A mysterious hippocampal region encodes social memory

The hippocampus encodes the story of our lives: the what, when and where. Overall, we understand relatively well how the hippocampus processes information, but the function of one small subregion called CA2 has always been a mystery.

Now, scientists from Columbia University have developed a way to selectively label CA2 neurons with fluorescent proteins that glow in the dark. To gain an understanding of CA2 function, they first mapped out the neuronal connections leading into and out of this enigmatic brain area with viral tracers. They found that anatomically, CA2 acts as a central computational “hub”: it receives incoming data from diverse brain areas, integrates these synaptic inputs and then transmits the output to other areas within the complex hippocampal circuitry.

Surprisingly, unlike its neighboring areas, neurons in CA2 didn’t care for information encoding the “where”. Instead, they seemed to process input regarding the “who”. When researchers inactivated CA2 neurons in mice, these animals performed just as well as normal mice on tasks requiring spatial recognition or navigation. However, CA2-inactivated mice suffered from a serious case of “social amnesia”. Generally, mice prefer to interact with complete strangers rather than old acquaintances. Mice with inactivated CA2 neurons lost this preference for new companions, spending a similar amount of time with both old and new pals. Importantly, these mice had no trouble with their vision or smell.

Scientists think that CA2 neurons are important for remembering social interactions. In fact, humans with hippocampal lesions also have impaired social memory. In autism and schizophrenia – both neuropsychiatric diseases associated with changes in social behavior ­– researchers have also previously observed decreased CA2 function.

It’s too early to say that CA2 transmission misshapes cause social faux pas. However, the next time you reintroduce yourself to an old associate, laugh off the awkwardness and blame it on a CA2 hiccup – you might just be right.

Hitti FL, & Siegelbaum SA (2014). The hippocampal CA2 region is essential for social memory. Nature PMID: 24572357