Spontaneous fluctuations in the brain’s internal state influence how ready the memory system is to store new information, according to a neuroscience study published in Nature Communications by researchers at the University of Tübingen. Led by Professor Andrea Burgalossi of the Institute of Neurobiology and the Werner Reichardt Center for Integrative Neuroscience (CIN), the research reveals that micro-states of alertness and arousal influence how mammalian brains form episodic memories.
Hippocampal Place Cells and Internal States
The research team used mice to investigate the neural mechanics of memory formation inside the hippocampus, a brain region essential for episodic memories. Mammalian hippocampi contain neurons known as place cells that represent experiences in the brain. According to Dr. Eduardo Blanco-Hernandez, a lead author of the study, observing these cells provides real-time insight into how memories form or reactivate.
Researchers recorded place cell activity while tracking the animals’ internal brain states during periods of immobility. Brains are dynamic systems that rapidly fluctuate between levels of alertness even at rest. “The internal brain state can be estimated from pupil size,” says CIN researcher and co-author Nicola Sartorato. “When we become more alert or engaged, our pupils dilate. The same is true in mice.”
The team discovered that these spontaneous fluctuations heavily influence which memory neurons fire. A specific subset of place cells activates during moments of heightened alertness. These changes also align with brain rhythms called theta oscillations, which strengthen or weaken depending on the internal state of the subject.
Did you know?
Pupil dilation serves as a reliable outward indicator of internal brain state changes in both mice and humans, signaling shifts between alertness and rest that govern neural activity.
Neural Plasticity and Memory Formation
Internal state fluctuations reflect windows of time when the brain becomes physically capable of change. “Plasticity is essential for memory because it is the process by which the brain physically stores experiences,” says Professor Burgalossi. Because episodic events happen only once without rehearsal, the brain requires fast, real-time mechanisms to capture and secure information.
To test whether internal states actively control these windows of readiness, the researchers stimulated individual memory-related neurons during specific states. This artificial stimulation successfully triggered new place-specific responses, generating fresh memory cells within the network. The hippocampus enters brief, natural periods where it readily rewires its connections.
Frequently Asked Questions
What brain region is responsible for forming episodic memories?
The hippocampus, located in the mammalian brain, is essential for forming episodic memories.
How do researchers track a mouse’s internal brain state?
Researchers track internal brain states by monitoring pupil size, as dilating pupils indicate increased alertness and engagement.
What are place cells?
Place cells are neurons located in the hippocampus that represent and map different environments in the brain.
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