The hippocampus plays an integral role in our ability to remember aspects of our daily lives, such as who we meet and what happened to us yesterday. Situated in the temporal lobe the hippocampus is recognized as one of the most important factors for the formation of social memory – an archive from which specific memories can be retrieved. Researchers at Columbia University Medical Center (CUMC) have now been able to pinpoint the exact part of the hippocampus that is responsible for social memory.
In 1953 it first became apparent that the hippocampus played a part in social memory. When Henry Molaison (HM) went in for an operation to try to alleviate his severe epilepsy surgeons removed part of his hippocampus, resulting in him being unable to form new memories. Molaison’s botched operation opened up a treasure trove to those studying memories and he is largely responsible for all we know today about memory formation (If you want to know more about HM´s life check out my post ‘The most important man in psychology?’). As time has passed since HM´s tragic case scientists have been trying to deduce exactly which part of the hippocampus is the focal point of social memory.
Researchers at CUMC may well have found the answer. Current research shows that the hippocampus can be divided into specific areas, each responsible for an exact function. Area CA1 is responsible for all forms of memory, whilst CA3 enables us to recall memories from partial clues. Area CA2 has until now remained a bit of a mystery. Some studies have suggested that CA2 is involved in social memory, especially as this region has a high response to a hormone that is responsible for social behaviours.
Lesions in the hippocampus restrict social memory formation not only in humans but rodents as well, making them the perfect lab rat so to speak. CUMC decided to test if the CA2 is responsible for social memory by creating a transgenic mouse, in which the neurons active in the CA2 can be inhibited in adults. When the neurons were inhibited, essentially shutting down the CA2 region, the mice were given a series of behavioral tests. Frederick L. Hitti, author of the paper published in Nature reporting the study commented on the results, “Normally, mice are naturally curious about a mouse they’ve never met; they spend more time investigating an unfamiliar mouse than a familiar one. In our experiment, however, mice with an inactivated CA2 region showed no preference for a novel mouse versus a previously encountered mouse, indicating a lack of social memory.”
To test that the transgenic mice suffered no serious inhibitions in social conduct from normal mice that may have affected the results, the mice were given two separate novel object recognition tests. The mice with the CA2 neurons inhibited showed a normal preference for an object they had not previously seen, demonstrating that the CA2 transgenic mice did not have a lack of interest in new objects. Researchers also tested if an inability to form social memories may be caused by problems in the mices’ sense of smell, which is crucial for their social interaction. However, the results showed no restriction in ability to discriminate social or non-social odours. So it would appear that the results are valid and that the CA2 area is indeed responsible for social memories.
How can this help those who are suffering from damage to the hippocampus or those who suffer from social disorders? With a high number of psychiatric disorders linked with problematic social behaviors the results put forward the possibility that CA2 dysfunction can be the cause, or at least a partial cause, of these behavioral changes. By targeting the CA2 area new treatments may be found for social disorders. Already it is known that schizophrenics have a decreased number of inhibitory neurons in the CA2, so with evidence trending towards the CA2 as a possible cause surely it is only a matter of time till a treatment can be discovered that targets the CA2 and reduces the suffering of those with debilitating social disorders.