How Our Brains Predict Eye Movements — and Why Afterimages Don’t Always Line Up
If you’ve ever been caught off guard by bright lights coming around a corner, or the sun suddenly hitting your eyes, you’ve likely quickly looked away and noticed that a faint shape remained.
That faint shape, known as an afterimage, is caused by the brain and the way our eyes process the world around us. Our eyes are continuously moving in rapid little jumps known as saccades. However, we rarely notice our eyes making these movements, and our overall vision remains pretty still because our brain provides that stability.
New research published in Science Advances examines how accurately our brains predict eye movements and why those predictions may not always be correct.
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Understanding Eye Movements
According to the study, Aristotle was among the first to document the phenomenon of afterimages, and, from there, the consensus is that afterimages follow wherever we direct our gaze. However, this notion raises an interesting point of dissociation. Even though our vision appears stable while our eyes constantly move scenes across our retinas, afterimages appear to drift across our image, even though the image is fixed on our retina.
This, researchers believe, may be one of the brain’s mechanisms to keep track of its own eye movements.
To understand these mechanisms, the research team conducted experiments in complete darkness, the opposite of the typical feedback our brains experience daily, helping to predict our eye movements.
For the experiments, the researchers had participants sit in the dark and exposed them to a bright flash that created an afterimage. Once the afterimage appeared, the participant would quickly look over at a second, briefly illuminated light source. When the next afterimage was created, the participant was exposed to several probe lights in different positions. From here, the participant had to report where the afterimage was, either to the right or to the left of the probe, or directly in line with it.
The team also used eye-tracking measurements to assess how closely eye movements matched participants’ perceptions.
Almost, But Not Quite 100 Percent
The results showed that the participants’ perception and tracked eye movements were highly accurate. The team noted that the larger the eye movement, the further the afterimage moved. While the results showed high accuracy, the predictions were not 100 percent.
“On average, the perceived shift of the afterimage reached about 94 percent of the actual eye movement,” said Richard Schweitzer, lead author of the study, in a press release. “In practical terms, perception follows eye movements very closely, but not perfectly.”
This small margin is known as hypometria, and the research team found it across the board in all participants, regardless of eye size and the directions they moved during the study. According to researchers, this likely indicates a slight inaccuracy in the brain’s processing rather than a random error.
Why Afterimages Seem To Drift With Our Gaze
To understand where the afterimage appears, the team believes that visual feedback from each slight eye movement could determine the perceived location of the afterimage. To test this, in some experiments, the light the participants were told to follow remained visible for a brief time after the eye landed on it. However, in other experiments, the team shifted the light very slightly to create misleading feedback.
Neither of these experiments changed where the afterimage was perceived. The team thinks this could be because the brain uses an efference copy, or an internal copy of a command that’s been sent to the eye muscles. However, there are other body mechanisms that can change perception as well. Saccades can adapt in response to eye muscle fatigue, possibly resulting in a smaller visual shift. Though these saccades can vary, the brain can still compensate for the shift.
In addition to adapting to saccades, the brain can use its knowledge of upcoming eye movements to predict where an object will appear on the retina. This could explain why afterimages appear to move with our gaze, even though they are on the retina. However, there is still much to learn about how the eyes and the brain work and adapt together. Understanding this connection could lead not only to advances in vision but also in robotics.
“Afterimages become a useful tool for studying how the brain keeps the visual world stable by predicting the sensory consequences of its own movements,” said Schweitzer.
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