Memories are consolidated from short-term to long-term in the hippocampus.Forgetting is as simple as walking through a doorway – that is the finding of a new study that experimented with memories and ways to walk through a home. Researchers asked participants to complete a simple task (exchanging one object for another) in either the same room or by walking through a doorway to another room. The result: people asked to complete the task in the other room were two to three times more likely to forget what they were supposed to do. “Entering or exiting through a doorway serves as an ‘event boundary’ in the mind, which separates episodes of activity and files them away,” said lead researcher Gabriel Radvansky, a psychologist at the University of Notre Dame. “Recalling the decision or activity that was made in a different room is difficult because it has been compartmentalized.”

As the doorway study suggests, memories can be fleeting. One moment, you are walking into the kitchen to grab a coffee mug. The next, you are standing in the kitchen, wondering why you walked into the room. But even if you recall a memory, there is no guarantee it is accurate. “Human memory is not like a computer where information is stored and retrieved in an identical fashion,” said Mary Spiers, neuropsychologist at Drexel University. “Our memories are more constructive and changeable.” In fact, Spiers told us when we recall a memory over and over again, it could change the original memory.

Making Memories

To understand why memories are fragile and changeable, you need to know how they are stored in the brain. It is a common misconception that memories are stored as single units, like a file in a computer. “Memories do not exist as packaged, isolated things stored somewhere in the brain,” explained Ricardo Gil da Costa, a neuroscientist at the Salk Institute. Instead, he told us, a memory is “represented by a neural network that includes multiple brain areas.” For example, a memory of a cat would be distributed among multiple areas of the brain. The visual of a cat is processed in the visual cortex, while the sound of a cat’s meow is processed in the auditory cortex, and the emotional feelings are processed in the limbic cortex, etc.

The processed information is then sent to the hippocampus, a part of the brain belonging to the limbic system, where it is consolidated into memory. Normally, memories are stored in the areas of the brain where they were processed (so, the visual of cat is stored in the visual cortex and so on). But consolidation can be a lengthy process. “Consolidation depends on neuronal changes,” explained Spiers, “and this may take hours, days, weeks, or months.”

The Man Without a Hippocampus

Henry Molaison, known as H.M., had two-thirds of his hippocampus removed in 1953.During this consolidation, memories can be lost or damaged by neurological injury. Or, in the case of Henry Molaison, famously known as H.M., memories can be lost by the removal of the hippocampus. In an attempt to lessen debilitating seizures, doctors removed two-thirds of Molaison’s hippocampus. It did alleviate Molaison’s seizures but the surgery had another curious side affect: he could not form new memories. Molaison could remember everything 1 to 2 years before the surgery but afterward, his memories were short term.

His condition might remind you of Dory from Finding Nemo, the animated regal tang fish with short-term memory loss. “She has a problem in remembering anything for more than a minute or so,” said Spiers. “Neuropsychologists term this ‘anterograde amnesia.’ It is a difficulty in encoding, and therefore learning new things.” Popular films, like The Bourne Identity, rely on a different type of amnesia: retrograde amnesia, where information before an event (like a head injury) is lost. Some films take it to the extreme, where a character cannot remember who he is, like Jason Bourne, but this is not an accurate depiction of retrograde amnesia. A person with amnesia would not lose memories of his name or identity and retrograde amnesia can usually be explained, Spiers said, as “a problem of incomplete consolidation of recently learned memory.”

Even though Molaison lost his ability to consolidate memories from short term to long term, he could still develop new motor skills. In one experiment, he was asked to draw within the lines of a star shape while looking in a mirror. It is a difficult task and researchers believed his anterogade amnesia would make him lose this newly gained skilled the next day. But he did not. As you can see, there is more to memory than we can explain in an article (or two or three) but if you have any questions, leave them in the comments and we will try to follow up!

Scientists are still experimenting with memory, and even though Molaison died in 2008, his contribution to science lives on: his brain, cut into slices, resides at the University of California, San Diego, for further study.

Add new comment