What do perceptual illusions mean

If the impression is wrong

Sometimes the world is not what it seems: Optical illusions leave us with impressions that often have nothing to do with reality. Its exploration is in full swing - and teaches us a lot about the way we see.

Scientific supervision: Prof. Dr. Uwe Ilg

Published: 12.12.2017

Level: difficult

  • Optical illusions are perfect for testing theories about vision. If the theory is correct, it must be able to explain the distorted picture exactly.
  • Many of the previous ideas about the human visual system did not hold up to well over a hundred different illusions.
  • The explanation of the Hermann grid proved particularly difficult. A long-standing theory was refuted in 2004, and since then there have been several explanations that have not yet been finally confirmed.
  • The Ebbinghaus illusion shows that various image objects are related to one another in the brain. The actual size is deduced from a size comparison.
  • Visual memory plays an essential role: What one has seen in previous life has a decisive influence on the perception of reality.
Birds can be optically deceived

Birds can also be optically deceived. This has been demonstrated in pigeons, African gray parrots, mistletoe thrushes and chickens. The gray dove bird, which lives in Australia and New Guinea, even uses its susceptibility to illusions for its courtship ritual: the bird builds an arbor from wood and twigs, behind which it gathers light-colored stones. The female looks through the arbor onto the Steinplatz like on a theater stage. The male then presents himself on this - each with a present in his beak, such as a shell, a crown cap or a snail. Now the male gray bower bird does not drape the stones at will. Rather, he places large stones further back and smaller ones closer to the arbor. This creates the effect of the so-called “forced perspective”: The space appears smaller, the presents larger. In addition, the Ebbinghaus illusion also seems to work, report researchers Laura Kelley and John Endler from Deakin University in Australia: Because the bird's gifts are always a little larger than the stones in its mating area, they look more impressive than if they were the same Size. In any case, the illusion makes an impression on the females, as Kelley and her colleague were able to show in a 2012 study: The courtiers were more successful, even if the optical illusion was more pronounced. (tak)

Some will have rubbed their eyes in amazement on full moon evenings: When the moon rises on the horizon, it looks inflated like a hot air balloon. In contrast, it looks as small as a soccer ball up in the sky. Is that still possible with the right things? Science knows it's all just a mirage. For them, optical illusions like the moon illusion are “a window into the world of vision”. This is how the famous perception researcher David Eagleman of the Bayor College of Medicine in Houston put it. Because the visual slip-ups put the scientific theories about the sense of sight to the test.

Even if the moon is an exception: “Our vision usually works amazingly well in fields, forests and meadows,” explains the vision researcher Michael Bach from the Freiburg University Eye Clinic. Almost all optical illusions are based on artificial images.

Geometric black and white structures such as the Hermann grid, named after the physiologist Ludimar Hermann (1838-1914), who was one of the first to mention them in 1870: when looking at a white grid appears on a black background, are among the oldest illusions a blurred spot on every white intersection, just not in the center of the field of view (see picture). Since the contrast between black and white is not correctly recorded, researchers speak of a contrast illusion.

If you look at the so-called Hermann grid, you will see blurry black spots at the white intersections. The exact cause of the deception is not yet known, and theories that have prevailed up to now have recently been refuted. Graphic: Dana Zymalkowski

Almost a century passed after Hermann's invention before scientists were able to come up with the first convincing theory to explain the phenomenon. The neurophysiologist Günther Baumgartner (1924 to 1991) placed microelectrodes in cats' optic nerves and recorded the electrical currents. He realized that the information from several light-sensing cells on the retina converge in just one ganglion cell, which processes it and forwards the result via the optic nerve. The abundance of signals from the photoreceptors is already condensed in the optic nerve. The circular catchment area of ​​a ganglion cell on the retina entered textbooks as a “receptive field”.

There are two types of ganglion cells in the retina: ON-center ganglion cells react particularly strongly when the inner area is stimulated in the receptive field, but the outer area is not. In the case of OFF-center ganglion cells, it is the other way round. The fact that signals from the edges of the receptive field can influence the information in the middle is called lateral inhibition (see: The retina: wafer-thin high-performance computer).

With this knowledge, it seemed, Baumgartner was able to explain the Hermann grid deception: If an ON center ganglion cell detects a white junction, it would be more irritated than at other points on the grid. This leads to a different offsetting of the visual impressions and thus to the washed-out spots. He even had an explanation for the lack of this in the middle of the gaze: In the middle of the retina, fewer photoreceptor impulses are bundled in a ganglion cell. The receptive field in the center of our gaze is therefore so small that it does not touch the black squares.

The Hungarian researcher Janos Geier created this variation of the Hermann grid: The illusion does not occur here - which called into question the explanatory sentences that had been accepted until then. © Michael Bach

Baumgartner's declaration was printed in textbooks for four decades. But in 2004 the Hungarian János Geier put an abrupt end to it - and thus amazed the world of researchers. He modified the Hermann grid slightly by distorting the white lines sinusoidally (see picture). The spots promptly disappeared; and that although the receptive fields still receive the same black information and thus, according to Baumgartner, would have to cause a deception. “I always thought that the Baumgartner model did not go far enough,” acknowledges vision researcher Bach. “I have my black key ring here in front of me and I should see stains on the edge if it were correct. But I see it razor-sharp. So the brain is able to work out the lateral inhibition, except with the Hermann grid. ”But why? The question remains unanswered to this day.

Bach considers the washed-out spots in the Hermann-Gitte to be a side effect of the constant brightness. Brightness constancy is the ability to see the brightest areas in the field of view. “To do this, our visual system does not have to absolutely measure how bright it is, but rather compare the different brightnesses,” explains Bach. The researchers David Corney, today at the City University of London, and Beau Lotto, today director of the "Lab of Misfits" at University College London, demonstrated in an impressive way in 2007 that this gift could produce the Hermann Grid Illusion as a side effect : You brought brightness constancy to an artificial neural network. The self-learning computer program reliably recognized the brightest areas in a display with colored leaves. However, the same program also generated the exact illusion with washed-out gray dots at the intersections from a Hermann grid.

The circle or Ebbinghaus illusion suggests different sizes. Graphic: dasGehirn.info

The repeated attempts to explain the Hermann grid illusion are not an isolated case. The controversy is similarly lively with other deception phenomena, such as the Ebbinghaus deception (see picture). It shows two circles of equal size, one of which is surrounded by smaller circles, the other by larger ones. This makes the latter appear significantly smaller than its twin.

In the 1980s, the American neurophysiologists Margret Livingstone and David Hubel postulated that the Ebbinghaus illusion, like other shape illusions, stems from the strong difference in brightness between black (circle) and white (environment and filling). The neuropsychologist Kai Hamburger from the university took up this thesis in 2007 and presented colored illusions to twenty students. However, they misled them just as much as the black-and-white drawings. "The assumption by Livingstone and Hubel is not tenable," Hamburgers conclusion.

I can see what you can not see

Indications for a new explanatory approach are now coming from an unexpected direction - from cultural psychology: For example, researchers found in 2007 that the indigenous Himba population, who are at home in northern Namibia and southern Angola, experienced the Ebbinghaus deception much weaker than Europeans. The Himba do not have a word for circle in their language. Round objects hardly play a role in their everyday life. So visual experiences seem to influence optical illusions. What we have already seen in life determines what we see.

Obviously, it also depends on the length of our experience: Children under the age of seven recognize the size of the Ebbinghaus circles almost correctly. They are less fooled than adults. “The error in perception is a consequence of late brain development,” comments psychologist Martin Doherty, today at the University of East Anglia. In 2010 he discovered children's visual talent for the first time. As the brain matures, Europeans evidently include more and more the context of the image, that is, the surrounding circles, and merge it into a “synthetic image”. Adults therefore inevitably always see the circles in relation to one another. Children look at them almost in isolation.

The picture comes from the head

More recent publications on the Ebbinghaus deception make it very clear how strongly the image context influences perception. The behavioral neurobiologist Farshad Nemati showed in 2009 at the Canadian University of Lethbridge that the deception turns out to be more drastic, the larger the white space around the Ebbinghaus circles. The Dutch psychologist Niek van Ulzen, who is currently doing research at the University of Verona, found out when the circles are filled with images. If the inner circle contains a negative motif, such as a weapon, a toilet, or a spider, and the outer circles show something pleasant, such as a rabbit or a sunflower, then the illusion is weaker. Ulzen explains: “Negative stimuli require a lot of attention. The information all around is faded out to a greater extent. As a side effect, the size of the circle is estimated more correctly. "

The misleading context also deceives us about the actual size of the moon: Close to the horizon it appears as big as a hot air balloon, because the brain involuntarily relates the celestial body to trees and mountains. At its zenith, the moon is small and lonely, embedded in a sea of ​​stars.

for further reading:

  • Bach, M .: The Hermann grid illusion: the classic textbook interpretation is obsolete. The ophthalmologist. 2009; 106 (10): 913-917 (to the abstract).
  • Doherty, M. et al .: The Ebbinghaus illusion deceives adults but not young children. Developmental Science. 2010; 13 (5): 714 - 721 (to the abstract).
  • Geier, J. et al .: Straightness as a main factor for the Herman grid illusion. Perception. 2008; 37 (5): 651 - 665 (to the abstract).
  • Illusion of the year; URL: http: // www .michael bach .de / ot / [status: 02/18/2004]; to the website.
  • Nemati, F: Size and direction of distortion in geometric- optical illusions: conciliations between the Müller-Lyer and Titchener configurations. Perception. 2009; 38 (11): 585 - 600 (to the abstract).
  • van Ulzen, N. et al .: Affective stimulus properties influence size perception and the Ebbinghaus illusion. Psychological Research. 2008; 72 (3): 304-310 (to the abstract).
  • Visual Phenomena & Optical Illusions; URL: http: // illusionoftheyear .com / [Status: 2005]; to the website.

Publication on: 05.03.2012
Update on: December 12th, 2017