Optical Illusions: The Pitfalls of Perception
For most people, the eye is the most important sense. Colors can warn us of danger, we can move effortlessly in three-dimensional space, and we can read complex emotions from faces in a matter of seconds. Our brains are constantly working at their best to handle this mass of information. Even small deviations from what we are used to can lead to us seeing something different than we actually expected. Many optical illusions use this trick. However, they also give us a glimpse of how our brain processes sensory stimuli. Since optical illusions can also be fun, we would like to take a look behind the scenes and introduce you to some of the most interesting phenomena.
Optical illusions are as irritating as they are fascinating.
How does perception work?
Every second, countless pieces of information bombard us through our various senses: we smell and taste the air that surrounds us. Our skin measures the temperature and feels air currents and touches. We hear the rustling of leaves in the wind and footsteps approaching us. And we watch dogs playing and people laughing. If we were to perceive all sensory impressions at the same time and without prioritizing, we would be hopelessly overwhelmed by this flood of information. Our brain filters this information and only presents us with the information that seems useful at the time. In most cases, we can distinguish whether the touch on our arm is from a gust of wind or an insect. The brain also interprets our perception. It helps us to distinguish whether the dogs are actually just playing with each other or whether they are perhaps aggressive and pose a danger to us. Even if the dogs are still relatively far away, we can still estimate their size quite well. To realize all of this, our brain relies primarily on experience. New visual impressions are then compared with experience and put into a suitable, meaningful context. This is also the reason why we can easily recognize figures or faces when we look at clouds. Or we can see triangles and squares where there aren't any.
Experience creates forms where there are none.
Spatial Illusions
Spatial perception itself is an illusion of our brain. The image captured by the retina is two-dimensional. The brain constructs our three-dimensional world from the slightly different viewing angles of the two eyes, vanishing lines, and the experience that objects further away appear darker. Many optical illusions play with these elements. Vanishing lines in particular reinforce the effect of perspective size illusions. Size is perceived depending on the surroundings. People further away appear larger to us than the image on the retina. This makes the following illusion possible. The three figures are exactly the same size. You can measure this if you like. But since the figures at the back appear to be further away, the brain thinks that these people must also be larger.
An illusion of spatial perception.
lines and contrasts
Anyone who has ever tried to hang a picture horizontally without any aids will know how difficult it is to keep an eye on an imaginary straight line. But even drawn lines are not always easy to follow. Strong contrasts and additional auxiliary lines can really mess up even the most precise eye. Maybe it was just a Monday job, but a lot seems to have gone wrong with the following tiled wall. None of the horizontal lines appear to run parallel to the next and even the individual tiles appear somehow distorted. In fact, however, they are exact rectangles arranged on parallel lines. This illusion is also known as the coffee house illusion because it was discovered on the facade of a café in Bristol in 1973. How exactly this illusion comes about is not yet fully understood.
The so-called coffeehouse illusion.
The crux of the matter
Strong contrasts can do more than just distort angles. They can also make us see something that isn't there. The scintillation grid consists of dark tiles separated by bright stripes. The intersections are highlighted by bright dots. If the grid pattern is viewed from the right distance, dark dots appear at the intersections, always exactly where you aren't looking closely.
Dark spots in the scintillation grid.
The greatest illusion – sharp vision
One possible explanation for this and many other optical phenomena could lie in the way our brain processes our visual stimuli. To do this, we need to take a further look behind the scenes. We always believe that we have a sharp image of our surroundings in our field of vision. But in fact the eye is not capable of producing such an image. Only a small area of the retina is capable of producing a really sharp image. Only the macula lutea, also known as the yellow spot, has enough photoreceptors to make this possible. However, the macula only covers a relatively small area of our field of vision. So the eyes are constantly moving and taking small snapshots. The brain then puts these together to form an overall image that appears sharp to us. For everything that happens at the edge of our field of vision, the brain simply makes predictions and fills in the image as it should logically look. This may sound unbelievable, but it can be demonstrated with a simple example. Very close to the macula is the so-called blind spot. This is where the optic nerve exits the retina and there are therefore no photoreceptor cells there. Normally there should be a black spot in our field of vision, but the brain simply fills in the missing information itself. Normally this works so reliably that we don't notice it. But such patterns - which are extreme for the eye - can really mess up this system. This can literally be taken to dizzying extremes and we can see movement where there is none.
Patterns can create movements.
Do optical illusions harm my eyes?
So if you come across an illusion, you don't have to worry about the health of your eyes. Many illusions actually only work with healthy eyes. On the other hand, some illusions can also be an indication of an eye disease. Age-related macular degeneration (AMD) in particular can lead to impairments that can be confused with optical illusions. This is because it leads to impairments in the visual function precisely in the area of the macula, i.e. the sharpest vision. The brain can no longer compensate for this misinformation. Distorted lines, holes or grey veils in the field of vision can be signs of AMD. Although it is of course no substitute for a visit to the doctor, the Amsler grid test can be used to test quite easily at home whether AMD might be present. Find out how it works here.
Optical illusions – confusing and exciting
Optical illusions confuse us, but they are also fascinating. They make us see shapes where there are none, they create movement in static images, or they change the way we perceive the extent of space. The reasons for optical illusions lie in the way our eyes and our brain work together: what we lack in optical information for a meaningful perception of the world is filled by the brain with experiences. Who do you now distrust more: your eyes or your brain?
