Background
Memory is how the brain takes information captured by our senses and turns it into experiences that can be stored and remembered in the future.
Instead of a perfect recording system, the brain selects a subset of sensory data for processing in short-term memory before storing it in long-term memory for future recall. This means remembering creates incomplete reconstructions of knowledge.
When learning, repetition and other techniques can enhance the quality of reconstructions by giving the brain more chances to supplement memories with more data. However, inaccurate details and factors such as age can also weaken them, creating false memories.
Memory Types
Memory is categorized into a system of types and subtypes involving different brain regions, each responsible for different functions (memory types, explained).
Based on the multistore memory model, information from our surroundings is constantly collected using sensory memory—memory unique to each sense (see examples). Like background conversations we do not remember, this data expires within two seconds.
However, paying attention to sensory data transfers it into short-term memory, where it can expire in about 30 seconds, be used by working memory (e.g., solving an equation after reading a problem), or be moved into long-term memory.
Given the variety of knowledge and skills that can be stored, long-term memory is divided into two categories: declarative (explicit) memory and nondeclarative (implicit) memory.
Declarative memory requires conscious effort and includes subtypes for remembering life experiences (episodic memory) and general knowledge (semantic memory). Nondeclarative memory is subconscious, like knowing how to walk without thinking about it (procedural memory).
Although this model has existed since 1968, researchers in 2024 found long-term memories could be created in rodents without first passing through short-term memory (watch explainer).
Making Memories
Creating and storing information in long-term memory requires reinforcing pathways between neurons—nervous system cells—that were activated by our environment when we experienced events (watch explainer).
First, neurons throughout the body are activated (e.g., those responsible for detecting spice activate when eating spicy food). These activations cascade onto other neurons via synapses—junctions between neurons—until they reach the hippocampus (see visualization).
The hippocampus is a structure in the brain that produces pulses matching those created by activated neurons while we sleep (1440 Topic: Sleep). These reactivations strengthen the synapses formed during our experiences, “storing” the memory in a network of linked neurons called an engram.
Researchers have also observed these pulses in conscious mice and suggested that the hippocampus may use them while we are awake to label experiences for future storage.
The hippocampus incorporates signals for fear, anxiety, and other feelings into memories as part of the limbic system, which manages your emotions, behaviors, and motivations.
In honor of the hippocampus, which is seahorse-shaped, the USA Memory Championship logo and trophy are shaped like seahorses.
Memory Recall
Remembering a long-term memory requires a retrieval cue—stimulus for a neuron within the memory's engram. Once triggered, neurons in the network activate, and the stored sensory details reconstruct the experience in short-term memory.
Because the hippocampus does not tag all details, reconstructions are always incomplete. Additional sensory information can change which neurons are associated with a memory, reinforcing or weakening it. Researchers are currently exploring this flexibility to edit or delete memories as treatments for PTSD.
Many memory problems are actually attention problems, preventing information from reaching the hippocampus. Memory loss may be caused by limited recollections, which weaken synapses, or mismatches in encoding and retrieval contexts, such as walking into a different room (read about the doorway effect here). Significant memory loss can occur due to memory disorders damaging relevant brain structures.
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