Memory is not one store — it is at least three stores with different capacity, duration and encoding. The Atkinson–Shiffrin multi-store model (1968) lines them up in a sequence: information enters the sensory register, is then attended to and passed to short-term memory (STM), and through rehearsal becomes part of long-term memory (LTM).
The sensory register
A brief, modality-specific buffer that holds raw sensory input for a fraction of a second so the brain can decide what to process. Iconic memory (visual) lasts about 250 ms; echoic memory (auditory) lasts about 2 seconds — long enough to make sense of the start of a sentence by the time you hear its end. Capacity is huge (essentially the whole sensory field), but most information is lost almost instantly because it isn't attended to. Sperling (1960) showed iconic memory using a 12-letter grid: with full report participants recalled ~4 letters; with partial report (cued by a tone) they recalled most letters from any row, suggesting all 12 were briefly stored.
Short-term memory
If information is attended to it transfers to STM. STM has:
- Capacity of 7 ± 2 items (Miller, 1956). Chunking — grouping items into meaningful units (e.g. 1066, MI5, IBM) — extends this.
- Duration of 18–30 seconds without rehearsal (Peterson & Peterson, 1959 — used trigrams and counted-back interference).
- Mostly acoustic encoding in adults (Baddeley, 1966).
STM is your "mental workbench" — you hold a phone number long enough to dial it, work out a sum, follow a sentence to its end.
Long-term memory
LTM is the durable store: capacity is essentially unlimited, duration up to a lifetime, and encoding mostly semantic (although procedural and episodic memory use other codes too). Material reaches LTM via maintenance rehearsal (repetition) or, more durably, elaborative rehearsal (linking to existing knowledge).
For an exam, learn one number for each store:
| Store | Capacity | Duration | Encoding |
|---|---|---|---|
| Sensory register | very large | ¼–2 sec | sensory (iconic, echoic) |
| STM | 7 ± 2 | ~18–30 sec | acoustic |
| LTM | unlimited | up to lifetime | semantic |
Why three stores not one?
Evidence for separate stores comes from:
- Capacity differences measured experimentally.
- Duration differences under controlled conditions.
- Different encoding types (Baddeley).
- Brain damage: patients like H.M. lost the ability to form new LTMs but kept STM intact, showing the two stores can be selectively impaired.
⚠Common mistakes— Common errors
- Confusing STM with "working memory" — for GCSE Psychology, treat STM as a single store; the working-memory model is A-Level material.
- Saying LTM is "infinite" — examiners want "potentially unlimited" or "very large".
- Not naming the encoding type when describing each store.
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