A team of researchers has been exploring how we maintain a consistent perception of our surroundings despite the constant movement of our eyes. Their findings reveal that quick and smooth eye movements operate through different mechanisms, and our sense of visual stability relies on specific signals of motion.
Perceiving visual stimuli requires immense processing power from the brain. Every second, our eyes take in over ten million bits of information and send them to the brain through thousands of nerve fibers. This allows us to experience our environment as stable, even while our eyes are in continual motion. Researchers believe that a unique compensatory mechanism within our visual system enables this effect, a topic that has been studied extensively but remains poorly understood. A team led by psychologist Prof. Markus Lappe from the University of Münster has been investigating how this stable perception is achieved from the rapidly changing visual signals that hit our retinas.
The researchers concentrated on how we perceive the motion of non-rigid objects, like fire or water, an area that has been largely unexplored. Contrary to previous beliefs, they discovered that engaging in smooth eye movements (known as smooth pursuit) is not possible for all types of visual motion. Furthermore, they revealed for the first time that the compensation mechanism typically used for quick eye movements (saccades) gets overridden when viewing certain types of non-rigid motions, resulting in a loss of visual stability. Their findings were published in the journal Science Advances.
For a long time, vision scientists assumed that quick and smooth eye movements reacted to the same motion signals. ‘Our findings distinctly clarify that these two systems function separately and follow different neural pathways,’ states Markus Lappe. In the study, the researchers introduced a newly identified visual motion illusion that disrupts spatial perception. To evaluate this new stimulus concept, fifteen participants were asked to visually track a simulated rotating vortex as it moved across a dotted background. ‘Typically, this task is straightforward, and the eyes fixate on the object, moving seamlessly at its speed. However, the participants struggled to keep up with the vortex, leading to periods where their eyes remained still,’ explains PhD student Krischan Alexander Koerfer. Approximately every 400 milliseconds, participants executed rapid eye movements to reposition the vortex at the center of their retinas. Each time this occurred, the vortex appeared to ‘jump’ forward. ‘The standard compensatory mechanism for quick eye movements didn’t function properly while tracking the vortex. Even though the movement was clearly visible, the eye couldn’t follow it, demonstrating a previously unrecognized scenario.’
The research team analyzed the relationship between the physical stimuli presented and the participants’ perceptions, such as the perceived jumps, while carefully tracking eye positions and movements using high-speed infrared cameras, known as ‘eye trackers.’ By illuminating the eyes with infrared light, they captured reflections off the cornea and pupil, allowing precise determination of eye position and movements.
The new insights gained from this foundational research hold great potential for cognitive and brain science. ‘The introduction of a movement scenario where the compensation mechanism fails allows us to test existing models and create new ones,’ says Markus Lappe. In the long run, this innovative stimulus concept may also aid in diagnosing and researching neurodegenerative diseases.
