are always identical to each other; that is, they blink together "yellow-blue-yellow-blue, etc." But the rectangles look as if they are not modulating together; rather, they appear to "wink" asynchronously.

If you wish to convince yourself that the two rectangles have the same color, click on and drag one rectangle to place it next to the other rectangle.

Click on the button to add thin bars on the top and bottom of the rectangles. When the bars are present, the perception of alternation disappears—the rectangles appear to modulate together.

Comments: In my first post on this blog, I presented an illusion similar to this one (the contrast asynchrony), so—you may be wondering—how is this illusion different from that one? The important point from this post is to show how little it takes to kill the perception of alternation.

The rectangles appear to modulate in phase as soon as the thin bars are added to the top and bottom of the rectangles. The lever on the side of the display allows you to adjust the length of the thin bars. See how small you can make the bars and still have the rectangles appear to modulate together. In the experiments conducted in my lab (link to abstract), the rectangles always appeared to modulate together once the bars were greater than 10 minutes of visual angle, with some effect occurring at 1 minute of visual angle (that is pretty small).

The illusion “works” because it juxtaposes two sources of information: the color of the rectangles, and the color contrast of the rectangles relative to the background. The color information modulates together, but the color contrast alternates. The effect is surprising because we don’t always take into account that our visual system can process both color and color contrast. The fact that thin edges can effectively shut down the perception of contrast is curious and will be the subject of some future posts.

Lastly, I would like to point out that the contrast asynchrony display is fundamentally different from “dynamic simultaneous contrast” displays, where the appearance of the center patch changes due to induction from the surround field. I will leave this as an open topic to return to later, but I discuss the differences between the two types of displays in the introduction to my 2008 paper “Separating color from color contrast.”

## Wednesday, July 16, 2008

### The FedEx Arrow

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.

## Monday, June 30, 2008

### Four Squares

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.