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 from a corner or the sun suddenly hitting your eyes, you’ve probably quickly looked away and noticed a faint shape remaining.
This faint shape, known as an afterimage, is caused by the brain and the way our eyes process the world around us. Our eyes continually move in small, rapid jumps called saccades. However, we rarely notice our eyes making these movements, and our overall vision remains relatively stable because our brain provides this stability.
New research published in Scientific advances examines how accurately our brains predict eye movements and why these 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. This notion, however, raises an interesting point of dissociation. Even though our vision appears stable as our eyes constantly move scenes across our retina, afterimages appear to drift across our image, even though the image is fixed on our retina.
According to the researchers, this could be one of the mechanisms the brain uses to track 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 the participants sit in the dark and exposed them to a flash of light that created an afterimage. Once the afterimage appeared, the participant quickly looked at a second, briefly illuminated light source. When the next afterimage was created, the participant was exposed to multiple probe lights in different positions. From there, the participant had to indicate where the afterimage was, either to the right or left of the probe, or directly in line with it.
The team also used eye-tracking measurements to assess how well eye movements matched participants’ perceptions.
Almost, but not quite 100%
The results showed that participants’ perception and tracked eye movements were very accurate. The team noted that the greater the eye movement, the further the afterimage traveled. Although the results showed high accuracy, the predictions were not 100 percent.
“On average, the perceived afterimage shift was approximately 94% of the actual eye movement,” Richard Schweitzer, lead author of the study, said in a press release. “Concretely, perception follows eye movements very closely, but not perfectly. »
This small margin is known as hypometria, and the research team found it in all participants, regardless of eye size and the direction they moved during the study. According to the researchers, this likely indicates a slight inaccuracy in brain 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 that participants had to follow remained visible for a brief period after the eye landed on it. However, in other experiments, the team shifted the light very slightly to create misleading feedback.
None of these experiments changed where the afterimage was seen. The team thinks this could be because the brain uses an efference copy, or an internal copy of a command sent to the eye muscles. However, there are other bodily mechanisms that can also alter perception. Saccades may adapt in response to eye muscle fatigue, eventually leading to less visual lag. Although these saccades may vary, the brain can always compensate for this change.
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 seem to move with our gaze, even though they are on the retina. However, there is still much to learn about how the eyes and brain work and adapt together. Understanding this link could lead not only to advances in vision, but also in robotics.
“Afterimages become a useful tool for studying how the brain maintains the stability of the visual world by predicting the sensory consequences of its own movements,” Schweitzer said.
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