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Troxler's Fading: An Early Optical Illusion

Posted May 22, 2017 12:00 AM by Hannes
Pathfinder Tags: Optical Illusion Troxler

This blog has occasionally focused on optical illusions and impossible constructs—here’s Part 1 and Part 2 of the impossible object series. This post will take a look at one of the earliest documented cognitive optical illusions, Troxler’s fading.

Ignaz Paul Vital Troxler (1780-1866) was a Swiss physician, philosopher and politician who worked in those capacities in Lucerne, Aarau, Basel and Berne. In 1804 Troxler noticed that when he focused his vision on a central object for a long period, visuals in his peripheral field faded and disappeared. He published his findings that same year in Ophthalmologische Bibliothek, the first ever ophthalmological journal.

Troxler’s fading exploits the adaptability of the brain’s sensory systems. In short, neurons strongly respond to a novel stimulus but become less responsive when the stimulus is deemed constant or static. To demonstrate the specific Troxler effect, a subject is instructed to focus on a central point in an image. Light reflected from a stimulus enters the eye and an inverted image is projected onto the retina. The retinal cells gradually decrease their sensitivity to the static stimulus peripheral to the point of focus and transmit information to the lateral geniculate nucleus (LGN), the area of the brain responsible for perceiving peripheral stimuli away from a fixation point. The LGN aids in the process of neural adaptation that causes a decrease in response to the constant stimulus. Finally, the LGN relays the visual image to the primary visual cortex, which fills in the area behind the disappeared stimulus. You can test it yourself by focusing hard on the dot in the image or the cross in the video.

A related tactile effect can be tested by placing a hand on a wall or other surface. When a person’s hand first touches the wall, the surface is felt most acutely, but as the hand is left in contact for a longer period the sensation fades.

On a biographical note, Troxler is all but forgotten except in regards to the optical illusion he documented, but he made some waves in early 19th century Europe. His 1828 magnum opus Naturlehre des menschlichen Erkennens oder Metaphysik [The nature of human knowledge or metaphysics] reflected the primitive state of vitalistic science at the time, but it also contains some surprisingly forward-looking psychological concepts of perception, consciousness and the nature of the self. He was also one of the first doctors to advocate for mass vaccination and federal oversight of physician exams.

All that and the disappearing dots, too.

Image credit: Mysid / CC BY SA 3.0

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#1

Re: Troxler's Fading: An Early Optical Illusion

05/22/2017 10:41 AM

It doesn't seem that surprising that staring at the blue circle fatigues the blue receptors in the eye. If you stare at a white surface, you will see a yellow circle, the complement of blue (white minus blue). If you continue staring at the blue circle, the yellow and blue combine to produce white (another way of looking at the blue receptors being fatigued), causing the blue circle to disappear.

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Re: Troxler's Fading: An Early Optical Illusion

05/23/2017 10:39 AM

It explains something I have noticed myself.

It's like the old wet film colour negative; you stare intently at it for a minute, and when you look away you temporarily see the colour of the print.

Or from watching TV credits that go on for ages. When the scrolling stops, the static screen and the whole TV set appears to be moving.

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Re: Troxler's Fading: An Early Optical Illusion

05/24/2017 5:10 PM

I've noticed the same thing myself after driving for hours, that when I stop it appears that objects are receding. It's called the "waterfall effect".

https://en.wikipedia.org/wiki/Motion_aftereffect

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Re: Troxler's Fading: An Early Optical Illusion

05/23/2017 12:10 PM

Is something similar occurring when, after inspecting steel surfaces for blemishes in a "green room" and the normal lighting is returned, the world is "rose colored" for a while?

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