Same visual system for all primates

The smallest primate in the world reveals the incredible preservation of our visual system through millions of years of evolution.

The gray mouse lemur, the smallest species of primate, has excellent eyesight. More than a fifth of its cerebral cortex is devoted to visual processing to accommodate enough pixel processing units. © UNIGE / Huber

Primates process visual information in front of their eyes, similar to pixels on a digital camera, using small computing units located in the visual cortex of their brains. In order to understand the origins of our visual abilities, scientists from the University of Geneva (UNIGE), in collaboration with the Max Planck Institute in Gottingen and the National Museum of Natural History in Paris, have now examined whether these computation units they adapt to large differences in size between primates. Madagascar’s gray mouse lemur (microcebus murinus) is one of the smallest and weighs just 60 grams. In a study published in the journal Current Biology, scientists compared the mouse lemur’s visual system to that of other primates and found that the size of these visual processing units is identical in all primates, regardless of their body size. As the mouse lemur is a very special species, sharing many traits with primate primates that evolved 55 million years ago, these findings suggest an incredible conservation of our visual system and underscore the importance of vision in our daily life. and that of our ancestors in the distant past.

For more than a century, the visual system of primates has been intensively studied. These studies found that, unlike other mammals such as rodents, visual information is processed by small, dedicated computing units located in the visual cortex. “As different primate species cover a wide range of sizes, we wondered whether this basic computation unit fits the size of the body or the brain. Is it simplified or miniaturized, for example, in the smallest primate in the world, the gray mouse lemur, ”asks Daniel Huber, professor in the Department of Basic Neuroscience at UNIGE’s Faculty of Medicine?

Don’t worry about the size

To answer this question, the mouse lemur’s visual system was studied using an optical brain imaging technique. The lemurs were presented with geometric shapes representing lines of various orientations and the activity of neurons that respond to visual stimuli was imagined. Repeating these measurements gradually made it possible to determine the size of the minimum processing unit. “We expected to see a small unit, proportional to the small size of the mouse lemur, but our data revealed that it measures more than half a millimeter in diameter,” says Daniel Huber.

In collaboration with Max Planck researchers, Huber compared hundreds of these units captured in the small brain of mouse lemur with data obtained for the visual circuits of other much larger primate species. The team made a startling discovery: Not only was the core processing unit nearly identical in size in the 60-gram mouse lemur, as in larger monkeys like macaques weighing around seven kilograms, or even in larger primates like we humans.

They also found that the way the units are arranged across the brain was totally indistinguishable, following the same rules with mathematical precision. The researchers also found that the number of nerve cells per visual unit was nearly identical in all primates studied so far.

The Göttingen physicist Max Planck Fred Wolf who had pointed out that universal mathematical principles can govern the evolution of the visual system ten years ago is amazed by the degree of invariance: “55 million years of separation in different continents is a very long evolutionary path to go through. I would have expected a mix of general similarities and characteristic differences between species in these neural modules. But the fact of the matter is simply: it’s practically impossible to tell them apart. “

The visual circuits are powerful and incompressible

These findings then provide insight into the origins of the primate view. First of all, the fact that this unit is so well preserved suggests that it probably evolved very early in primate history, indicating that when it comes to forming vision our primate ancestors had similar visual abilities to ours from the very beginning.

Second, the discovery by UNIGE scientists and their collaborators reveals that this part of the visual system cannot be compressed or miniaturized. A fixed number of neurons therefore seems necessary to ensure optimal functionality. “For tiny primate species with excellent eyesight, such as the mouse lemur, the visual system must therefore be relatively large, relative to the size of the entire brain, to accommodate enough visual processing units,” says the neuroscientist from Geneva. In fact, more than a fifth of this lemur’s cerebral cortex is devoted to visual processing. By comparison, vision-related neural circuits take up just 3% of the human brain.

“This study also highlights the crucial importance of preserving the habitat of primate species such as the mouse lemur, particularly in the forests of Madagascar. These habitats are disappearing at an alarming rate, bringing with them precious keys to understanding our origins, “concludes Daniel Huber.