Summary: Is the brain born as a “blank slate” (tabula rasa) or a “full slate” (tabula plena)? Researchers found that when it comes to the hippocampus, the brain’s memory and navigation hub, nature chooses the full slate.
The study reveals that hippocampal networks actually start out incredibly dense and “noisy” after birth. Instead of building new connections to learn, the brain spends its development pruning and streamlining this initial abundance into a refined, efficient map.
Key Facts
- The Pruning Model: Contrary to the intuitive idea that brains get denser as we learn, the CA3 hippocampal network actually becomes sparser over time. It starts “full” and is then optimized through selective removal of connections.
- Exuberant Connectivity: In the early stages after birth (days 7–8 in mice), neuronal connections appear random and extremely dense. By adulthood (days 45–50), the network is far more structured and refined.
- The Integration Advantage: A “full slate” allows neurons to link different types of information, visual, auditory, and olfactory, immediately. If the brain started as a blank slate, neurons would be too far apart to “find” each other quickly enough to function.
- Precision Mapping: Using the patch-clamp technique and laser-based microscopy, researchers were able to measure tiny electrical signals at specific connection points, confirming that the “thinning out” of the network is what actually creates intelligence and memory capacity.
Source: ISTA
Imagine a blank sheet of paper in front of you. There’s nothing on it so you start writing, adding more and more information. This is the principle of tabula rasa—the “blank slate.”
It’s a different story when the sheet already contains marks: new information must be added to, or overwrite, what is already there. That describes tabula plena—the “full slate.”
At the heart of this philosophical concept lies a fundamental question: Is everything pre-set from the very beginning or do experiences shape who we become?
Biology reflects this controversy as well—between genes that provide the basic blueprint and environmental factors that sculpt the final organism.
Neuroscientists in the Jonas group at the Institute of Science and Technology Austria (ISTA) addressed precisely this question in the context of the hippocampus—the brain region that forms memories and guides spatial navigation. Specifically, they asked: How does the hippocampal network evolve after birth? Is it linked to tabula rasa or tabula plena?
First more, then less
The study focused on a central hippocampal network made up of interconnected CA3 pyramidal neurons. These cells store and recall memories through a process known as plasticity—the ability of neurons to constantly change, for example, by strengthening or weakening their connections or by reshaping their structure.
For his project, ISTA alum Victor Vargas-Barroso examined mouse brains at three developmental stages: early after birth (day 7–8), adolescence (day 18–25), and adulthood (day 45–50).
To analyze the networks, he applied the patch-clamp technique. This allows researchers to measure tiny electrical signals in specific parts of neurons—such as at their signal-sending ends (presynaptic terminals) or at the branching sites that receive signals (dendrites). In addition, advanced microscopy and laser-based techniques were used to observe processes inside the cells and to activate individual connections with high precision.
The results: Early on, the CA3 network is very dense, and the connections appear random. As the animals mature, however, the configuration shifts—the network becomes sparser but more structured and refined.
“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.”
An efficient network thanks to tabula plena?
Why this happens remains a matter of speculation. Jonas suspects that an initially widespread network allows neurons to connect quickly and efficiently—a crucial advantage in the hippocampus. This region does not just store visual, smell, or sound information—it links all these together.
“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, on the other hand, the network started as a true tabula rasa—with no preexisting connections—neurons would be too far apart and would need to ‘find’ one another first, making efficient communication nearly impossible.
Key Questions Answered:
A: Not smarter, but perhaps more “connected.” A baby’s brain is like a giant, unorganized block of marble. It has the potential for everything, but it lacks the specific “sculpture” of knowledge. Adulthood is the finished statue, leaner, more functional, and specialized for the real world.
A: It’s a survival strategy. By starting with a “full slate,” the hippocampus ensures that no matter what environment a creature is born into, the hardware is already there to start linking sights and smells. It’s much faster to delete a wrong connection than to grow a new one from scratch.
A: It’s a partnership. The genes provide the “tabula plena” (the full slate), but your experiences are the “chisel” that decides which connections stay and which are pruned. Without the environment, the brain would remain a chaotic, inefficient mess.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this memory and neuroscience research news
Author: Veronika Oleksyn
Source: ISTA
Contact: Veronika Oleksyn – ISTA
Image: The image is credited to Jose Guzman / Jonas group at ISTA
Original Research: Open access.
“Developmental emergence of sparse and structured synaptic connectivity in the hippocampal CA3 memory circuit” by Victor Vargas-Barroso, Jake F. Watson, Andrea Navas-Olive, Alois Schlögl & Peter Jonas. Nature Communications
DOI:10.1038/s41467-026-71914-x
Abstract
Developmental emergence of sparse and structured synaptic connectivity in the hippocampal CA3 memory circuit
Hippocampal CA3 pyramidal neurons (PNs) form the largest autoassociative network in the mammalian brain. Whether CA3–CA3 recurrent connectivity is genetically preconfigured or environmentally shaped during ongoing memory storage is currently unknown.
o address this question, we performed multicellular patch-clamp-based circuit mapping of up to eight CA3 PNs in the mouse hippocampus at multiple postnatal time points (P7–8, P18–25, and P45–50).
Here, we show that the hippocampal CA3 network undergoes a developmental transformation from local, dense, and random connectivity to a distributed, sparse, and structured configuration.
Thus, sparse and structured connectivity may emerge via experience-dependent mechanisms. In parallel, the strength of single synapses is downregulated; single synaptic events are sufficient to trigger postsynaptic spiking early in development, whereas spatial summation of several inputs is required at later time points.
Biologically inspired models of memory storage by Hebbian synaptic plasticity and retrieval via pattern completion suggest that developmental changes improve specific aspects of memory storage and retrieval.
Our results imply a developmental transformation of the neuronal code and the memory functions in the hippocampal CA3 network.
