In 1953 a surgeon removed parts of both medial temporal lobes from a 27-year-old man, Henry Molaison, to treat his severe epilepsy. The seizures eased, but he could no longer form new long-term memories of facts or events. Until his death in 2008 he was known in research as patient H.M. Yet when the psychologist Brenda Milner had him practice tracing a star in a mirror, he got better day after day, while insisting each time that he'd never done it before. One man, two kinds of memory: that's the shape of this lesson.
Encoding, storage and retrieval
Memory is usually split into three processes. Encoding gets information in, storage keeps it, and retrieval gets it back out. A failure at any stage looks the same from the inside: "I forgot." Mira's "How memory works" deck covers forgetting and study habits; this one covers the systems and types.
The multi-store model
Atkinson and Shiffrin's 1968 model describes three stores:
- Sensory memory holds a brief copy of what your senses take in. Visual (iconic) memory lasts less than a second; auditory (echoic) memory lasts a few seconds.
- Short-term memory holds what you're attending to. Without rehearsal it fades within roughly half a minute. George Miller's 1956 paper put its capacity at about seven items, plus or minus two; later estimates, such as Nelson Cowan's in 2001, suggest closer to four chunks.
- Long-term memory holds information for minutes to a lifetime.
Working memory
In 1974 Alan Baddeley and Graham Hitch argued that short-term memory is an active workspace, not a waiting room. Their working memory model has a central executive that directs attention, a phonological loop for words and sounds, and a visuospatial sketchpad for images and layouts. In 2000 Baddeley added an episodic buffer that links them to long-term memory.
How deeply you encode
Some information, like where you sat this morning, is encoded automatically. Most of what you study takes effortful processing. Craik and Lockhart's 1972 levels of processing idea says meaning-based (semantic) processing produces stronger memories than shallow processing of how a word looks (structural) or sounds (phonemic). Relating something to yourself helps further: the self-reference effect. Repeating a phone number (maintenance rehearsal) keeps it alive for a moment; connecting it to things you know (elaborative rehearsal) helps it last.
Types of long-term memory
- Explicit (declarative) memory is what you can consciously recall. Endel Tulving split it in 1972 into episodic memory (personal events) and semantic memory (facts).
- Implicit (nondeclarative) memory shows in behavior: procedural skills, priming, and conditioned responses.
H.M. lost the ability to form new explicit memories while keeping implicit learning, which is why his case is evidence that the two rely partly on different brain systems. The hippocampus is key for forming new explicit memories.
Getting memories back
Recall (fill in the blank) is usually harder than recognition (multiple choice), and relearning is faster than learning the first time. Retrieval cues help most when they match how the memory was encoded, Tulving and Thomson's encoding specificity principle. Your mood can act as a cue too: mood-congruent memory.
Common mix-ups
- Episodic vs semantic. Remembering your trip to Paris is episodic; knowing Paris is France's capital is semantic.
- Anterograde vs retrograde amnesia. Anterograde blocks new memories; retrograde loses memories from before the injury.
- Short-term vs working memory. Short-term is a store; working memory is the model of what you do with it.
Where the research is shaky
The multi-store model is a useful simplification, not a map of the brain. "Depth" in levels of processing is hard to measure independently of how well people remember, which makes the idea partly circular. And flashbulb memories feel vivid but aren't reliably accurate: after the 1986 Challenger disaster, Ulric Neisser and Nicole Harsch found that many students' later accounts differed from what they'd written soon after, despite high confidence.
Frequently asked questions
What are the three stages of memory?
Encoding, storage and retrieval. The multi-store model adds three stores: sensory, short-term and long-term memory.
What is the difference between explicit and implicit memory?
Explicit memory is what you can consciously recall and describe, such as facts and events. Implicit memory shows in what you do, such as skills and priming.
How many items can short-term memory hold?
Miller's famous estimate was seven, plus or minus two. Many researchers now think it's closer to four chunks.
Is a photographic memory real?
There's little good evidence that adults can store pages like a photograph. People with remarkable memories usually rely on practiced techniques.
Try it
Pick five new words. For two, just repeat them. For the other three, ask "does this describe me or someone I know?" Tomorrow, write down all five without looking, and see which ones survived.