Without our noticing it, our perception is constantly corrected. Because a special system compensates for the constant rapid eye movements with which we involuntarily scan our environment. “Blurred” image passages are simply suppressed. Scientists have now found out where this so-called Saccadic suppression takes place and how it works. Her experiments clearly show for the first time that this vision correction takes place directly in the retina and not only in downstream parts of the vision system.
Our eyes never stand still. Instead, they are constantly moving back and forth in rapid succession – about two to three times a second. Only these so-called saccades ensure that we perceive a sharp picture of our environment. Because through these involuntary movements, our eyes scan the surroundings like a scanner: they let everything visible in our field of view run over the high-resolution area in the center of the retina. The strange thing is: We don’t notice anything from this constant jerking. We see a stable, calm image because our vision system composes the visual impression from countless sharp still images, but suppresses the signals of the rapid movement phases. This also means that, as many experiments have shown, we simply overlook very short visual stimuli that happen in one of these saccades. “But how this saccadic suppression works is still controversial,” said Saad Idrees from the University of Tübingen and his colleagues.
Retina or movement signals?
There are currently two competing hypotheses to explain the saccadic stimulus suppression. One assumes that this effect is closely linked to eye movement: According to this hypothesis, the nerve signals that control the eye muscles are also supposed to coordinate the optical suppression of movement impressions. “From this popular point of view, the motor commands for eye movement are a necessary prerequisite for saccadic suppression – the whole thing would therefore be a movement-related signal,” the researchers explain. But there is another hypothesis that the retina itself causes this selective stimulus suppression – regardless of eye movements. Accordingly, the retina automatically recognizes from the image sequences registered by its sensory cells that the images are “blurred” and corrects this – similar to an image stabilizer in a digital camera.
To find out which hypothesis is correct, Idrees and his colleagues conducted several experiments. In one, the test subjects should look at pictures of coarse or fine surfaces and follow a rapidly and rapidly moving marker point. This created controllable saccade movements of her eyes. At various times within this saccade, the researchers faded in an extremely short, bright point of light at one point in the texture and asked the test subjects to indicate this when they saw it. The times when the participants did not see the point allowed them to determine when the Saccadic suppression began and how long it lasted.
The images themselves control the correction
The experiments showed that the brief “blindness” created by the Saccadic suppression was clearly detectable in all cases. However, their duration depended on the texture: the coarser the pattern, the earlier the suppression started and the longer it lasted. “The fact that the strength and length of the suppression depended on the textures depicted can only mean that the trigger must be of a visual nature,” says working group leader Ziad Hafed from the University of Tübingen. “The suppression clearly depends on the visual images.” But how exactly does this happen? To find out, the scientists conducted electrophysiological experiments with the retina of mice and pigs in culture. Using electrodes, they derived the signals that the sensory cells emitted when a high-contrast texture was quickly moved back and forth in front of the retina. Instead of the eyes, the texture performed the saccades. The measurement results confirmed that the retina already corrects the “wobble” of the image and suppresses the stimuli from the movement phase.
The visual signal is therefore already suppressed when it leaves the eye. “The retina therefore directly contributes to our stable visual impression,” says Idrees ’colleague Thomas Münch. “It recognizes that the world is rushing by and regulates the sensitivity for a short time.” Accordingly, the results largely support the second hypothesis. However, with one small restriction: When the researchers subjected their subjects to a counter test, in which the texture moved but not the eyes, there were subtle differences from the previous attempt: The saccadic suppression was longer without eye movements than with. The scientists conclude that the selective “jerk correction” of our vision originates from the retina and is independent of the eye muscles and their signals. But the eye movements cause a kind of additional “fine tuning” by influencing the length of the suppression. Hard and his team could now have resolved the dispute over the mechanism behind the Saccadic suppression.
Source: Saad Idrees (University of Tübingen) et al., Nature Communications, doi: 10.1038 / s41467-020-15890-w