Actually, everything stands still and yet we see movements: This illusion is also caused by optical effects in the fruit fly, researchers have established. Using this model animal of science, it was also possible to investigate which neuronal mechanisms could be the basis of the perception phenomenon. Accordingly, unbalanced reactions from nerves of the visual system seem to trigger the impression of movement in the fly. Presumably this is similar in humans, say the scientists.
Ultimately, the image of the world is created in our brain: the raw visual data is recorded, evaluated and interpreted – this is how we record patterns, colors and movements in our field of vision. As is well known, the brain can be fooled – we perceive something that does not correspond to reality. A prime example of this is the illusion of movement as it is conveyed in the example illustration: the repetitive patterns and contrasts create the illusion of rotating movements in the rings shown. The neurological basis of this deception is not yet fully understood.
Deceived baptisms turn
However, it was already suspected that lighter areas in the patterns are processed more quickly, so that a sequence of shades creates the illusion of movement. There is also evidence that fine eye movements – so-called microsaccades – play a role in the perception phenomenon. It is clear that not only humans recognize movement in certain shading patterns where there is none at all: Experiments show that this perception phenomenon is typical of vertebrates. But how is it with insects? The researchers working with Damon Clark from Yale University in New Haven answered this question through studies on the fruit fly (Drosophila melanogaster) followed up. The scientists used the well-known effect that the insects adapt their body alignment to a perceived movement: They turn in the corresponding direction.
For the experiments, the test animals were first fixed with a fine wire. The researchers then placed them on a tiny movable ball that the insects could spin with their feet. In this constellation, the flies were then presented with shading patterns that give humans the illusion of a directed movement to the right or to the left. It turned out that the insects turned the ball with their feet in a way that corresponded to their need to adapt to the direction of the respective illusion. In other words, you too succumbed to the optical illusion. “It was exciting to see that flies can sense movement in static images,” says Clark.
Evidence of neural mechanisms
In addition to this finding, the results opened up the possibility of further investigations. Because a lot is already known about the visual perception system of Drosophila and the model animal offers a comparatively large number of genetic and neuronal experimentation possibilities. As the researchers report, it was already known that two categories of nerve cells in flies are involved in the perception of movement: T4 and T5 neurons. When examining the neural activity of these nerves, the researchers found a characteristic reaction pattern that is evoked by the illusion-creating images.
Further experiments then showed: By switching off both types of neurons, the flies could no longer perceive the illusion. If, on the other hand, only one version was selectively inactivated by genetic means, flies developed that could only perceive movement illusions in one direction. According to the researchers, this finding suggests that an imbalance in the response of the two neural motion detectors is underlying the optical illusion. The researchers conclude that the perception of illusions is probably a by-product of the brain’s strategies for processing movements.
Since there are known similarities between the visual processing systems of humans and flies, the researchers suspect that our perception of the illusion of movement is based on similar neuronal mechanisms as in the insect. As they report, they have also carried out tests with human subjects, the results of which further support this assumption. “The last common ancestor of the fly and man lived half a billion years ago, but evolution seems to have led to similar strategies for the perception of movement in both living things,” says Clark. “Understanding these common strategies could help us to better understand the human visual system,” says the scientist with a view to future studies.
Source: Yale University, Article: PNAS, doi: 10.1073 / pnas.2002937117