Here’s what you’ll learn when you read this story:

• For a long time, people have debated whether our brains are blank at birth or are already processing large amounts of information.
• By studying mice throughout their lives, scientists revealed that a network of neurons in the hippocampus starts out dense and chaotic, but later thins out and becomes more ordered.
• More research needs to be done on human brains, but it’s clear that even though most of us don’t remember ourselves as babies, our brains were working furiously.

Are we born with brains that are essentially blank slates, tabula rasas that neurons only begin to write on after birth? Or does our brain function emerge more like a tabula plena, a full slate that’s already been written on and continues to be overwritten?

When a research team—led by neuroscientists Peter Jonas and Victor Vargas-Barroso of the Institute of Science and Technology Austria (ISTA)—set out to answer this question, they decided to focus on the hippocampus. It’s the primary region of the brain linked to forming memories, and is also essential to learning and spatial recognition. The scientists specifically zeroed in on the network of CA3 neurons exclusive to the hippocampus, which have the plasticity to encode, store, recall, and update memories.

The CA3 neural network is thought to store large amounts of information through synapses—the spaces between nerves through which messages are transmitted—that are highly plastic and easily adapt to change. Previous research has suggested that these neurons are spread out rather than densely packed, but how they connect to each other after birth is still being investigated. Under one hypothesis—the tabula rasa model—there would be few connections between neurons to begin with, and synapses would keep forming over time. The opposite is true of the pruning model, which suggests that the brain starts out full of synapses and the number gradually decreases, ultimately creating connections that are farther apart, but much more specific.

“Tabula rasa and pruning models make different predictions of how connectivity will change over developmental time,” Jonas and Vargas-Barroso said in a study recently published in Nature Communications. “Synaptic connectivity in mature hippocampus and neocortical networks is not random […] but how [it] is generated is not clear.”

To see how CA3 synapses develop from birth to adulthood, the researchers studied mice soon after birth (7-8 days old), during adolescence (18-25 days old), and in adulthood (45-50 days old)—periods before, during, and after the time when the hippocampus shows the highest level of plasticity. Using the patch-clamp technique allowed for the precise recording and measurement of electrical signals passing through neurons, which made it possible to measure signals when they reached different parts of the neurons, from one extreme (presynaptic terminals, which are the ends of neurons that convert electrical signals to chemical signals by releasing neurotransmitters) to the other (dendrites, the branched extensions that receive signals and send them to the soma, or body, of a neuron).

When the team analyzed the recordings, they found that mice were born with a vast abundance of connections between CA3 neurons. Those connections decreased as the animals matured, with CA3 synapses gradually becoming more structured and less random. Both observations support the pruning model, indicating that the CA3 network starts from a tabula plena state rather than a tabula rasa state. The researchers also found that individual synapses were surprisingly strong in young mice (capable of triggering spikes on their own), whereas in mature animals, many weaker inputs had to combine simultaneously to fire a neuron. Microscopic analysis of the same neurons further supported the pruning theory: axons grew shorter and developed fewer branch points over time, while dendrites grew longer and developed greater density.

So, it seems the neonatal brain—at least, in mice—is more tabula plena than tabula rasa. From birth to adulthood, the CA3 neural network in the hippocampus went from dense and random to more spaced out and structured. Mature CA3 neurons also fire less often than immature ones. Whether the human brain undergoes the same processes, however, remains to be seen, since the mechanisms that drive synapse pruning are still not well understood on a cellular or molecular level.

“These changes have been interpreted as a transition of hippocampal higher-order computations,” the researchers said. “Such changes could be related to transitions from dense [and] random to sparse [and] structured CA3 connectivity. Direct testing of these hypotheses will require more work in the human hippocampus.”

You may not remember anything from infancy, but that doesn’t necessarily mean your brain was empty back then.