![]() 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. 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.Įye tremors make it so that different regions in an image do not easily succumb to Troxler's effect. All the cells in the retina become equally sensitive, roughly speaking.Įven when we think we are keeping our eyes fixed, there are small involuntary movements, called tremors, that are always taking place. As our eyes move around to take in everything around us, the photosensitive cells achieve an average sensitivity. 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. We don't generally notice Troxler's effect in everyday life, and the reason is that our eyes are always moving. (See our lesson on Color Perception for more details of the color-specific processes in the retina.) eye movements Eventually, the entire visual field becomes a uniform grey. 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. After enough time has passed (usually a few seconds), all portions of the image appear uniformly bright. 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. (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. 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. In this way, photosensitive chemicals are constantly being broken down by the absorption of light and reconstituted by enzyme action. Eventually, enzymes in the eye reconstitute the chemical from its components. During this process, some of the photosensitive chemical in the cell is used up, and the cell is temporarily less sensitive to light. 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. retinal, derived from vitamin A) and a protein (either opsin or photopsin). 2 Certain cells in the retina contain a photosensitive chemical constructed from two components: retinaldehyde (a.k.a. "adaptation"), which occurs primarily in the retina. Troxler's effect is a result of "desensitization" (a.k.a. 1 The phenomenon is now known as 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. Stare at the X without moving your eyes until the cat fades away, leaving only the smile. ![]() Click on the thumbnail to bring up a full-size image.
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