The visual perception of optical stimuli demands high performance from the brain. Every second, the eyes absorb more than ten million pieces of information and transmit them to the brain via thousands of nerve fibres. This leads us to perceive the world as stable, even though we are constantly moving our eyes. Experts suspect that this is made possible by a special compensation mechanism of the visual system, which has been studied for a long time but is still not understood. A research team led by psychologist Prof Markus Lappe from the University of Münster has investigated how this stable perception of the world arises from a highly dynamic visual input signal on the retina.
They focussed on the motion perception of non-rigid objects such as fire or water, which is virtually unexplored. The researchers found that, contrary to previous assumptions, smooth eye movements (smooth pursuit) cannot be performed for all types of visual motion. In addition, they were able to demonstrate for the first time that the compensation mechanism for rapid eye movements (saccades) is overridden when we see certain types of non-rigid movements. As a result, visual stability is lost. The results of the study have been published in the journal Science Advances.
Vision science has long assumed that rapid and smooth eye movements respond to identical motion signals. ‘Our results show a clear separation of the two systems. They are functionally distinct and run along different neuronal pathways,’ explains Markus Lappe. In the study, the scientists presented a newly discovered visual motion illusion that leads to a disruption of spatial perception. In order to test the new stimulus concept, fifteen subjects had to follow a simulated rotating vortex with their eyes as it moved across a field of dots. ‘Normally this is an easy task, and the eyes remain fixed on the object. Thus, they move continuously at the speed of the object. However, the vortex could not be followed, so the eyes remained static for a period of time,’ explains PhD student Krischan Alexander Koerfer. A phase of rapid eye movement occurred approximately every 400 milliseconds, which brought the vortex back to the centre of the retina. Each time the subjects made such an eye movement, the vortex seemed to jump forward. ‘The usual compensatory mechanism for rapid eye movements failed when the vortex moved. Although the resulting movement was clearly perceived, the eye could not follow it. This is a previously unknown combination.’
The research team examined the relationship between the presented physical stimuli and the corresponding perception, such as perceived jumps, while precisely measuring the eye position and eye movements synchronously using high-speed infrared cameras, so-called ‘eye trackers’. In this process, the eyes are illuminated with infrared light and the reflections on the cornea and pupil are filmed and analysed. The reflections allow the exact position and movement of the eyes to be determined.
The new findings from basic research are particularly useful for cognitive and brain research. ‘The fact that for the first time a movement is presented in which the compensation mechanism fails means that old models can be tested and new ones developed,’ says Markus Lappe. In the long term, the new stimulus concept can also be used to diagnose and research neurodegenerative diseases.