By Gregory Brown, MD/PhD Candidate
Over my 20 years of schooling, I have taken many tests in my life (many, many tests). And I am always fascinated by how I am able to strengthen memories over the course days to years (or hours, depending on how much I am cramming).
I begin barely knowing the material, then I recognize it and realize I should have known that fact. And finally, I am so familiar with the material that I can spout off 5-10 related facts. This is the point where I become drastically less fun to be around. “Know-it-all” would be a euphemism.
The processes of forming memories has been a vigorous topic of research and two models (an old and new) have been proposed to help explain this crazy process we call memory.
The Standard Model of Memory
Much of the knowledge about the hippocampal role in memory is from a case study around 1955 of H.M. His really name is Henry Molaison and he was an assembly line worker with severe seizures. But his initials (H.M.) were used for his privacy and H.M. has become famous in memory science, although many people don’t even know his real name.
To treat his debilitating seizures, H.M. underwent surgery to remove the portion of the brain that was acting erratically. This region was called the hippocampus. This is not an uncommon procedure for patients suffering from seizures that completely derail their functioning. Although risky, removal or lesioning of a small portion of the crian can be compensated for by other regions. The crazy thing was that after the surgery, H.M. couldn’t remember anything from the last 20 years and couldn’t form new memories. Otherwise the surgery was a success (except for the whole short-term memory loss).
H.M. also couldn’t remember any new facts. This was a real-life version of 50 First Dates. If you haven’t seen that movie, it is one of Adam Sandler’s 2,374 movies, and it is fine.
The scientists studying H.M. were William Beecher Scoville and Brenda Millner. This case along with other cases of hippocampal lesions, formed the basis of the standard model of memory, which states new information goes to the hippocampus and then is sent to a storage place in the neocortex. The neocortex (Latin for new brain) is the recently evolved higher-order part of the mammalian brain.
Through multiple cycles of retrieval and reconsolidation the information is consolidated in the neocortex and can eventually be retrieved from the neocortex without the hippocampus. This model explains why HM, without a functioning hippocampus, retained memories from his childhood, but could not retain recent memories, which had not been fully consolidated, or form new memories.
However, HM had memory loss spanning twenty or more years, which is a long time for concepts to be converted to long-term memory. Therefore, a new model has been proposed called the multiple trace theory.
The Multiple Trace Theory
The multiple trace theory proposes that each memory has a spot in the hippocampus that points (by neuronal connections) to a spot in the neocortex. [6] This is like having a bunch of filing cabinets scattered throughout the neocortex, and then there is one directory (the hippocampus) that tells you where everything is stored. Each time that memory is retrieved and then reconsolidated, a slightly different memory (or trace) is formed. As more traces are formed, there are more traces which can be activated to recall the memory, leading to a stronger memory.
For example, there are multiple different ways to remember the date. Maybe you saw it on your computer, or on a calendar, or on the newspaper, or somebody mentioned it. By having many slightly different traces of the memory, recalling the fact (the date) is easier.
This could explain why HM had memory loss dating back many years, and the only memories that persisted were ones with many traces.
What this means for memory
The idea is that memory is highly distributed throughout the brain. Certain memories (i.e. your mother’s name) are very robust; however, other ones (i.e. a name you heard once) are not as strong. But the hippocampus is vital for calling upon those memories.
Attention plays a role in making memories stronger. You rarely make a conscious effort of where you put your keys, so you often forget where you left them.
Also having multiple types of input can be helpful. For this reason, PowerPoint lectures can be very effective, due to the combination of both audio and visual input.
The highly distributed nature of memories is also promising in cognitive rehabilitation after a stroke. The brain has a profound ability to compensate for damage.
So now when you forget someone’s name at your next cocktail party, just tell them that you haven’t formed a sufficient number of traces for accurate recall.