A gently lobbed baseball is easy to see. The same ball, however, can seem to vanish from the hand of a skilled pitcher, whizzing invisibly into the catcher’s mitt.
Given enough acceleration, moving objects become too fast to see. Yet this visual speed limit isn’t universal – some people are apparently better at seeing in high-speed.
According to a new study, the secret may lie in subtle eye movements known as saccades. These rapid motions of the eyes shift our focus between various points of interest, and are regarded as the most frequent movement the human body makes. By some calculations they occur two to three times every second, adding up to around 10,000 times every waking hour.
Saccades take place naturally constantly without our thought or even awareness, but they don’t occur at the same speed in everybody. The quickness of each person’s saccades may determine their visual speed limit, the authors report, enabling those with faster saccades to perceive faster motion.
The findings seem to demonstrate how our perception depends not just on the sensory limits of our eyes, but also on how our eyes behave as they observe. From bow hunters to baseball players, those with exceptional skills in fast visual tasks might be unwittingly capitalizing on speedy saccades.
“What parts of the physical world we can sense depends fundamentally on how good our sensors are,” says lead author Martin Rolfs, a vision scientist at Humboldt University of Berlin.
Human eyes aren’t sensitive to infrared light, for example, so we can’t see it. This is an in-built limitation, illustrating how the mechanics of our sensory organs dictate our perception.
“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,” Rolfs says.
As we scan a scene or read text, our eyes naturally dart around to different focal points, briefly pausing on each before flitting to the next. While this helps our vision cover more territory, it risks disrupting the entire retinal image, effectively blinding our perception for fractions of a second at a time.
Fortunately our brain’s visual system edits out this disrupted motion in real-time, providing us with a seamless visual input by selectively excluding the careening effect of saccades.
“Even though visual processing remains operational during saccades, this saccade-induced retinal motion is subjectively invisible during natural vision – a phenomenon referred to as saccadic omission,” the researchers write.
Saccades occur with incredible speed, outpacing most ordinary moving stimuli. Yet when we do encounter unusually fast objects, those similar enough to our own saccade motions may be edited from our perception.
In the new study, Rolfs and his colleagues used high-speed video projections to display fast-moving objects that either matched or deviated from the motion of saccades. For each stimulus, observers performed perceptual tasks that were only possible if they’d seen the trajectory of motion.
Stimuli following the specific movement patterns of saccades became invisible to observers, the study found.
“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 says.
Considering how active our eyes are, this highlights the importance of factoring that movement into our understanding of the broader visual system.
“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,” Rolfs says.
“Our visual system and motor system are finely tuned to each other, but this has long been ignored.”
The study was published in Nature Communications.
