How the retina synchronizes different visual signals regardless of their speed

The human brain builds mental representations of the world based on the signals and information detected via the human senses. While we perceive simultaneously occurring sensory stimuli as being synchronized, the generation and transmission speeds of individual sensory signals can vary greatly.

Researchers at the Institute of Molecular and Clinical Ophthalmology Basel (IOB), University of Basel and Eidgenossische Technische Hochschule (ETH) Zurich recently carried out a study aimed at better understanding how the human visual system achieves this synchronization, regardless of the speed at which visual signals travel. Their paper, published in Nature Neuroscience, reports a previously unknown mechanism through which the retina synchronizes the arrival times of different visual signals.

“We can see because photoreceptors in the retina at the back of our eyes detect light and encode information about the visual world in the form of electrical signals,” Felix Franke and Annalisa Bucci, senior author and first author of the paper, respectively, told Medical Xpress.

“The retina needs to send these signals to the visual areas of the brain, where it achieves the axons of retinal ganglion cells that connect each part of the eye to the brain. However, our vision is not uniform across the visual field. Only in a small area in the very center of our vision—the fovea—can we see sharply, read, and recognize faces. Axons cannot cross over the fovea because they would blur our central high-resolution vision, so they need to bend around and avoid that region.”

This recent study was inspired by a simple anatomical observation, namely that while axons in the retina cannot cross the fovea (i.e., small specialized region of the retina that supports detailed vision and the detection of color), visual signals picked up by different photoreceptors leave the eye via different pathways. As the pathways followed by the signals also vary in length, the researchers tried to determine how the signals are kept in sync, enabling a smooth vision of the world.

“The idea behind our study was simple, but the implications touched on how the brain preserves precise timing from the very first stages of sensory processing,” said Franke. “Our goal was to find out whether the retina itself helps coordinate the timing of visual signals before they even reach the brain.”

To better understand how the brain synchronizes sensory information, Franke, Bucci and their colleagues used a variety of experimental techniques. Since the timing of sensory signals is a network effect, to study it, researchers must collect both precise local measurements of the times when the signals arrive and the global organization of the network.

“To bridge these different scales—from microsecond-precise measurements of traveling electrical signals within individual axons to reconstructing the wiring pattern of axons across the entire human eye—we relied on human organ donations from which we could retrieve entire eyeballs,” explained Franke.

“A major achievement was to keep this tissue in such a high quality during the experiments, that the retina was functionally still active, i.e., the neurons in the retina of these eyes were still sending signals,” said Bucci.

In their experiments, the researchers used high-density microelectrode arrays, devices that record videos of electrical fields with a temporal resolution of 20kHz, to record the electrical signals leaving the fovea of human participants. Using the same technique, they could also determine the speed at which these signals were traveling, which were found to vary significantly depending on the length of the axons carrying them.

“Because the speed of axons can be influenced by the diameter of the axons, we used transmission electron microscopy—a technology that can measure nanometer-precise anatomical details—to estimate the diameter of the axons in different parts of the human retina,” said Bucci.

“Using labeling techniques and high-resolution microscopy, we made images of the axonal pathways across the entire human eye. We then constructed a model of the human eye and used mathematical theory to understand the precise details of the wiring pattern and the lengths of each individual axon.”

By collectively analyzing all the data they collected, the researchers were able to relate the length of axons with their thickness and the speed at which they transmitted signals. This allowed them to unveil a compensatory mechanism that takes place in the human eye, which appears to support the synchronization of visual information.

“We showed that longer axons are thicker and therefore transmit faster to compensate for their increased length,” explained Franke. “To see if this anatomical fine-tuning of transmission speed is important for human vision—i.e., if it has perceptual consequences—we employed yet another technology.”

The additional technology used by the researchers is known as adaptive optics scanning laser ophthalmoscopy (AOSLO). Franke, Bucci and their colleagues teamed up with a lab led by Wolf Harmening in Bonn, who is highly skilled in the experimental application of this technique.

“The AOSLO technique allowed us to image individual photoreceptors in the back of the eye of healthy participants,” said Franke.

“We also used this technique to stimulate these individual photoreceptors with brief flashes of light and asked the participants to press a button as fast as possible after seeing the flash. We showed that human reaction times to single photoreceptor stimulation are remarkably uniform across the fovea—a result that is only possible if the signals from different parts of the fovea are precisely synchronized.”

Overall, the findings of this recent study suggest that the human retina employs a specific mechanism to ensure that visual signals remain in sync before they even leave the eye. This finding is particularly significant because axons in the human retina are unmyelinated (i.e., they lack myelination).

“Myelination is a fat layer that the brain wraps around axons, providing electrical insulation and massively increasing their transmission speeds,” explained the authors. “It is thought that myelination is one of the major ways in which the brain influences and coordinates axonal transmission speeds. However, myelination is visually opaque (which is the reason why the white matter in the brain is white) and would obfuscate our vision.

“This discovery has two important implications: First, throughout the nervous system, unmyelinated axons may contribute substantially to temporal synchronization. Second, it suggests that the retina plays a more active role in fine-tuning temporal precision than previously thought.”

The researchers hope that their findings will inform new studies aimed at better understanding the new mechanism they uncovered. In the future, their efforts could enrich the present understanding of early sensory processing, while also potentially informing the treatment of diseases or medical conditions that impact visual processing.

“Our next step is to explore what happens when this finely tuned system breaks down,” said Bucci. “Now that we’ve built a model of the retinal nerve fiber layer—showing how axon length, thickness, and conduction speed are matched to preserve timing—we can begin to ask how disease might disrupt that balance.

“In glaucoma, for instance, retinal ganglion cells with longer axons often degenerate first. These cells are more vulnerable because longer axons require more energy to maintain, depend on efficient long-range transport, and are subject to greater mechanical strain at the optic nerve head—especially where they bend to exit the eye.”

As part of their future research, Franke and Bucci also hope to shed light on how the synchronization mechanism they discovered develops or, in other words, how the retina “knows” the speed at which individual signals should travel. In addition, they could try to determine whether the nervous system employs any other similar synchronization strategies.

© 2025 Science X Network

The human brain builds mental representations of the world based on the signals and information detected via the human senses. While we perceive simultaneously occurring sensory stimuli as being synchronized, the generation and transmission speeds of individual sensory signals can vary greatly.

Researchers at the Institute of Molecular and Clinical Ophthalmology Basel (IOB), University of Basel and Eidgenossische Technische Hochschule (ETH) Zurich recently carried out a study aimed at better understanding how the human visual system achieves this synchronization, regardless of the speed at which visual signals travel. Their paper, published in Nature Neuroscience, reports a previously unknown mechanism through which the retina synchronizes the arrival times of different visual signals.

“We can see because photoreceptors in the retina at the back of our eyes detect light and encode information about the visual world in the form of electrical signals,” Felix Franke and Annalisa Bucci, senior author and first author of the paper, respectively, told Medical Xpress.

“The retina needs to send these signals to the visual areas of the brain, where it achieves the axons of retinal ganglion cells that connect each part of the eye to the brain. However, our vision is not uniform across the visual field. Only in a small area in the very center of our vision—the fovea—can we see sharply, read, and recognize faces. Axons cannot cross over the fovea because they would blur our central high-resolution vision, so they need to bend around and avoid that region.”

This recent study was inspired by a simple anatomical observation, namely that while axons in the retina cannot cross the fovea (i.e., small specialized region of the retina that supports detailed vision and the detection of color), visual signals picked up by different photoreceptors leave the eye via different pathways. As the pathways followed by the signals also vary in length, the researchers tried to determine how the signals are kept in sync, enabling a smooth vision of the world.

“The idea behind our study was simple, but the implications touched on how the brain preserves precise timing from the very first stages of sensory processing,” said Franke. “Our goal was to find out whether the retina itself helps coordinate the timing of visual signals before they even reach the brain.”

To better understand how the brain synchronizes sensory information, Franke, Bucci and their colleagues used a variety of experimental techniques. Since the timing of sensory signals is a network effect, to study it, researchers must collect both precise local measurements of the times when the signals arrive and the global organization of the network.

“To bridge these different scales—from microsecond-precise measurements of traveling electrical signals within individual axons to reconstructing the wiring pattern of axons across the entire human eye—we relied on human organ donations from which we could retrieve entire eyeballs,” explained Franke.

“A major achievement was to keep this tissue in such a high quality during the experiments, that the retina was functionally still active, i.e., the neurons in the retina of these eyes were still sending signals,” said Bucci.

In their experiments, the researchers used high-density microelectrode arrays, devices that record videos of electrical fields with a temporal resolution of 20kHz, to record the electrical signals leaving the fovea of human participants. Using the same technique, they could also determine the speed at which these signals were traveling, which were found to vary significantly depending on the length of the axons carrying them.

“Because the speed of axons can be influenced by the diameter of the axons, we used transmission electron microscopy—a technology that can measure nanometer-precise anatomical details—to estimate the diameter of the axons in different parts of the human retina,” said Bucci.

“Using labeling techniques and high-resolution microscopy, we made images of the axonal pathways across the entire human eye. We then constructed a model of the human eye and used mathematical theory to understand the precise details of the wiring pattern and the lengths of each individual axon.”

By collectively analyzing all the data they collected, the researchers were able to relate the length of axons with their thickness and the speed at which they transmitted signals. This allowed them to unveil a compensatory mechanism that takes place in the human eye, which appears to support the synchronization of visual information.

“We showed that longer axons are thicker and therefore transmit faster to compensate for their increased length,” explained Franke. “To see if this anatomical fine-tuning of transmission speed is important for human vision—i.e., if it has perceptual consequences—we employed yet another technology.”

The additional technology used by the researchers is known as adaptive optics scanning laser ophthalmoscopy (AOSLO). Franke, Bucci and their colleagues teamed up with a lab led by Wolf Harmening in Bonn, who is highly skilled in the experimental application of this technique.

“The AOSLO technique allowed us to image individual photoreceptors in the back of the eye of healthy participants,” said Franke.

“We also used this technique to stimulate these individual photoreceptors with brief flashes of light and asked the participants to press a button as fast as possible after seeing the flash. We showed that human reaction times to single photoreceptor stimulation are remarkably uniform across the fovea—a result that is only possible if the signals from different parts of the fovea are precisely synchronized.”

Overall, the findings of this recent study suggest that the human retina employs a specific mechanism to ensure that visual signals remain in sync before they even leave the eye. This finding is particularly significant because axons in the human retina are unmyelinated (i.e., they lack myelination).

“Myelination is a fat layer that the brain wraps around axons, providing electrical insulation and massively increasing their transmission speeds,” explained the authors. “It is thought that myelination is one of the major ways in which the brain influences and coordinates axonal transmission speeds. However, myelination is visually opaque (which is the reason why the white matter in the brain is white) and would obfuscate our vision.

“This discovery has two important implications: First, throughout the nervous system, unmyelinated axons may contribute substantially to temporal synchronization. Second, it suggests that the retina plays a more active role in fine-tuning temporal precision than previously thought.”

The researchers hope that their findings will inform new studies aimed at better understanding the new mechanism they uncovered. In the future, their efforts could enrich the present understanding of early sensory processing, while also potentially informing the treatment of diseases or medical conditions that impact visual processing.

“Our next step is to explore what happens when this finely tuned system breaks down,” said Bucci. “Now that we’ve built a model of the retinal nerve fiber layer—showing how axon length, thickness, and conduction speed are matched to preserve timing—we can begin to ask how disease might disrupt that balance.

“In glaucoma, for instance, retinal ganglion cells with longer axons often degenerate first. These cells are more vulnerable because longer axons require more energy to maintain, depend on efficient long-range transport, and are subject to greater mechanical strain at the optic nerve head—especially where they bend to exit the eye.”

As part of their future research, Franke and Bucci also hope to shed light on how the synchronization mechanism they discovered develops or, in other words, how the retina “knows” the speed at which individual signals should travel. In addition, they could try to determine whether the nervous system employs any other similar synchronization strategies.

© 2025 Science X Network