Side by side, two faces look no different. But slowly, why are their shadows inverse?
The answer is actually quite simple: the right side of the face is not raised, but a sunken model.
Image courtesy of David Mack, fineartamerica.com
Take a closer look at the different angles of the model, and fine, now you know exactly what the sunken face is. Look at the model facing you in the middle, do you see a sunken face?
For many people, the answer is probably no. How do these two faces still look like they are all bulging?
A concave face that cannot be seen
This phenomenon has a very straight name: the hollow-face illusion, which has been known for more than 200 years, but it is still a very concerned by researchers. This is because the effect of the concave face illusion is really very domineering.
The original signal received by the retina is a two-dimensional projection, and the vision system has many ways to restore the depth information of the three-dimensional world. But the "lever handle" techniques of these visual systems— "binocular parallax" and "kinematic parallax," all fail in the face of the concave face illusion. Some optical illusion devices may be exposed by slightly changing the perspective, but in front of the concave face mask, even if we move left and right, the illusion effect is always there. Even when the concave face model is rotated, people tend to only see the sunken face at extreme angles, while at other angles, the recessed side will look like a convex face moving in the opposite direction.
Recorded by: The Royal Institution
Researchers have found that unless we walk within a meter, we can see the true face of the concave face. Change the light source? Useless. Even if the light is shot in the opposite direction from the light source we think, the illusion remains.
People usually default to the light source from top to bottom, if the reverse light will make people feel some objects bumpy changes, but this trick is not useful for concave face masks. The objects from left to right in the image above are: fruit packaging material (which is actually dented), a prominent face model, and a sunken face model. After the light changes, it is easy to feel the concave and convex changes in the packaging material, but the concave face mask always looks like a bulge. Recorded by: Weipeng Liu
The brain wants to be lazy again
What we see depends not only on the original signals received by the retina, but also on past experiences in the brain that influence the process of visual perception, and the concave face illusion is one such example.
There is a general tendency to "see objects as raised" that can partially explain the concave face illusion, but this is far from the answer to the problem.
When viewed from a certain distance, other objects can produce similar illusions, but the sunken face is still special, and the illusion it produces is particularly strong. Experiments have found that the sunken face model produces a significantly stronger illusion than a casually shaped recessed object, and the observer must get closer to eliminate the effects of the illusion [1]. If noise is gradually added to the 3D face model to distort it, it will also weaken the concave face illusion [2]. Inverting the face model also reduces the tendency to see the depression as a bulge [3]. These results all show that "familiarity" affects people's visual perception.
The addition of noise can make the concave illusion gradually less useful. Image credit: Hill, H., & Johnston, A
People have two basic ways of processing visual information, top-down processing and bottom-up processing.
We can understand it this way: there is a place in the brain that resembles a "registry" that stores all kinds of things that we have seen. When encountering something we are familiar with, the brain will search at the "registration office", and when the matching template is found, the brain will no longer bother to dwell on the details of the thing in front of us, but directly report to the lower level perception system: "This is the face, the next question", which is the "top-down" processing.
As humans, our sensitivity to "faces" is so high that you simply can't ignore things that are a bit like human faces...
When faced with new things, the brain cannot identify against the existing template, so it has to carefully process the things in front of it and then combine the details to form a perception of new things. This is bottom-up machining, which is more time-consuming and laborious. Finally, the brain stores new things in the registry so that the next time it can be processed from the bottom up in a time-saving and labor-saving way.
The emergence of the concave face illusion can be seen as the process of "laziness" of the brain. Because the brain easily does not want to use time-consuming and laborious bottom-up processing, so every time it will search the registry to find a perfect match, it is naturally good, but if it cannot find a perfect match, it will also ignore the details of some things and the template, and hard to the template, thus eliminating bottom-up processing. Even if the light and shadow information is contradictory, the brain still allows us to perceive the protruding human face through experience, so that it can save the processing of concave face cognitive resources. And in order to do this, our brains even make us think that the sunken face is in reverse motion to cover up the secret that "it is not a normal human face".
Can't feel the illusion of a concave face, is this a disease?
The concave face illusion is very powerful, but it has been found that some people are not confused by this illusion – such as schizophrenics.
This is actually related to the two cognitive processing methods mentioned above. Schizophrenia is not only a mental illness, but also abnormalities in the corpus callosum (used to connect the left and right hemispheres of the brain) in patients, resulting in the left and right brains not communicating well [4]. Top-down processing is a high-level overall processing that requires the various parts of the brain to communicate with each other. Therefore, the cognitive processing of schizophrenic patients has to rely more on the "bottom-up" model, so that the elements needed to produce the concave face illusion are not available [5].
So, if a person can't see the illusion of a concave face, does it mean that they have schizophrenia? Some people have indeed proposed such an idea, but this small test is not yet a universal diagnostic or rehabilitation indicator. It has been found that approximately 30 percent of patients with schizophrenia differ significantly from the general population, and that the duration of illness, age of illness, and severity of severe illness are not associated with the manifestation of distinguishing the illusion of concave faces [6].
Extras: Eyes lock on you!
The small dinosaur paper mold that often appears on the Internet will always stare at people, and it can actually be seen as the embodiment of the illusion of a concave face.
Recorded by brusspup
The head of this paper mold is actually concave, and we tend to think of it as a more familiar bump. If the dinosaur's face is normal, when we move a few steps to the left, we should see more part of the dinosaur's right face, and the left face will be blocked. But because dinosaurs are actually sunken down, when we move to the left, we actually see more of the dinosaur's left face, and the right face is blocked instead. Under this contradiction, the explanation given by the brain is that the face of the dinosaur must have moved with you, and it was even larger than the amplitude of your movement.
As a result, people can't get rid of the gaze of this annoying little guy.
Turning to an extreme angle, the sunken face is finally revealed. Recorded by brusspup
bibliography
[1]http://journals.sagepub.com/doi/abs/10.1068/p231335
[2]http://journals.sagepub.com/doi/abs/10.1068/p5523
[3]https://link.springer.com/article/10.3758/BF03212186
[4] Beaumont, J. G., & Dimond, S. J. (1973). Brain disconnection and schizophrenia. The British Journal of Psychiatry.
[5] Dima, D., Roiser, J. P., Dietrich, D. E., Bonnemann, C., Lanfermann, H., Emrich, H. M., & Dillo, W. (2009). Understanding why patients with schizophrenia do not perceive the hollow-mask illusion using dynamic causal modelling. Neuroimage, 46(4), 1180-1186.
[6] Wichowicz, H. M., Ciszewski, S., uk, K., & Rybak-Korneluk, A. (2016). Hollow mask illusion–is it really a test for schizophrenia?. Psychiatr. Pol, 50(4), 741-745.
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