How we learned to anticipate: The crucial role of the cerebellum in predicting the world

In 2018, the journal Springer Nature published a very curious clinical case : a 43-year-old woman suffered a stroke due to an abnormal tangle of blood vessels in her brain. To save her, doctors performed an embolization, a procedure that blocks the defective vessels to prevent further bleeding.

But after the procedure, something changed. The woman began to experience strange symptoms: she lost coordination, had difficulty concentrating and finding words, acted inappropriately (such as laughing at funerals or work meetings), and became impulsive, aggressive, and irritable. She even began experiencing hallucinations, such as seeing flamingos running around her house.

Her husband claimed that all this was new. Before the embolization, she was a successful lawyer with no psychiatric history. We know, thanks to famous cases like that of Phineas Gage (the worker who survived an iron bar piercing his frontal lobe), that personality depends on the brain. But here was something interesting: the damage was not in the cerebral cortex, but in the cerebellum.

It turns out that this "cauliflower," hidden beneath the occipital lobes, has historically been underappreciated because it was believed to only control movement coordination. However, growing evidence suggests a much broader role. In recent years, we've even seen headlines presenting the cerebellum as the new trophy that makes us human , taking some of the spotlight away from the frontal lobe.

But to understand why the cerebellum is so important, we must go back to its origins. It arose from more primitive structures called, rather ingeniously, “cerebellum-like structures.” These structures still exist in some primitive fish, such as sharks, rays, and electric fish.

Their function is key: they help differentiate between external and internal sensory stimuli. For example, sharks hunt by detecting the electrical fields of their prey, but there's a problem: they themselves also produce electrical fields that can interfere with those signals.

According to a study published in Behavioral Neuroscience in 2019, cerebellum-like structures solve this dilemma with a brilliant mechanism: an adaptive filter. This system uses a copy of internal motor commands to predict and cancel self-generated sensory information. In other words, they filter out internal noise and leave only relevant external information.

The cerebellum evolved from the duplication of these structures, allowing its predictive capacity to extend to new functions, such as improved coordination of movements. In fact, its development is closely linked to the emergence of jaws and paired fins in vertebrates, suggesting that its evolution was driven by the need for greater motor and sensory control.

As vertebrates developed more complex nervous systems and advanced cognition, their new functions also benefited from the cerebellum. According to a review published in Cerebellum in 2015, its role is to detect repetitive patterns, creating "internal models" that allow it to predict the future and anticipate what will happen next.

For example, when we listen to a familiar song, we can anticipate the next notes or lyrics effortlessly. This happens because the cerebellum has identified the sequence and built a model of how it sounds. If the melody changes unexpectedly, it adjusts its prediction without us being aware of it.

Its importance is also reflected in its anatomy. Although the human cerebellum represents only 10% of brain volume, it houses 80% of its neurons. Throughout mammalian evolution, it grew in proportion to the rest of the brain, but in apes, and especially in humans, its expansion was enormous. In our species, it is 31% larger than expected.

Interestingly, most of its projections are connected to areas of the cerebral cortex involved in cognitive tasks. A 2018 article published in the journal Frontiers in Cellular Neuroscience suggests that the cerebellum was key to the evolution of qualities that gave rise to humankind, such as tool use, language, and culture.

Millions of years ago, early humans began making stone tools, such as axes and knives. This task required fine motor coordination and the ability to predict how the stone would break when struck. Thanks to its ability to detect sequences and predict outcomes, the cerebellum allowed this technique to be refined over time.

Furthermore, more recent research has highlighted its role in social intelligence, especially in the more advanced stages of theory of mind. Imagine someone hides a toy in a drawer and then leaves the room. While they're gone, you move the toy to another location. When that person returns, where will they look for the toy? If you answer "in the drawer," you understand that the person has a "false belief." This type of reasoning, crucial for social interaction, depends on a specific region of the cerebellum called Crus I and II.

However, we're not the only ones with an extraordinary cerebellum. In dolphins, this structure is also exceptionally large relative to their total brain size, due to their need to process complex sensory information, especially auditory information. In particular, a region called the paraflocculus stands out, which is enormous in dolphins. Its function is to integrate sound information and transform it into precise motor responses, a key mechanism for echolocation, which allows them to "see" with sound.

Another mammal notable for its large cerebellum is the elephant . These animals have the largest cerebellum relative to brain size of any mammal studied so far. It accounts for about 18.6% of their brain mass, almost double that of humans, suggesting it plays a crucial role in their lives.

We know that the cerebellum is essential for fine motor control, and this is especially important in elephants due to the complexity of their trunks. This versatile and ultra-sensitive organ allows them to feed, drink, communicate, and manipulate objects with astonishing precision. Furthermore, elephants use infrasound to communicate over long distances, and their cerebellum may also play a key role in this.

If the cerebellum made us human, it also made elephants into elephants and dolphins into dolphins. And if we can now hold conversations, sculpt, and travel to space, it's because many millions of years ago, the first vertebrates learned to predict the future.