What is the science behind optical illusions

Optical illusion for scientific purposes

Delusions and loss of reality are major symptoms of schizophrenia. It is assumed that these are caused, among other things, by a faulty predictive ability of the brain: The patients cannot correctly match new sensory perceptions with their memories and the predictions they have learned from their memory. One way to examine this limitation in the perception of people with schizophrenia in more detail is through their response to optical illusions. Because they often perceive them differently than healthy people.

A well-known example of an optical illusion is a three-dimensional cube, the Necker cube. If we look at it for a long time, our perception changes again and again between two views: We see the throw either “from below” or “from above”. Such an optical illusion is called a bi-stable stimulus and this creates a bi-stable perception. "Interestingly, studies by our network partners at the Charité in Berlin led by Dr. Katharina Schmack show that the process of bi-stable perception has different characteristics in people with schizophrenia than in healthy people. And this can be an indication of a changed neural activity in the brain of those affected, ”explains the mathematician Professor Gaby Schneider from the Goethe University in Frankfurt. She is a project manager in the interdisciplinary junior network “PsychoSys”, which is funded by the Federal Ministry of Education and Research (BMBF).

Who changes perspective and how often?

For example, patients with schizophrenia show a spontaneous change between the perceptual perspectives somewhat less often when a bi-stable stimulus is presented. If, on the other hand, the researchers present the bi-stable stimulus with short interruption intervals, the number of changes in perspective in schizophrenics is slightly higher than in healthy test subjects. According to the scientist, it was observed that patients with schizophrenia, as well as healthy people, can perceive a single perspective in a stable manner, even over a longer period of time. But there are also periods of time in which you switch between perspectives in rapid succession. “In general, people with schizophrenia are much more likely to be in such an unstable state of perception than healthy people. This fits in with the observation that patients with schizophrenia are more susceptible to being distracted by irrelevant information, ”describes Schneider.

Bringing math and practice into harmony

One of the goals of the PsychoSys junior network was to first design a mathematical model that could precisely describe these changes in the perception of patients with schizophrenia. Such a model then allows predictions about the underlying neural processes. Thanks to the close cooperation between researchers from mathematics, medicine and psychology, Schneider and her doctoral student Stefan Albert have succeeded in developing a theoretically sound mathematical model that is so simple that it can also be used in practice. “That may sound banal, but that was exactly the difficulty. Because sometimes the theoretical models in their mathematical beauty and elegance cannot be reconciled with the data measured on patients. Or mathematical models are created that can provide explanations for processes in the brain and describe all kinds of data, but which have so many levers that an effect can no longer be clearly assigned, ”explains Schneider.

With the help of the model, the scientists can now relate empirical data on the perception of optical illusions to possible neuronal processes in the brain. “The differences between healthy people and patients with schizophrenia enable us to derive hypotheses about the underlying neuronal processes,” says Schneider.

Junior alliances in systems medicine

With the funding of junior networks in systems medicine, younger scientists from various disciplines should be given the opportunity to implement highly innovative research projects in systems medicine in an interdisciplinary team. This should make it easier for them to network across the boundaries of their specialist disciplines, to build up scientific expertise and to establish themselves in system-oriented medical research. The BMBF has been funding nine interdisciplinary junior associations with around 16 million euros since 2014.

Competition between neurons is the basis for the model

For the construction of their mathematical model for optical illusions, the scientists took advantage of the fact that neuronal processes with bi-stable perception can often be described with the following model: Two groups of nerve cells are in “competition”. If group A wins, i.e. the neurons in this group are more active than those in the other group, we perceive perspective A. If, on the other hand, group B wins, we perceive perspective B. In the second step of the model, another, similar competition on a higher neural level is assumed, which causes the change between stable and unstable perception. “In the simplest case, our model describes this competition between neurons with only two parameters. In this way we can describe the resulting change in perspective and - with additional parameters - also the state of stable or unstable perception. Comparable models usually require significantly more parameters for this, which makes it difficult to adapt to empirical data sets, ”describes Schneider.

The mathematical model offers an important starting point to better understand the cognitive limitations of schizophrenia in the future. It could also help develop diagnostic tests that help determine whether or not a person has schizophrenia.

Inspiring the next generation for systems medicine

The PsychoSys junior network in Frankfurt offers a special course in order to inspire young academics for the interdisciplinary research area of ​​systems medicine and to introduce them to the application of mathematics in practice. “Our statistical internship is an extremely profitable event for everyone - for mathematics students, but also for teachers and researchers. It allows various analysis approaches for current data sets and questions to be developed and tested together. At the same time, we establish contact between students and users, ”says Schneider.

What is systems medicine?

“Systems biology” aims to understand the dynamic life processes of cells, tissues, organs and organisms and to map the underlying interactions on a molecular level in networks. To do this, she applies methods of mathematical modeling and the power of modern computers. "Systems medicine" wants to transfer the knowledge and methods of systems biology to medicine and make them usable for patients. The aim of systems medicine is to apply the new insights into the processes of life in order to diagnose diseases more precisely and to treat them better. Because whether a person is healthy or sick depends on many factors, be it genetic differences, changes in molecules or environmental influences. The question is how all these factors and systems interlock and how they can be influenced.

Contact person:
Prof. Dr. Gaby Schneider
Institute for Mathematics
Goethe University
Robert-Mayer-Strasse 10
60325 Frankfurt am Main
[email protected]