Where are memories stored?
There are three areas of the brain involved in the storage of memory: the hippocampus, the neo-cortex and the amygdala.
The hippocampus holds our episodic memories. These are memories from specific episodes in our lives, like the coffee we had with a friend last week.
How do we know this? In the 1950s, a patient named Henry Molaison had his hippocampus and part of the medial temporal lobe surgically removed to treat his epilepsy. Unfortunately after the surgery he was only able to remember recent events (episodic memories) for a few minutes, and was unable to permanently remember new information. Henry's memory became limited almost only to events that occurred years before his surgery, in the distant past. Interestingly, he was still able to improve his performance on motor tasks over days or weeks, but he had no memory of having ever encountered or practised those tasks. This indicated that the hippocampus and medial temporal region are not the sites of permanent memory storage.
Subsequent research suggests that over time information from multiple specific memories held in the hippocampus can be transferred to the neo-cortex, which overlies the hippocampus, in the form of general knowledge. Certain areas in the neocortex are important for storing semantic memory—factual knowledge—and knowledge about how this is used from day to day. Researchers think this transfer from hippocampus to neocortex happens when the two structures communicate during sleep.
Another important memory structure is the amygdala. The amygdala is important for attaching emotional significance to memories. Strongly emotional memories are difficult to forget—the permanence of these memories suggests that interactions between the amygdala, hippocampus and neocortex crucially determine how "stable" a memory is; that is, how likely it is to be retained over time.
In addition to modulating memories, the amygdala also appears to be the brain region where memories for fear are stored. For this reason, researchers including Professor Pankaj Sah and Dr Timothy Bredy believe that understanding how fear memories are formed in the amygdala may help in treating conditions such as post-traumatic stress disorder.
Concepts and definitions
Hippocampus – Brain structure located in the temporal lobe. Important for storing episodic and semantic memories, as well as for navigating space. Animal studies of hippocampal memory often test "spatial memories".
Neocortex – is the largest part of the cerebral cortex, the sheet of neural tissue that forms the outside surface of the brain, distinctive in higher mammals for its wrinkly appearance. In humans, the neocortex is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning and language.
Amygdala – Almond-shaped structure in the brain’s temporal lobe. Important for emotional processing.