## Cheshire Cat Illusions

posted on 8 May 2013 by guy
last changed 3 Jun 2015

 5 Average: 5 (2 votes) Select ratinghated itdidn't like itliked itreally liked itloved itCancel rating Your vote (click to rate)ages: 10 to 99 yrs budget: $0.00 to$0.00prep time: 0 to 0 min class time: 5 to 30 min Cheshire cat illusions are images that fade slowly away as you stare at them. They result from desensitization in the eye (and brain) when you view them without moving your eyes. This lesson explains the mechanism behind the disappearance, and provides several examples of optimized images. Images are attached in lecture slides for easy presentation.For more examples of optical illusions caused by visual desensitization, check out our summary curriculum on After Images and Optical Illusions.
subjects: Biology, Psychology
keywords: cheshire cat, Troxler, desensitization, negative, afterimage, blindness

## CheshireCat.pdf

Fig 1: A low-contrast fuzzy image optimized for the Troxler effect. Click on the thumbnail to bring up the full-sized image, then stare at the X in the center without moving your eyes. The background image should start to fade away after a few seconds.

Fig. 2: Another Troxler illusion. Click on the thumbnail to bring up a full-size image. Close your left eye and stare at the X without moving your right eye. The disk at the left should fade.

Fig. 3: Not the Troxler effect. This disk should not fade, no matter how long you spend staring at the X. Small involuntary tremors in your eye guarrantee that the photosensitive cells in your retina near the boundary of the disk do not desensitize.

Fig. 4: The Cheshire cat illusion. Click on the thumbnail to bring up a full-size image. Stare at the X without moving your eyes until the cat fades away, leaving only the smile.

## Troxler's effect

In 1804, a Swiss physician by the name of Ignaz Paul Vital Troxler observed that stationary images in the periphery of the field of view can sometimes fade away.1 The phenomenon is now known as Troxler's effect.

Troxler's effect is a result of "desensitization" (a.k.a. "adaptation"), which occurs primarily in the retina.2 Certain cells in the retina contain a photosensitive chemical constructed from two components: retinaldehyde (a.k.a. retinal, derived from vitamin A) and a protein (either opsin or photopsin). When this chemical absorbs light, it generates an electrical signal that carries visual information to the brain, and then becomes unstable and falls apart into its original components. During this process, some of the photosensitive chemical in the cell is used up, and the cell is temporarily less sensitive to light. Eventually, enzymes in the eye reconstitute the chemical from its components. In this way, photosensitive chemicals are constantly being broken down by the absorption of light and reconstituted by enzyme action.

The temporary desensitization of the retinal cells is responsible for the negative afterimages we see when we stare at an image for a while and then shift our gaze to a uniform white background. (See or lesson on Yang and Yin - Negative Afterimages for more details.) The same desensitization is responsible (at least in part) for Troxler's effect. When an image is projected on the retina, cells in the bright portions of the image desensitize more severely, so that those portions appear relatively dimmer. After enough time has passed (usually a few seconds), all portions of the image appear uniformly bright.

The effect also works in color, so that blue portions of the image appear less blue, green portions less green and red portions less red. Eventually, the entire visual field becomes a uniform grey. (See our lesson on Color Perception for more details of the color-specific processes in the retina.)

## eye movements

We don't generally notice Troxler's effect in everyday life, and the reason is that our eyes are always moving. A cell may start to become desensitized while a bright portion of an image is projected onto it, but in the next instant a dark portion may cover it and it will start to resensitize. As our eyes move around to take in everything around us, the photosensitive cells achieve an average sensitivity. All the cells in the retina become equally sensitive, roughly speaking.

Even when we think we are keeping our eyes fixed, there are small involuntary movements, called tremors, that are always taking place. These tremors are very small rapid jittery motions, typically about a quarter of a minute of arc at a frequency of about 50 times a second.

Eye tremors make it so that different regions in an image do not easily succumb to Troxler's effect. During a tremor, if a cell in a bright region crosses over to a dark region of the image, the dark region will appear even darker because the cell has been previously desensitized. Similarly, if in a dark region crosses over to a bright region, the bright region will appear even brighter than normal to that highly sensitive cell. In this way, cells moving around among different regions of light and dark, or among different colors, retain an average sensitivity.

Overcoming the effect of tremors is challenging. Historically, researchers in Troxler's effect have used a couple of methods to present a static image to the retina. One is to mount a miniature projector on a contact lens, so that when the eye moves, the lens and the projector move with it. Another is to track movements of the eye by looking at its reflections, and use a computer to adjust how the image is presented, usually by adjusting the orientation of mirrors.

## cheshire cat images

Still another way to get arround the effect of tremors in Troxler's effect is to use an image with low contrast and no sharp edges. That way, even if each retinal cell wanders over a small region of the image, it will see little variation in the amount of light shining on it, and will still desensitize appropriately.

Troxler's fading is strongest in the periphery of the retina because the visual area of the receptor cells there is larger than for receptor cells in the center of the retina. As long as the eye tremors are small compared to the field of view of the retinal cells, they will do little to prevent Troxler's fading.

Figure 1 shows an image contrived to enhance the Troxler effect. Click on the thumbnail to bring up a full-size image and stare at the X in the center. Keep your eye focused on the X without moving (this may be quite hard to do, but practice helps). After several seconds, the periphery should start to fade away. As soon as you move your eyes, the image should reappear.

You can appreciate the importance of using images with fuzzy boundaries by comparing figures 2 and 3. For these figures, close your left eye and stare directly at the center of the X with your right eye. (Using your right eye prevents your blind spot from getting in the way.) The fuzzy disk in figure 2 should disappear fairly easily, but the sharp-edged disk in figure 3 should remain visible. Eye tremors are constantly moving retinal cells back and forth across the sharp edge of the disk so that they never get a chance to fully desensitize.

After you've practiced with the preceding images, you can try your eye at the true Cheshire cat image in figure 4. In this image, the smile contains higher contrast and finer resolution than the rest of the image. If you fixate on the nose, everything in the periphery should slowly disappear, with the smile fading last of all.

## viewing tips

Troxler's effect depends in large part on reducing contrast. If you're looking at the images on a computer screen, make sure your line of sight is perpendicular to the screen. Many screens change color and contrast dramatically as the screen is tilted.

It's also important to make the image as big as possible. The larger the image, the smaller the effect of eye motion. Either project the image on a big screen, or if you're stuck with a small display, view it from close range.

## teaching notes

This lesson works best after students have learned a little about the structure of the eye (see our lesson on Anatomy of the Eye for ideas). It goes hand in hand with a discussion of negative afterimages, which are also caused by desensitization in the retina. See our lesson on Yang and Yin - negative afterimages for examples.

## further resources

Susan Martinez-Conde, Stephen L. Macknik and David H. Hubel have written a nice review of the role of eye movements in visual perception in "The Role of Fixational Eye Movements in Visual Perception." Nature Reviews - Neuroscience 5 (March 2004) 229-240.

• 1. Troxler, D. (1804). "Über das Verschwinden gegebener Gegenstände innerhalb unseres Gesichtskreises" [On the disappearance of given objects from our visual field]. In Himly, K.; Schmidt, J.A. Ophthalmologische Bibliothek 2 (2): 1–53.
• 2. There is evidence that Troxler's effect is generated by desensitization processes in the brain as well as in the retina. See, for example: Clarke, F. J. J. A study of Troxler’s effect. Optica Acta 7, 219–236 (1960).