The study was led by Milan Becker, Nimrod Leberstein, and Dr. Mark Shein-Idelson, researchers in the Department of Neurobiology and the Sagol School of Neuroscience at Tel Aviv University. The study was published in the prestigious journal Science Advances.

A Shared Ancestry

The researchers explain that reptiles and mammals diverged from a common ancestor approximately 320 million years ago. Since that time, the mammalian brain and the cerebral cortex in particular — has undergone dramatic development, becoming complex, large, and folded. The reptile brain, by contrast, is regarded as simpler and more like the common ancestor of reptiles and mammals. Therefore, when a sophisticated computational mechanism in mammals is discovered also in the brain of a turtle, it suggests that this mechanism already existed in the brains of the ancestral amniotes – the first animals that completed the move onto land.

Research team (Left to right): Milan Becker, Nimrod Leberstein & Dr. Mark Shein-Idelson.

How the Turtle Brain Sees

In the study, the researchers focused on the turtle’s dorsal cortex, a region considered an evolutionary homolog of the mammalian cerebral cortex. Using neural recordings in awake animals, along with eye-movement tracking, the researchers examined how the turtle brain responds to repeatedly presented visual stimuli compared with “deviant” stimuli that appear in unexpected locations in the visual field.

Dr. Shein-Idelson: “The truly surprising result emerged when we examined what happens when the turtle moved its head or eyes. Such movements shift the image on the retina and can create ‘confusion’ in the visual system. Yet in turtles, the response to both the deviant and the regular stimulus remained consistent, despite frequent changes in the viewing angle. In simple terms, the turtle’s brain ‘understands’ that something new has occurred in the environment, even if the image is seen from a different angle and no longer falls on the exact same spot on the eye.”

The researchers also found that the turtle’s self-generated movements, such as shifts of the head or eyes, hardly elicit any brain response, even though they substantially alter the image received by the eye. In contrast, a small but unexpected change in the external environment strongly activates the brain. This indicates an ability to distinguish between stimuli resulting from self-motion and new information that requires attention.

Rethinking Brain Evolution

According to the researchers, these findings change the way we understand brain evolution. Until now, it was believed that view invariance is hierarchically computed as information travels from low to high visual areas as observed in monkeys and humans. The new study presents a different picture: even in the brain of early terrestrial vertebrates with a simple cortex, like those of the turtle’s ancestors, there already existed an ability to detect important events in the environment invariantly of viewing angle.

The researchers believe that this ability helped animals understand their spatial environment, learn, and survive complex terrestrial environments. Remarkably, even without a large and folded cerebral cortex, turtles possess a smart system capable of recognizing when something truly important is happening around them.

Dr. Shein-Idelson concludes: “This study demonstrates how the brains of turtles offer a unique window into the evolutionary past. Because turtles and mammals diverged from a common ancestor hundreds of millions of years ago, the discovery of advanced brain mechanisms in turtles suggests that these abilities either evolved hundreds of millions of years ago or convergently evolved due to similar environmental pressures in both lineages. The findings suggest that the ability to detect new and important occurrences in the environment, without being influenced by self-generated head and eye movements, is one of the cornerstones upon which the cortex evolved and points to the importance of this essential computation.”