The topic of illusions is one of a kind. It shows that, as humans, our bodies and senses are not perfect and we can make mistakes when it comes to perceiving objects, things, and so on. In this article, we dive deep into what hollow face illusion is and how it is related to schizophrenia.

The following topics will be discussed in the article:

• What is Schizophrenia?
• What are Visual Illusions?
• Hollow Face Illusion
• Schizophrenia’s relation to Hollow face illusion

What is Schizophrenia?

Before we start talking about illusions, it is crucial to touch upon what schizophrenia is.  Schizophrenia is a mental disorder where the person struggles with what is real and what is not real. For instance, some symptoms are lack of insight, hallucination, delusions, social withdrawal, loss of motivation etc. People who have schizophrenia can experience different types of hallucinations (e.g., auditory hallucinations, visual hallucinations) [7].

If you would like to learn more about Schizophrenia, please click here to read Denisa’s article, where you can find more detailed information about this disorder, as well as the DSM-5 criteria.

What are Visual illusions?

A visual illusion is an error made by our visual system, where perception does not align with the physical truth of the object. Illusions are ambiguous phenomena that can result in distinct but mutual explanations. There are many famous visual illusions [6]. Some of them are Müller-Lyer illusion, Ebbinghaus illusion, Ponzo illusion, Poggendorf illusion, Oppel Kundt illusion, Sander Parallelogram, Kaniza’s Triangle also known as Gestalt effect, Hermann Grid, contrast-contrast illusion, Roelof effect, tilt illusion and many more can be listed [5].

‍

Visual illusions are an important research tool for scientists. By seeing how people perceive things in their environment, we learn more about how our brain processes information and how it reacts to certain things, such as object-based illusions.

Hollow face illusion‍

‍

(These images are taken from this video)

Hollow face mask illusion is a tool utilised to test if people have schizophrenia or not (Note: this is one test among others.) It should be noted that this is still being debated and it is not a definitive test: if you fail, it does not mean that you suffer from schizophrenia.

Our brains can struggle with telling the answer to the question of: is this (on the right) the front or the back of the mask? Thus, the hollow mask illusion confuses our brains when deciding whether the image is concave or convex. It is a type of optical illusion where the concave mask you see above is perceived as a normal convex face.

How does it work? The mask on the left is normal, whereas the one on the right is a result of the illusion, as it is lit from below. When this mask is turned, we don’t perceive it the way it should be perceived. We imagine the nose sticking out when, in fact, it is not sticking out. The left image is the front of the mask, and the right image is the back of the mask. Hollow face mask illusion is one of the strong examples of top-down knowledge: our knowledge inhibits how we should see the image. Furthermore, we are used to thinking that a face is supposed to be concave, another reason why why this illusion happens. You can check out this video to see the illusion yourself [2].

An interesting finding about hollow face illusion is that, it can be harder to spot the illusion when the image changes form or features [2]. For example, the illusion is stronger when the mask is rotated 90 degrees from the forehead. Second, an experiment demonstrated that depth inversion illusion was higher for textured objects and weaker for stereoscopic information-based objects [4]. Lastly, a study showed that patients are more likely to see things such as line length, motion direction, and brightness based on what is more probable in the visual world. This is in line with Bayesian theory [3].

Schizophrenia and Hollow face illusion

The Predictive Coding Model of Hallucinations and Delusions states that our system is differentiated in terms of its metacognitive components (i.e., the estimate of beliefs’ accuracy rather than the beliefs’ actual accuracy). Thus, when there is a disturbance in the estimates this causes an error to occur. Delusions can be seen as false and inflexible beliefs that can come from these errors. Essentially, you “choose” to believe in the delusion that comes from your sensory information rather than going with your prior knowledge. This is one of the theories that link visual illusions and psychotic symptoms in patients. All in all, delusions might be coming from prior beliefs generated by a high number of sensory signals[6].

Patients with schizophrenia do not perceive this illusion the same way normal control groups do. Usually, the illusion occurs because the image is perceived in a wrong way in our brain, but for schizophrenic patients the face is seen as a hollow face. Schizophrenic people are less likely to see an object as concave due to the decreased susceptibility to this feature[4].

An experiment was conducted to establish the susceptibility of the hollow face illusion (i.e., control groups vs schizophrenic patients). It was found that controls were not able to differentiate between concave and convex faces, but they were not fooled by the illusion. The authors thought that this occurred due to the mask design not being based on a real face. Instead, for schizophrenic patients, it was found that they had impairments related to the hollow face illusion [1].

In conclusion…

A debate about hollow mask illusion still exists to this day. Can it be used to test if people have schizophrenia or not? Is it valid? These are considerations to take into account.

Even though we cannot give a definitive answer to this question, one study did find that the correct perception of the illusion was present in 30% of patients who suffered from the paranoid and undifferentiated types of schizophrenia. Compared to the general population, this is 3 times higher [8].

Are you still curious to know more about the topic? Consider watching these videos:

1.       https://www.youtube.com/watch?v=6YIPtJlCbIA&t=73s&ab_channel=SkillBoosters

2.       https://www.youtube.com/watch?v=G_Qwp2GdB1M&ab_channel=Blackmoon

3.       https://www.youtube.com/watch?v=drrxA5YOodc&ab_channel=Blackmoon

References

1. Dima, D., Dillo, W., Bonnemann, C., Emrich, H. M., & Dietrich, D. E. (2011). Reduced P300 and P600 amplitude in the hollow-mask illusion in patients with schizophrenia. Psychiatry Research: Neuroimaging, 191(2), 145-151.

2. Hill, H., & Johnston, A. (2007). The hollow-face illusion: Object-specific knowledge, general assumptions or properties of the stimulus?. Perception, 36(2), 199-223.

3. Kaliuzhna, M., Stein, T., Rusch, T., Sekutowicz, M., Sterzer, P., & Seymour, K. J. (2019). No evidence for abnormal priors in early vision in schizophrenia. Schizophrenia research, 210, 245-254.

4. Keane, B. P., Silverstein, S. M., Wang, Y., & Papathomas, T. V. (2013). Reduced depth inversion illusions in schizophrenia are state-specific and occur for multiple object types and viewing conditions. Journal of abnormal psychology, 122(2), 506.

5.King, D. J., Hodgekins, J., Chouinard, P. A., Chouinard, V. A., & Sperandio, I. (2017). A review of abnormalities in the perception of visual illusions in schizophrenia. Psychonomic Bulletin & Review, 24(3), 734-751.

6.Notredame, C. E., Pins, D., Deneve, S., & Jardri, R. (2014). What visual illusions teach us about schizophrenia. Frontiers in integrative neuroscience, 8, 63.

7.Picchioni, M. M., & Murray, R. M. (2007). Schizophrenia. BMJ (Clinical research ed.), 335(7610), 91–95. https://doi.org/10.1136/bmj.39227.616447.BE

8.Wichowicz, H. M., Ciszewski, S., Żuk, K., & Rybak-Korneluk, A. (2016). Hollow mask illusion–is it really a test for schizophrenia. Psychiatr Pol, 50(4), 741-5.