Experimenting with Adelson’s Checkerboard Illusion

Can you change how things seem? Phenomenological slideshow test below! Scroll past all the text if you must!

Edward H. Adelson‘s (1995) Checkerboard Illusion, or “Checkershadow Illusion“, remains one of the best lessons on how deceptive our perceptions can be, and it affords a great opportunity for phenomenological experimentation, which I promote with a slideshow below. Above is an inspired variation on Adelson’s original by a digital artist who goes by butisit at DeviantArt.

If you have never seen Adelson’s original image before, look at it now (here below), and ask yourself, “Is the parallelogram region of this image labeled A darker than the parallelogram region labeled B?” If your answer is Yes, you are incorrect. If you insist, if you claim that region A is obviously darker than region B, you are still incorrect. If you think this means merely that A is not as much darker than B as it seems to be, then you are nevertheless incorrect. Regions A and B are the same exact shade. The same holds for butisit’s version above.

Any newcomer to this illusion who rejects the claim that region A is no darker than region B is justified—at least initially—in doing so. For, given that you are able to view the image and to read these words, you are relying on a perceptual system that has served you sufficiently well—perhaps better than anything else—when it comes to information about shades in the external world. (Most of us don’t regularly walk around with photometers.) So far, you have no reason to give less weight to your perception that A is darker than B than to my assurance that it isn’t. Nevertheless:

Remember this feeling you have of high certainty. Remember the strength of the trust you are putting in your perceptions. For you are about to lose that confidence. Keep this in mind on future occasions when your perceptual confidence is as high as it is now. You now have the opportunity to recalibrate that confidence. You may even have the opportunity retrain your perceptual system.

Initial Considerations

We tend to take Adelson’s image as depicting a mint (seafoam?) green cylinder sitting on, and casting a shadow over, a 5×5 checkerboard of dark and light grayscale tiles. We know, of course, that we are not actually looking at such an arrangement of things. There is no real cylinder, board, or tiles, and there is no real light source causing a cylinder to cast a shadow across them. What appears to be the bottom right corner of a checkerboard is not necessarily closer to us than what appears to be the the top left corner. We view only a 2D image, varied illuminated patches on the screen of our computer or smartphone, and yet our visual system somehow converts the image into a phenomenologically 3D experience.

This much the modern viewer of realistic images understands even in early childhood. We are accustomed to this disparity between appearance and reality. Notably:

We can simultaneously believe with virtual certainty that what we see is in fact 2D and admit with fitting humility that how it appears is 3D, often stubbornly so.

Perhaps, seeing is not believing.

Let’s return now to the parallelogram regions A and B. One might think that while the illusion of a 3D scene persists, we can see that there is no such 3D scene. Thus, one might argue, the appearance of a 3D scene is not like the appearance of A’s being darker than B. In other words:

One can sincerely say, “It looks like a 3D scene, but I can clearly see that it isn’t really a 3D scene.”

But one cannot so easily say, “It looks like A is darker than B, but I can clearly see that A is not darker than B.”

So again, if you remain unconvinced that region A is truly no darker than region B, your confidence may yet be justified.

However, the slideshow in the next section allows one to demonstrate to oneself that region A is no darker than region B. By incrementally occluding the original image, the slideshow diminishes and removes contextual cues, regarding the appearance of a shadow, of contrast, and of uniformity of tiles, until at some point in the series, you will see that regions A and B are “obviously” the same shade. (Although once you reach this point you may have reached such a lowered opinion of your ability to compare shades that you doubt even that conclusion.)

Phenomenological Exercises

As you progress through the series in the slideshow below using the ‘>’ button, ask yourself when exactly you become convinced that regions A and B are the same shade. Note which slide that is. See then if you can train yourself to see A and B as the same shade earlier in the progression. Note how far you get. Try moving backward from the end to see when exactly the illusion reasserts itself. Count toggles between two slides: the slide in which A and B appear to have the same shade and the slide in which they do not. Assuming at some point the apparent difference in shade goes away, note how many times you toggled between these two slides before it did. Note at which point, if any, the illusion stubbornly persists despite these exercises.

If, by the way, you suspect trickery on my part, that is if you think I might have incrementally altered the tile shades one slide at a time, you can use the buttons below to jump from any of the first slides to any of the last slides.

Checkerboard Illusion Slideshow

Questions Only You Can Answer

Are you able to alter your phenomenological experience in any way? Can you overcome the illusion that region A is darker than region B in any of the early slides? Can you regain that illusion, then dispel it again? How much control do you have over how this image seems to you? If you tracked numbers as I suggested above, what are they? Can you improve them?

If you can summon and dispel the illusion, are you aware of any difference (intended or not) in how you interpret the contextual cues? For example, we normally take the parallelograms to indicate a flat surface seen at an oblique angle, but does that change as you dispel the illusion? Is that change intended?

Interpreting The Phenomenological Experiments

Assuming you have been able to note a difference in how the image seems, how should we interpret that event?

For me there is a slide (not always the same one) for which my (to use a neutral term) sense that the regions are the same shade wavers: they sometimes seem the same, and other times don’t seem the same. This sense of wavering is not unlike what we experience when looking at the Necker Cube. 

Necker Cube, introduced as an optical illusion by Swiss crystallographer Louis Albert Necker in 1832

This image may appear as a transparent/wireframe cube placed to below you to your right or as a transparent/wireframe cube placed above you to your left. (You may even see it as a 2D object resembling a tilted shield with a diamond center.)

Assuming (or at least preferring) that you have dutifully carried out the phenomenological exercises above, I will leave you to ponder the following question:

In both the Checkerboard and Necker Cube cases, when we have the impression that “the way an image looks changes,” what exactly is changing?

Before answering, consider two points I’d push further, time and room permitting.

First, a purely physical explanation will remain incomplete unless it explains how the events you have just witnessed are purely physical. And a partially physical explanation will ultimately need to bridge the Explanatory Gap between physical explanation and undeniable phenomenological conscious experience.

Second, everything neuroscience tells us strongly suggests that you could have experiences such as these without the existence of the physical image. Presently, the physical image on the screen of whatever device you’re using stimulates your visual systems and consequently (somehow) you have these experiences. But had your visual systems been stimulated in that way by other means (as in The Matrix) you would have the same experiences. Thus, unless this argument fails, whatever it is exactly that changes, those changes cannot be explained in terms of changes in your relation to the physical image. After all, there may be no physical image to which you can change your relation.

Taking these two points together, I suspect a complete explanation will require a level of phenomenological sophistication that has been rare in contemporary analytic philosophy and cognitive neuroscience.