Troxler's Fading: An Early Optical Illusion

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 19^th 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