A recent study on mice by MIT has revealed that memories in the brain are formed simultaneously in two different parts. It was previously believed that short-term memories are stored in the hippocampus region, some of which are later on transferred to the brain's cortex for long-term storage. Researchers believe that these findings, which contradicts the decades-old hypothesis, could apply to humans as well and might help understand Alzheimer's disease.

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According to a study of the neural circuits that underlie this process, memories are formed simultaneously in the hippocampus and the long-term storage location in the brain's cortex. However, the long-term memories remain "silent" for about two weeks before reaching maturity.

Susumu Tonegawa, the Picower Professor of Biology and Neuroscience and the lead author of the study, said that this and other findings in the paper provide a comprehensive circuit mechanism for consolidation of memory.

Beginning in the 1950s, studies of the famous amnesiac patient Henry Molaison (then known only as Patient H.M.) revealed that the hippocampus is essential for forming new long-term memories. Molaison's hippocampus was damaged during an operation, which was meant to help control his epileptic seizures. He was no longer able to store new memories after the operation, but he could still access some memories that had formed before the surgery. This suggested that long-term memories of specific events are stored outside the hippocampus. Scientists believed these memories are stored in the neocortex - the part of the brain also responsible for cognitive functions such as planning and attention.

The new findings, however, suggest that traditional theories of consolidation might not be accurate because memories are formed rapidly and simultaneously in the prefrontal cortex and the hippocampus on the day of training and then they go separate ways.

Paul Frankland, a principal investigator in the Neurobiology Laboratory at the Hospital for Sick Children in Toronto, stated that this paper clearly shows that from the get-go, engrams are formed in the prefrontal cortex and challenges the notion of the movement of the memory trace from the hippocampus to the cortex, while showing that these circuits are engaged together at the same time.

However, further studies are needed to show whether memories fade completely from hippocampal cells or if some traces remain. Right now, the researchers can monitor engram cells only for about two weeks, but work is underway to adapt the technology to operate for a longer period.

This research was funded by the RIKEN Brain Science Institute, the Howard Hughes Medical Institute, and the JPB Foundation.