Too fast to see: Eye movements predict speed limits in perception
If you quickly move a camera from object to object, the abrupt shift between the two points causes a motion smear that might give you nausea. Our eyes, however, do movements like these two or three times per second. These rapid movements are called saccades, and although the visual stimulus during a saccade shifts abruptly across the retina, our brain seems to keep it under the hood: we never perceive the shift. New research shows that the speed of our saccades predicts the speed limit in our vision when an object becomes too fast to see. According to a study published in Nature Communications by researchers from the Cluster of Excellence Science of Intelligence (TU Berlin), visual stimuli ––think a chipmunk darting around or a tennis ball hit with full force–– become invisible when they move at a speed, duration, and distance similar to those of one of our saccades. This suggests that the properties of the human visual system are best understood in the context of the movements of our eyes.
When does a moving stimulus become too fast to see?
The limits of how fast an object can be before it becomes invisible to us is directly related to the speed of our own eye movements. Beyond a certain speed, a moving stimulus becomes too fast for us to see. As a result, the speed of our eye movements across a specific distance can be used to predict at what speed a moving stimulus becomes invisible to us. And since the speed of our eye movements changes from person to person, people who make particularly rapid eye movements can also see objects moving at higher speeds than people with slower eye movements. This might mean that the best baseball batters, action video game players, or wildlife photographers are the ones with quicker eye movements.
Our movements shape our perception
This result is exciting as it provides first evidence of the idea that our body movements fundamentally shape the abilities of our perceptual system. “What parts of the physical world we can sense depends fundamentally on how good our sensors are,” explains Martin Rolfs, the lead author of the study. “For example, we don’t see infrared light because our eyes are not sensitive to it, and we fail to see flicker on our screens because they flicker at higher frequencies than our eyes can resolve. In this paper, however, we show that the limits of seeing are not just defined by these biophysical constraints but also by the actions and movements that impose changes on the sensory system. To show this, we used the body’s fastest and most frequent motions, i.e. the saccadic eye movements that people make more than a hundred thousand times a day.”
A motion we don’t perceive
Much like a camera movement causes motion in a movie, saccades create movement patterns on the retina. “But we never consciously perceive that motion,” says Rolfs. “We have shown that stimuli that follow the same (very specific) movement patterns as saccades (while people are holding their eyes still) also become invisible. So we are basically suggesting that the kinematics of our actions (here, saccades) fundamentally constrain a sensory system’s access to the physical world around us.” Rolfs explained that this is to be considered an intelligent trait of the visual system, because it remains sensitive to fast motion, but only up to speeds that result specifically from saccades, and these speeds are not seen consciously albeit available to the brain. “In simple terms,the properties of a sensory system such as the human visual system are best understood in the context of the kinematics of actions that drive its input(in this case, rapid eye movements),” said Rolfs.
A finely tuned machine
“Our visual system and motor system are finely tuned to each other, but this has long been ignored,” says Martin Rolfs. “One of the issues is that the people who study motor control are not the same ones who study perception. They attend different conferences, they publish in different journals –– but they should be talking!”
This study suggests that our visual system can recognize when a stimulus moves in a way that is similar to our own eye movements, and then filters out the conscious perception of this movement. This also introduces a new mechanism to explain why we do not see visual motion smear on the retina during eye movements as we would if we were using a camera.
In brief:
- Objects moving at speeds, durations, and distances similar to those of our saccades can become invisible to us—even when our eyes are still.
- People with faster saccadic eye movements can perceive faster-moving objects better than those with slower ones.
- The ability to perceive motion is not just about sensory limits but also shaped by the movements that drive sensory input.
- Our visual and motor systems are tightly coordinated, yet often studied separately—to understand perception, we need to understand action.
Photo: Eastern Chipmunk in Erindale Park, Ontario, Canada, Wikimedia, CC BY Oleksii Voronin, right side of image adapted.
