“Huh? I feel like I’ve seen this person before, but I can’t remember where.” Have you ever experienced a sudden feeling of familiarity when you see someone on the street? This might be because the hippocampal CA2 area of your brain has become active. The CA2 is a subregion within the hippocampus, and although there isn’t a vast amount of research on its functions, most studies suggest it might be involved in processing socially relevant information and memories.



This paper investigates how the CA2 region’s neural activity differs when mice interact with familiar versus novel mice. The study uses support vector machines (SVM), a method from machine learning, to analyze the neural data. They found that CA2 activity can identify which mouse is being interacted with. It also determines the location of the mouse, indicating that this brain area represents information related to identity and location, which are both important for social interactions.



Interestingly, the researchers found that while interacting with novel mice, CA2 neurons primarily processed information about identity and location, separately. However, when interacting with familiar mice, CA2 neurons could handle both identity and location information simultaneously (for example, Mouse A is on the left, Mouse B is on the right). This distinction likely arises because social memories are activated during interactions with familiar mice. Taking humans as an example, when we encounter strangers, we may only need to discriminate their identity; but with people we know well, we must retrieve various pieces of information related to that person, making the processing of information more specific and complex.

Reference
Boyle, L. M., Posani, L., Irfan, S., Siegelbaum, S. A., & Fusi, S. (2024). Tuned geometries of hippocampal representations meet the computational demands of social memory. Neuron, 112(8), 1358-1371.e9. https://doi.org/10.1016/j.neuron.2024.01.021