Scientists found the brain doesn’t start blank, it starts full

Picture a completely empty sheet of paper. You begin writing on it, gradually filling it with information. This idea reflects the concept of tabula rasa, or the "blank slate."

Now imagine a page that already has marks on it. Any new information must fit around or replace what is already there. This represents tabula plena, or the "full slate."

This long-standing debate asks whether we begin life with everything prearranged or whether our experiences shape who we become. In biology, this question appears as the balance between genetic instructions and environmental influences that shape development.

The research team at ISTA applied this idea to the hippocampus, which is responsible for memory and spatial awareness. They wanted to understand how its internal network changes after birth and whether it behaves more like a blank slate or a full one.

Studying the Brain's Memory Network

The scientists focused on a key hippocampal circuit made up of CA3 pyramidal neurons. These cells are critical for storing and retrieving memories. They rely on plasticity, the brain's ability to adapt by strengthening or weakening connections or by altering structure.

ISTA alum Victor Vargas-Barroso studied mouse brains at three stages of development: early after birth (day 7-8), adolescence (day 18-25), and adulthood (day 45-50).

To examine how these networks function, he used the patch-clamp technique, which measures tiny electrical signals within specific parts of neurons, including presynaptic terminals and dendrites. The team also used advanced imaging and laser-based methods to observe activity inside the cells and to activate individual neural connections with precision.

From Dense and Random to Refined and Efficient

The findings revealed a surprising pattern. Early in development, the CA3 network is extremely dense, with connections that appear largely random. As the brain matures, this network becomes less crowded but more organized and efficient.

"This discovery was quite surprising," says Jonas. "Intuitively, one might expect that a network grows and becomes denser over time. Here, we see the opposite. It follows what we call a pruning model: it starts out full, and then it becomes streamlined and optimized."

Why the Brain Starts Full

Researchers are still exploring why this pattern occurs. Jonas suggests that beginning with a highly connected network may allow neurons to link up quickly, which is especially important in the hippocampus. This region must combine different types of information, including sights, sounds, and smells, into cohesive memories.

"That's a complex task for neurons," Jonas explains. "An initially exuberant connectivity, followed by selective pruning, might be exactly what enables this integration."

If the brain started as a true tabula rasa, with no built-in connections, neurons would first need to locate and connect with one another. That process could slow communication and reduce efficiency, making it harder to form memories effectively.

Overall, the findings suggest that the brain begins not as a blank slate, but as a richly connected network that becomes more precise over time by trimming away unnecessary links.