There is no single thing called "memory." What gets called memory in everyday speech is at least seven different cognitive systems, supported by different brain circuits, with different storage durations, different mechanisms of forgetting, and different patterns of age-related decline. The neuroscientist Larry Squire summarized the field's mature taxonomy in a 2004 review in Neurobiology of Learning and Memory. The picture has not changed much since.
This matters in practice because the systems do not rise and fall together. People with severe amnesia can still learn new motor skills. People with normal episodic memory can have severe deficits in semantic knowledge. The aging brain loses some types of memory rapidly and others not at all. Knowing which type is involved in any given complaint is the first step in doing anything useful about it.
This post is a field guide to the seven, with the cleanest example of each, and a brief note on what the cognitive-aging literature shows about which decline.
The short answer: Memory is organized in two layers. Short-term and working systems hold information briefly for active use. Long-term systems store it durably and come in four flavors: episodic (events), semantic (facts), procedural (skills), and priming. Each is supported by different brain regions and ages differently.
What are the short-term and working memory systems?
The first layer of memory is short-term, in the second-to-minutes range. There are three systems worth distinguishing.
1. Sensory memory
Sensory memory is the briefest of all. It is the persistence of a sensory impression after the stimulus is gone, on the order of about 250 milliseconds for visual (iconic memory) and 2 to 4 seconds for auditory (echoic memory). It is the reason a sentence still seems to be hanging in the air after the speaker stops. The classic demonstration is George Sperling's 1960 partial-report paradigm, in which subjects could read off any row of a briefly-flashed grid of letters, but only if asked within roughly a quarter-second.
Sensory memory is automatic, unconscious, and pre-attentional. It does not benefit from effort or strategy. Its job is to give the rest of the system a brief window to grab whatever is worth holding.
2. Short-term memory
Short-term memory is what most people mean when they say short-term memory. It is the brief conscious holding of information for active use, with a capacity classically estimated at about 7 plus-or-minus 2 items (George Miller's "magical number") and now usually pegged at about 4 chunks for purely passive storage. Duration is about 20 seconds without rehearsal.
The phone-number-you-just-heard is short-term memory. So is the start of the sentence you are currently reading.
3. Working memory
Working memory is short-term memory with active manipulation. It is what you use when you are not just storing information, but doing something with it: mental math, holding a multi-step direction in mind while executing it, or comparing two ideas in real time.
The standard framework is Baddeley and Hitch's 1974 multi-component model, refined by Baddeley in 2000 with the addition of an "episodic buffer." Working memory has a phonological loop (the inner voice that lets you rehearse verbal material), a visuospatial sketchpad (holds visual and spatial layouts), and a central executive that allocates attention across them. Working memory is the system most cognitive-training research targets.
Capacity is usually pegged at 4 chunks of actively manipulated information, less when the load is heavy.
What are the long-term memory systems?
The second layer is long-term, durable storage. There are four major systems, and they fall into two superfamilies. Declarative memory is conscious, explicit, and verbalizable: facts and events you can describe. Non-declarative memory is implicit and skill-based: things you know how to do without being able to describe how.
4. Episodic memory
Episodic memory is autobiographical, time-and-place tagged memory for events you have personally experienced. The neuroscientist Endel Tulving introduced the concept in 1972 and refined it across his career, ultimately arguing that episodic memory is the cognitive ability that allows mental time travel: the reliving of a specific past event with awareness that you are remembering, not just knowing.
Episodic memory depends critically on the hippocampus and surrounding medial temporal lobe structures. These are the regions most vulnerable to age-related shrinkage and to Alzheimer's pathology, which is why episodic memory is the type that declines most reliably with age and is the type that fails most dramatically in dementia.
"Episodic memory is the kind of memory that allows people to consciously re-experience past experiences."
Endel Tulving, 2002, Annual Review of Psychology
The story of yesterday's dinner, the location of where you parked this morning, the punchline of the joke your colleague just told you, all episodic.
5. Semantic memory
Semantic memory is general world knowledge, decoupled from any specific event of acquisition. The capital of France, the meaning of the word "ergonomic," the boiling point of water. You know these things, but you typically cannot remember when or where you learned them.
Semantic memory depends on more distributed cortical networks, particularly in the lateral and ventral temporal lobes. It is largely preserved with age, often into the eighth and ninth decades, and is one of the few cognitive abilities that can continue to grow across the lifespan in healthy adults.
The tip-of-the-tongue phenomenon is mostly a semantic memory failure: the meaning is intact, the access to the lexical form is temporarily blocked.
6. Procedural memory
Procedural memory is the memory for how to do things. Riding a bike, typing without looking, the pattern of movements in a sport, the chord progression you have played a thousand times. It is supported primarily by the basal ganglia and cerebellum, with extensive motor cortex involvement.
Procedural memory is famously dissociable from episodic memory. The patient HM, after surgery for epilepsy that destroyed his medial temporal lobes, could not form new episodic memories of any kind. He could still learn new motor skills, including a mirror-tracing task on which he became progressively more skilled across days, while having no episodic recollection of ever having done it before.
Procedural memory is robust to age and to most forms of brain damage. It is one of the last memory systems to fail.
7. Priming and perceptual memory
Priming is the speeded or biased processing of a stimulus due to prior exposure, even when the prior exposure was not consciously remembered. Show someone the word "doctor," then ask them to complete the word stem "nu___," and they are more likely to write "nurse" than someone not primed. They typically have no awareness that the prior exposure influenced them.
Priming is supported by perceptual cortices and operates implicitly. Like procedural memory, it is largely preserved with age and with damage to the medial temporal lobe.
How do these systems decline with age?
The pattern is reasonably consistent across hundreds of cognitive-aging studies. Park and Reuter-Lorenz, 2009, in Annual Review of Psychology, gives the canonical summary.
- Episodic memory declines most reliably, beginning in midlife and accelerating after 65. This is the system most worth supporting with the modifiable lifestyle factors.
- Working memory shows moderate decline, with capacity dropping by roughly 30 percent between young and older adulthood. The spacing effect and effortful retrieval can partially compensate.
- Processing speed declines steadily across adulthood. Speed-of-processing training is one of the few interventions with strong randomized evidence behind it.
- Semantic memory is largely preserved, often growing through midlife. Older adults outperform young adults on vocabulary tests.
- Procedural memory is largely preserved.
- Priming is largely preserved.
The asymmetry matters. The "memory problems" people complain about in healthy aging are mostly episodic and working-memory problems. They are not failures of all memory.
What this means for you
A few practical translations:
- Be specific about which type is failing. "I can't remember where I put my keys" is episodic. "What's the word for that thing" is semantic access. "I forgot why I came in here" is more like a working-memory and prospective-memory issue. The interventions that help differ by type.
- Most age-related complaints are episodic. Episodic memory benefits most from sleep, exercise, and effortful retrieval practice.
- You cannot really train long-term memory directly. What you can train is the encoding and retrieval processes that build it: attention, working memory, organization, retrieval practice.
- Procedural and semantic systems are robust. This is good news. The skills and knowledge you have built are the most durable parts of you.
For the longer view of how these systems work together and what that means for memory across a lifetime, see our complete memory 101 guide.