A study published in Nature Communications by researchers at the Institute of Science and Technology Austria (ISTA) examined the development of neural connections in the CA3 region of the mouse hippocampus. The findings indicate that the network of connections among CA3 pyramidal neurons becomes sparser and more structured over time, rather than denser.
Methodology and Key Findings
The research team, led by Professor Peter Jonas, analyzed mouse brain tissue at three distinct developmental stages: shortly after birth (postnatal days 7–8), during adolescence (days 18–25), and in adulthood (days 45–50). Using patch-clamp techniques, advanced microscopy, and laser-based methods, the scientists measured electrical signals and observed neuronal connections.
The study reported the following observations:
- In newborn mice, the CA3 network was dense, with a high number of seemingly random connections.
- By adolescence, the network had become more refined and organized.
- In adulthood, the network was streamlined and sparser.
- In young brain tissue, a single input could trigger a neuron to fire. In mature networks, multiple inputs were typically required for a neuron to fire.
"The system is not a tabula rasa, as we thought originally... Rather, it starts out as a tabula plena [full slate] and then becomes sparser and specifically connected." — Peter Jonas, ISTA
The researchers propose a "pruning model" of development, where the brain begins with an excess of connections and subsequently eliminates unnecessary ones, rather than building new connections over time. The authors suggest that the initial overconnectivity may be genetically programmed, with experience later refining the wiring.
Implications for Memory and Development
The findings challenge the idea that the hippocampus begins as a "blank slate" (tabula rasa). The researchers describe the initial state of the region as a "full slate" (tabula plena). The early, easily triggered neuronal firing results in overlapping activity patterns. Consistent with prior behavioral studies, the authors note that young animals form less specific contextual memories (e.g., fear generalization) compared to adults.
"There is plenty of developmental psychology work that suggests that memory becomes more specific with age. And so it's kind of interesting that now, at the circuit level, we're also seeing that the connectivity patterns are becoming sparser." — Hauður Freyja Ólafsdóttir, Donders Institute
Additional Notes
Whether these findings apply to human brains is currently unknown. Regarding the possibility of prenatal experiences leaving lasting neural traces, Ólafsdóttir stated they likely leave a trace but may not resemble mature hippocampal memories, while Jonas considered them more likely a byproduct of development than true memories.