Shared symmetry detection with divergent temporal dynamics in marmosets and humans

Symmetry is a fundamental language of the visual world, a pattern our brains seem wired to recognize instantly. It is the mirror image of a face, the balanced wings of a butterfly, or the repeating tiles on a floor. For decades, scientists have known that the human brain has a dedicated network for spotting these patterns, lighting up in specific areas at the back of the head when we see them. But a central question in the biology of how we think has remained unanswered: is this ability a special human invention, or is it a shared trait inherited from our distant primate ancestors? To find out, researchers needed to look inside the brains of other primates, but doing so without invasive surgery has been a major hurdle. While previous studies in monkeys relied on brain scans that show where blood flows, they could not capture the split-second electrical timing of how the brain processes a pattern.

A team of researchers has now bridged this gap by comparing how humans and common marmosets—a small, New World monkey—react to visual symmetry. They focused on two distinct types of patterns: reflection, which is like a mirror image, and rotation, where a shape repeats as it turns. By using a non-invasive method that records electrical signals from the scalp, they discovered that both species share a rapid, automatic ability to detect mirror symmetry. However, when it came to rotation, the two species diverged. While humans maintained a steady, sustained electrical response to rotating patterns, the marmosets' brains responded only briefly before settling down. This finding suggests that the basic, fast machinery for spotting symmetry is ancient and shared across primates, but the more complex, lingering processing that humans use to fully elaborate on these patterns may be a later development.

To make this comparison, the scientists had to overcome a significant technical challenge: recording clear brain signals from a monkey without surgery that would damage the brain or require daily electrode placement. They designed a custom, flexible grid of thirty-two tiny sensors that was surgically attached directly to the skull of three adult male marmosets. This device, which weighed only six grams, acted as a permanent window into the brain's electrical activity, allowing the animals to be tested repeatedly over many sessions. The marmosets were trained to sit calmly in a chair while watching a screen, where they were rewarded with a drop of juice for staying still. At the same time, a group of thirteen human volunteers watched the exact same images on a screen while wearing a standard cap of sensors.

The images shown to both groups were carefully crafted textures based on mathematical rules of symmetry. Some patterns repeated by mirroring themselves across a line, known as double reflection. Others repeated by rotating around a center point, known as four-fold rotation. To ensure the brain was reacting to the symmetry itself and not just random changes in the picture, the researchers alternated these symmetrical images with scrambled versions that looked similar but lacked the pattern. They measured the brain's response by looking for a rhythmic electrical pulse that matched the speed at which the images flickered. Because the brain responds differently to the symmetry part of the image versus the simple change of the image, the researchers could separate the two signals.

The results revealed a striking similarity in how the two species handled mirror symmetry. In both the marmosets and the humans, the brain produced a strong, steady electrical signal that lasted as long as the mirrored pattern was on the screen. This signal was strongest over the back of the head, in the visual processing areas, and its timing and strength were nearly identical across the species. This suggests that the ability to detect reflection is a conserved trait, likely evolving hundreds of millions of years ago to help primates recognize faces, bodies, and other living things in their environment.

However, the story changed when the researchers looked at the rotating patterns. While the human brain continued to hum with a steady electrical rhythm as long as the rotating pattern was visible, the marmoset brain responded differently. The monkeys' brains fired a quick, sharp signal when the rotating pattern first appeared, but this signal faded away quickly, leaving the brain quiet for the rest of the time the image was on the screen. This difference was specific to the rotating patterns; the mirror patterns still produced the long, steady response in the monkeys. The researchers ruled out the possibility that this was due to the monkeys not paying attention or moving their eyes, as the patterns were large and the eye movements were tracked and controlled.

The study points to a specific divergence in how primate brains process visual information. The initial, fast detection of symmetry appears to be a shared, feedforward mechanism that works the same way in both species. But the ability to maintain a sustained representation of that symmetry, particularly for more complex rotations, seems to rely on a different kind of brain activity that is robust in humans but weaker in marmosets. This sustained activity likely involves signals looping back and forth between different parts of the brain, a process that may be less developed in marmosets than in humans or their close relatives, the macaques.

By establishing that marmosets can be used to study these complex visual processes with the same precision as humans, the study opens a new door for neuroscience. The marmoset's brain is smooth and accessible, unlike the folded brains of macaques where these visual areas are buried deep inside. This means that in the future, scientists can combine these surface recordings with direct, deep measurements inside the brain to understand exactly how these circuits work. The findings confirm that while we share a common, ancient foundation for seeing symmetry, the human brain has added a layer of sustained processing that allows us to hold onto and elaborate on these patterns in ways that other primates may not.