Study topic · about 4 minutes

How memory works

Memory is not kept in one place. Explicit memories of facts and events need the hippocampus to form, and are then stored across the cortex. Implicit memories, like riding a bike, rely on the cerebellum and basal ganglia, and the amygdala makes emotional memories stronger.

Key points

Memory works in three steps: encoding (taking information in), storage (keeping it) and retrieval (getting it back).
Explicit memory includes episodic memory (events) and semantic memory (facts); implicit memory includes procedural memory (skills).
The hippocampus turns new experiences into lasting memories; without it, people can't form new explicit memories.
Memories are physical changes at synapses: long-term potentiation makes connections stronger with repeated use.
Working memory, what you hold in mind right now, depends on the prefrontal cortex.

Encoding, storage and retrieval

Encoding: turning what you experience into a form the brain can store. Paying attention, and linking new information to what you already know (elaborative rehearsal), make encoding stronger.
Storage: keeping it. Sensory memory lasts a split second, short-term memory about 20 to 30 seconds unless you rehearse it, and long-term memory can last a lifetime.
Retrieval: getting it back, by recall (remembering without clues) or recognition (picking it out from options). Testing yourself strengthens a memory more than rereading does, which is called the testing effect.

Types of memory

Type	What it holds	Example	Key brain areas
Episodic (explicit)	Events you lived through	Your last birthday	The hippocampus, then the cortex
Semantic (explicit)	Facts and meanings	Paris is the capital of France	The temporal lobes and other cortex
Procedural (implicit)	Skills and habits	Riding a bike, typing	The cerebellum and basal ganglia
Emotional	Fear and feelings tied to events	Fear of a dog that once bit you	The amygdala
Working	What you hold in mind right now	A phone number while you dial it	The prefrontal cortex

There is also prospective memory: remembering to do something in the future, like taking a medicine at noon.

The amygdala lit up in front of the hippocampus inside a see-through 3D brain — The amygdala (yellow) sits just in front of the hippocampus. It makes emotional moments stick.

The hippocampus: where memories are made

The hippocampus, curled inside each temporal lobe, binds the sights, sounds and feelings of an experience into one memory. Information arrives from the entorhinal cortex, loops through the hippocampus, and leaves through the fornix toward the mammillary bodies and the thalamus. Over weeks to years, memories are gradually moved into the cortex, a process called consolidation, much of it during sleep.

The most famous memory patient, Henry Molaison (known as H.M.), had most of both hippocampi removed in 1953 to stop severe epilepsy. Afterward he could not form new memories of facts and events (anterograde amnesia), yet he could still learn new motor skills. That showed that different kinds of memory use different parts of the brain.

A 3D slice across the hippocampus, with the dentate gyrus curled inside the band of the CA fields and the pathways that join them — A slice across the hippocampus. Signals from the entorhinal cortex (orange) reach the dentate gyrus (green), then CA3 (pink) and CA1 (teal), and leave through the subiculum (blue).

Explore the hippocampal circuit in 3D

What a memory is made of: long-term potentiation

When two connected neurons fire together again and again, the synapse between them gets stronger, so the next signal passes more easily. This long-term potentiation (LTP) was first found in the hippocampus, and it is thought to be how memories are stored: learning physically changes the connections between neurons.

Inside the hippocampus, signals travel a loop of three synapses, each passing a "go" signal with glutamate: from the entorhinal cortex to the dentate gyrus, from there to CA3, and from CA3 to CA1. In 1973, Bliss and Lømo found LTP at the first of these synapses, in rabbits.

Why we forget

The forgetting curve: we forget fastest right after learning, then more slowly. Hermann Ebbinghaus showed this in the 1880s.
Encoding failure: the information was never stored, often because we weren't paying attention.
Interference: old memories get in the way of new ones (proactive interference), or new ones get in the way of old ones (retroactive interference).
Retrieval failure: the memory is there but you can't reach it, like a word on the tip of your tongue.
Amnesia: damage to the memory system. Anterograde amnesia is losing the ability to form new memories; retrograde amnesia is losing memories from before the injury.

Spaced review beats the forgetting curve: going over something just before you would forget it makes it last longer each time. The daily review in the 3D app is scheduled exactly this way.

When memory fails

Alzheimer's disease often damages the hippocampus and the entorhinal cortex first, which is why trouble remembering recent events is an early sign. Korsakoff syndrome, caused by a lack of vitamin B1 (thiamine), often with heavy drinking, damages the mammillary bodies and causes severe memory loss.

Quick quiz

5 questions. Your answers are saved and come back in your daily review.

Which structure is needed to form new memories of facts and events?
Show the answer
Hippocampus. The hippocampus binds new experiences into memories that the cortex later stores.
Riding a bike is an example of which kind of memory?
Show the answer
Procedural. Skills are procedural (implicit) memories, which rely on the cerebellum and basal ganglia.
Which structure makes emotional memories, like fear, stronger?
Show the answer
Amygdala. The amygdala boosts the storage of emotional events.
What is long-term potentiation?
Show the answer
Synapses getting stronger with repeated use. LTP strengthens the connections between neurons that fire together, and is thought to store memories.
After his surgery, what could patient H.M. still do?
Show the answer
Learn new motor skills. His procedural memory still worked, because it does not depend on the hippocampus.

In these study guides

MCATSleep, drugs, memory, language and reward
AP Psychology2.3 to 2.7 Memory
Intro psychologyMemory (sections 8.1 to 8.3)
NursingMovement disorders and memory
Medical schoolLimbic system and hypothalamus

Take it further in 3D

MemoryThe hippocampus and its circuit. A short guided lesson in the 3D app.
Emotion and rewardFear, drive and the value of things. A short guided lesson in the 3D app.

Sources

Types of memory, Queensland Brain Institute, University of Queensland
Where are memories stored in the brain?, Queensland Brain Institute, University of Queensland
Introduction to Behavioral Neuroscience, 18.4 Synaptic Mechanisms of Long-Term Memory, OpenStax
The Anatomy of the Hippocampus, Cerebral Ischemia (Exon Publications), NCBI Bookshelf

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