How long are memories stored in the hippocampus

The findings, which appear in Science on April 6, may force some revision of the dominant models of how memory consolidation occurs, the researchers say. Beginning in the s, studies of the famous amnesiac patient Henry Molaison, then known only as Patient H. Molaison, whose hippocampus was damaged during an operation meant to help control his epileptic seizures, was no longer able to store new memories after the operation. However, he could still access some memories that had been formed before the surgery.

This suggested that long-term episodic memories memories of specific events are stored outside the hippocampus. Scientists believe these memories are stored in the neocortex, the part of the brain also responsible for cognitive functions such as attention and planning.

Neuroscientists have developed two major models to describe how memories are transferred from short- to long-term memory. The earliest, known as the standard model, proposes that short-term memories are initially formed and stored in the hippocampus only, before being gradually transferred to long-term storage in the neocortex and disappearing from the hippocampus. A more recent model, the multiple trace model, suggests that traces of episodic memories remain in the hippocampus.

These traces may store details of the memory, while the more general outlines are stored in the neocortex. Until recently, there has been no good way to test these theories. Most previous studies of memory were based on analyzing how damage to certain brain areas affects memories. This allows the researchers to trace the circuits involved in memory storage and retrieval. They can also artificially reactivate memories by using optogenetics, a technique that allows them to turn target cells on or off using light.

In the new Science study, the researchers used this approach to label memory cells in mice during a fear-conditioning event — that is, a mild electric shock delivered when the mouse is in a particular chamber. Then, they could use light to artificially reactivate these memory cells at different times and see if that reactivation provoked a behavioral response from the mice freezing in place.

The researchers could also determine which memory cells were active when the mice were placed in the chamber where the fear conditioning occurred, prompting them to naturally recall the memory. Download printable poster. Skip to menu Skip to content Skip to footer. Site search Search. Site search Search Menu. Where are memories stored in the brain? The parts of the brain involved in memory Illustration by Levent Efe. Memory Types of memory How memories are formed Where are memories stored?

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Nader decided to revisit the concept with an experiment. That was easy—rodents learn such pairings after being exposed to them just once. Afterward, the rat freezes in place when it hears the tone. But if memories have to be at least partially rebuilt every time they are recalled—down to the synthesizing of fresh neuronal proteins—rats given the drug might later respond as if they had never learned to fear the tone and would ignore it.

If so, the study would contradict the standard conception of memory. It was, he admits, a long shot. Nader, who looks slightly devilish in his earring and pointed sideburns, still gets giddy talking about the experiment.

But the data struck a more harmonious chord with some psychologists. After all, their experiments had long suggested that memory can easily be distorted without people realizing it.

In a classic study led by Elizabeth Loftus, a psychologist then at the University of Washington, researchers showed college students a series of color photographs depicting an accident in which a red Datsun car knocks down a pedestrian in a crosswalk.

The students answered various questions, some of which were intentionally misleading. Later the researchers asked all the students what they had seen—a stop sign or yield sign? To Nader and his colleagues, the experiment supports the idea that a memory is re-formed in the process of calling it up.

Hardt and Nader say something similar might happen with flashbulb memories. People tend to have accurate memories for the basic facts of a momentous event—for example, that a total of four planes were hijacked in the September 11 attacks—but often misremember personal details such as where they were and what they were doing at the time. Hardt says this could be because these are two different types of memories that get reactivated in different situations. Television and other media coverage reinforce the central facts.

But recalling the experience to other people may allow distortions to creep in. Some experts think he is getting ahead of himself, especially when he makes connections between human memory and these findings in rats and other animals. Daniel Schacter, a psychologist at Harvard University who studies memory, agrees with Nader that distortions can occur when people reactivate memories.

The question is whether reconsolidation—which he thinks Nader has demonstrated compellingly in rat experiments—is the reason for the distortions.

Alain Brunet, a psychologist, is running a clinical trial involving people with post-traumatic stress disorder PTSD. The hope is that caregivers might be able to weaken the hold of traumatic memories that haunt patients during the day and invade their dreams at night. There are multiple aspects and types of memories. But there are also important short-term and sensory memory processes, which are required before a long-term memory can be established. Memory is generally divided into two broad categories: explicit declarative and implicit non-declarative memory.

Implicit, or non-declarative, memories are behaviours that we have learned, but cannot verbalise. These memories typically operate without conscious awareness, encompassing skills, habits and behaviours. These behaviours run on auto-pilot — for example, tying your shoelaces.

Multiple areas of the brain form implicit memories as they involve a variety of responses to be co-ordinated. Additionally, the cerebellum at the back of the skull plays a vital role in the timing and execution of learned, skilled motor movement. Explicit, or declarative, memories can be verbally expressed. These include memories of facts and events, and spatial memories of locations.

These memories can be consciously recalled and can be autobiographical — for instance, what you did for your last birthday — or conceptual, such as learning information for an exam. These memories are easy to acquire. However, they are also easy to forget as they are susceptible to disruption during the process of forming and storing the information.