Long-term memory (LTM) is the final stage of the multi-store memory model proposed by Atkinson-Shiffrin, providing the lasting retention of information and skills.
Theoretically, long-term memory capacity could be unlimited, the main constraint on recall being accessibility rather than availability.
Duration might be a few minutes or a lifetime. Suggested encoding modes are semantic (meaning) and visual (pictorial) in the main but can be acoustic also.
Using the computer analogy, the information in your LTM would be like the information you have saved on the hard drive. It isn’t there on your desktop (your short-term memory), but you can pull up this information when you want it, at least most of the time.
Key Takeaways
- Effectively Unlimited: Long-term memory has no known limit on capacity or duration. Most forgetting reflects a retrieval problem, not a lost memory trace.
- Availability vs Accessibility: A memory can be stored yet briefly hard to reach. The right cue often brings it back to mind.
- Two Main Types: Explicit memory (episodic and semantic) can be consciously recalled. Implicit memory, such as procedural skills, priming, and conditioning, shows itself through behaviour instead.
- Meaning-Based Encoding: Long-term memory mainly stores information by meaning rather than sound. Deep, meaningful processing builds stronger memories than rote repetition.
- Evidence From Amnesia: Patients such as HM could not form new declarative memories, yet they still learned new motor skills. This shows declarative and procedural memory rely on separate brain systems.
- Lifelong Retention Is Possible: Bahrick’s classic study found people still recognised former classmates’ names and faces decades later, with some memories holding up after 48 years.
- Memories Can Change on Recall: Retrieving a memory can briefly destabilise it before it is “reconsolidated,” showing long-term memories are dynamic rather than fixed forever.
Types of Long-Term Memory
Long-term memory is not a single store and is divided into two types: explicit (knowing that) and implicit (knowing how).
One of the earliest and most influential distinctions of long-term memory was proposed by Tulving (1972), who separated episodic memory from semantic memory. He added procedural memory as a third system only in 1985, arguing that episodic memory is embedded within semantic memory, which is itself embedded within procedural memory (Tulving, 1985).
Procedural Memory
Procedural memory is a part of the implicit long-term memory responsible for knowing how to do things, i.e., memory of motor skills.
It does not involve conscious (i.e., it’s unconscious-automatic) thought and is not declarative. For example, procedural memory would involve knowledge of how to ride a bicycle.
Semantic Memory
Semantic memory is a part of the explicit long-term memory responsible for storing information about the world. This includes knowledge about the meaning of words, as well as general knowledge.
For example, London is the capital of England. It involves conscious thought and is declarative.
The knowledge that we hold in semantic memory focuses on “knowing that” something is the case (i.e. declarative). For example, we might have a semantic memory for knowing that Paris is the capital of France.
Episodic Memory
Episodic memory is a part of the explicit long-term memory responsible for storing information about events (i.e. episodes) that we have experienced in our lives.
It involves conscious thought and is declarative. An example would be a memory of our 1st day at school.
The knowledge that we hold in episodic memory focuses on “knowing that” something is the case (i.e. declarative). For example, we might have an episodic memory of knowing that we caught the bus to college today.
Priming and Conditioning
Priming and conditioning are two further forms of implicit long-term memory. In priming, earlier exposure to a stimulus makes it easier to process a related stimulus later, with no conscious memory of that earlier exposure (e.g. seeing the word “doctor” speeds up recognising “nurse”).
Classical and operant conditioning create learned associations and emotional responses that shape behaviour automatically. A person can show a conditioned fear response with no conscious memory of the event that produced it.
Cohen and Squire (1980) drew a distinction between declarative knowledge and procedural knowledge.
Procedural and declarative knowledge map onto the implicit and explicit types described above. Procedural knowledge (“knowing how”, e.g. riding a bike or brushing your teeth) runs automatically with little conscious effort. Declarative knowledge (“knowing that”, e.g. that London is the capital of England) requires consciously bringing information to mind.
Evidence for the distinction between declarative and procedural memory has come from research on patients with amnesia. Typically, amnesic patients have great difficulty retaining episodic and semantic information following the onset of amnesia.
Their memory for events and knowledge acquired before the onset of the condition tends to remain intact, but they can’t store new episodic or semantic memories. In other words, it appears that their ability to retain declarative information is impaired.
However, their procedural memory appears to be largely unaffected. They can recall skills they have already learned (e.g. riding a bike) and acquire new skills (e.g. learning to drive).
A landmark example is patient HM (Henry Molaison), who lost the ability to form new episodic and semantic memories after surgery for epilepsy. He still learned new motor skills, such as mirror-drawing, despite having no conscious memory of ever practising them (Scoville & Milner, 1957).
Bahrick et al. (1975)
Bahrick, Bahrick, and Wittinger (1975) investigated what they called very long-term memory (VLTM). Nearly 400 participants aged 17 – 74 were tested.
Participants were asked to list the names they could remember of those in their graduating class in a free recall test.
There were various conditions including: a free recall test, where participants tried to remember names of people in a graduate class; a photo recognition test, consisting of 50 pictures; a name recognition test for ex-school friends.
Results of the study showed that participants who were tested within 15 years of graduation were about 90% accurate in identifying names and faces. After 48 years they were accurate 80% for verbal and 70% visual.
Participants were better at photo recognition than free recall. Free recall was worse. After 15 years it was 60% and after 48 years it was 30% accurate.
They concluded that long-term memory has a potentially unlimited duration.
A strength of this study is that it used meaningful stimuli. Bahrick et al. tested people’s memories from their own lives by using high school yearbooks. The study has higher external validity when compared to studies using meaningless pictures (where recall rates tend to be lower).
But the study did not control for confounding variables (they may have rehearsed their memory of the photos over the years), so any real-world application should be applied with caution.
Critical Evaluation of Long-Term Memory Research
Long-term memory is a well-evidenced concept, but several of its central claims attract legitimate criticism. Four issues are worth noting:
- Untestable claims: “Unlimited” capacity and “permanent” duration cannot be proven directly, only inferred from the fact that no limit has ever been observed.
- A blurred episodic/semantic boundary: Semantic knowledge is often abstracted from episodic experience, so the two may not be fully separate systems.
- Systems versus process debate: Dissociations credited to separate memory systems could instead reflect different types of processing, not separate stores.
- Reconstruction, not reproduction: Episodic memories are rebuilt at recall, shaped by schemas and open to distortion, rather than played back intact.
Untestable Claims About Capacity and Duration
Neither an infinite capacity nor a truly permanent memory trace can be tested directly. Both are inferences drawn from the fact that no limit has ever been observed.
Because retrieval failure can always look like storage loss, the claim that long-term memories are never truly erased is difficult to falsify.
Is the Episodic/Semantic Split Genuinely Clean?
The boundary between episodic and semantic memory can blur. Semantic knowledge is often abstracted from episodic experience, and autobiographical memories mix both types.
This has led some researchers to question whether episodic and semantic memory are truly separate systems, rather than two points on a continuum within the same declarative system.
Systems Versus Process Views
The dominant view treats long-term memory as a set of distinct neural memory systems (Tulving; Squire). Process-based accounts disagree.
They argue the same dissociations reflect differences in the processes engaged, such as conceptual versus perceptual processing, rather than separate stores in the brain.
Episodic Memory Is Reconstructive, Not Reproductive
Bartlett (1932) showed with his War of the Ghosts studies that recall is an active reconstruction shaped by schemas, not a faithful replay. Loftus and Palmer (1974) showed that memory can also be rewritten by information encountered after an event.
Long-term memory should not be pictured as a video archive: what is retrieved is reconstructed at the moment of recall.
This has direct consequences for eyewitness testimony, since misleading post-event information can distort what a witness “remembers”.
Contemporary Research
Long-term memory is not a fixed archive. Modern neuroscience shows it can be updated, replayed, and even reshaped at the level of individual brain cells.
Memory Reconsolidation
Retrieving a stored memory can return it to a fragile, changeable state. The memory must then be “reconsolidated,” or restabilised, to survive.
Nader, Schafe, and LeDoux (2000) found that blocking protein synthesis during this reconsolidation window weakens a reactivated fear memory in rats. This reframes long-term memory as dynamic rather than fixed, since a stored trace can be altered every time it is recalled.
This discovery has opened new treatment approaches for weakening maladaptive memories in conditions such as PTSD and addiction, by targeting the fragile window that follows retrieval.
Engram Cells
Liu et al. (2012) used optogenetics to reactivate the exact hippocampal neurons active during fear learning in mice. Stimulating these “engram cells” triggered the fear memory directly, without the original cue.
This gave the first direct evidence that a specific memory can be traced to a specific, identifiable group of neurons, rather than being distributed vaguely across the brain.
Related studies have gone further, implanting and reversing false memories in mice by manipulating the same engram cells. Together, this work gives a cellular-level account of both how memories are stored and how they can be distorted.
Sleep and Consolidation
During slow-wave sleep, the hippocampus “replays” newly learned information and gradually transfers it to the neocortex for lasting storage (Squire, Genzel, Wixted, & Morris, 2015).
This gives a biological explanation for why sleep, rather than last-minute cramming, secures long-term learning, and it connects the classic consolidation debate to systems-level neuroscience.
References
Bahrick, H. P., Bahrick, P. O., & Wittinger, R. P. (1975). Fifty years of memory for names and faces: A cross-sectional approach. Journal of Experimental Psychology: General, 104(1), 54–75.
Cohen, N. J., & Squire, L. R. (1980). Preserved learning and retention of pattern-analyzing skill in amnesia: Dissociation of knowing how and knowing that. Science, 210(4466), 207–210. https://doi.org/10.1126/science.7414331
Liu, X., Ramirez, S., Pang, P. T., Puryear, C. B., Govindarajan, A., Deisseroth, K., & Tonegawa, S. (2012). Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature, 484(7394), 381–385.
Nader, K., Schafe, G. E., & LeDoux, J. E. (2000). Fear memories require protein synthesis in the amygdala for reconsolidation after retrieval. Nature, 406(6797), 722–726.
Scoville, W. B., & Milner, B. (1957). Loss of recent memory after bilateral hippocampal lesions. Journal of Neurology, Neurosurgery, and Psychiatry, 20(1), 11–21.
Squire, L. R., Genzel, L., Wixted, J. T., & Morris, R. G. (2015). Memory consolidation. Cold Spring Harbor Perspectives in Biology, 7(8), a021766.
Tulving, E. (1972). Episodic and semantic memory. In E. Tulving & W. Donaldson (Eds.), Organization of memory (pp. 381–403). Academic Press.
Tulving, E. (1985). Memory and consciousness. Canadian Psychology, 26(1), 1–12.
