Memory Systems — Sensory Memory, STM & LTM
📊 Topic Overview
| Papers Appeared | 16/28 (57.1%) | Priority | 🔴 HIGH |
🎯 Must-Know Concepts
- Three types: Sensory (iconic/echoic), STM, LTM
- STM: 7±2 capacity (Miller), 15–30 sec duration, acoustic encoding
- LTM: Unlimited, lifelong, semantic encoding
- Types of LTM: Declarative (episodic + semantic) vs Implicit/Procedural
- Miller's magic number (short note topic)
- Sensory memory — Sperling's iconic memory experiments
Previous Year Questions
| Year-Session | Question | Marks |
|---|---|---|
| 2011-June | Short note: Short-term memory | 3 |
| 2015-June | Differentiate between STM and LTM (B section) | 6 |
| 2015-December | Short note: Sensory memory | 3 |
| 2018-June | Discuss the nature of STM and LTM (B section) | 6 |
| 2019-December | Short note: Differences between STM and LTM | 3 |
| 2021-June | Short note: Sensory memory | 3 |
| 2021-December | Differentiate STM and LTM (B section) | 6 |
| 2022-December | Short note: Miller's magic number | 3 |
| 2023-June | Explain sensory memory, STM, and LTM (B section) | 6 |
🌟 Section B Answer: STM vs LTM (~300 words)
Memory can be broadly classified into Short-Term Memory (STM) and Long-Term Memory (LTM), which differ fundamentally in capacity, duration, encoding, and neural basis.
Short-Term Memory (STM)
- Capacity: Approximately 7±2 chunks (Miller, 1956 — "The Magical Number Seven")
- Duration: 15–30 seconds without rehearsal (Peterson & Peterson, 1959 — demonstrated rapid decay using the Brown-Peterson distractor technique)
- Encoding: Primarily acoustic/phonological — Conrad (1964) showed acoustically similar words (cat, mat, hat) were confused in STM even when presented visually
- Forgetting: Decay over time + displacement by new items
- Maintenance: Subvocal rehearsal prevents decay
Long-Term Memory (LTM)
- Capacity: Essentially unlimited
- Duration: Potentially permanent (lifetime)
- Encoding: Primarily semantic (meaning-based) — Baddeley (1966) showed semantically similar words (large, big, wide) confused in LTM
- Forgetting: Proactive interference (old interferes with new) and retroactive interference (new interferes with old); also retrieval failure
Types of Long-Term Memory (Squire, 1987)
Long-Term Memory
├── Explicit (Declarative)
│ ├── Episodic — personal events ("my 10th birthday")
│ └── Semantic — world knowledge ("Delhi is India's capital")
└── Implicit (Non-declarative)
├── Procedural — motor skills ("cycling", "typing")
└── Priming — unconscious facilitation
Comparison Table
| Aspect | STM | LTM |
|---|---|---|
| Duration | 15–30 seconds | Potentially permanent |
| Capacity | 7±2 chunks | Unlimited |
| Encoding | Acoustic | Semantic |
| Retrieval | Immediate, easy | Requires cues/effort |
| Brain basis | Prefrontal cortex | Hippocampus → Cortex |
| Forgetting | Decay + displacement | Interference + retrieval failure |
Evidence for the Distinction
- Serial position effect: Primacy (first items recalled) = LTM; Recency (last items recalled) = STM — double dissociation
- Neuropsychological: H.M. had intact STM but no new LTM formation (hippocampal damage)
- Patient K.F.: Opposite — impaired STM, intact LTM — complete double dissociation
📝 Short Note: Sensory Memory (~150 words)
Sensory memory is the earliest stage of memory, holding raw, unprocessed sensory information for an extremely brief period before it either decays or is passed to short-term memory through attention.
Two main types:
- Iconic memory (visual sensory store): Holds a complete visual image for approximately 0.5 seconds (Sperling, 1960). Sperling's classic partial-report technique showed that participants could recall any row from a 3×4 letter matrix when signaled immediately after — demonstrating a large but rapidly decaying visual buffer.
- Echoic memory (auditory sensory store): Holds auditory information for 2–4 seconds — longer than iconic memory. This explains why we can "replay" something just said even when not initially attending.
Sensory memory has large capacity but minimal processing — it is pre-attentional. The information it holds is lost if not selected by attention for transfer to STM.
📝 Short Note: Miller's Magic Number (~150 words)
George Miller (1956) published "The Magical Number Seven, Plus or Minus Two," one of the most cited papers in psychology. He reported that the capacity of short-term memory is approximately 7±2 items across many different types of information — digits, letters, words, or more complex units.
The key concept is chunking — organizing individual items into meaningful units. By chunking, we can hold more information in STM despite the 7±2 limit. For example:
- 7 individual digits: 1-9-4-5-1-9-1-8 (difficult)
- 2 meaningful chunks: 1945, 1918 (World War end years — easy)
Miller's discovery was significant because it established that STM has a fixed, quantifiable capacity — a mentalistic claim that helped trigger the cognitive revolution. Modern research suggests the limit may be closer to 4 chunks (Cowan, 2001) when rehearsal is prevented.
Related Content:
🧠 Memory Aids
STM "7±2" — Miller's Magic Number; 15–30 seconds; Acoustic encoding
LTM types: Declarative → Episodic + Semantic; Implicit → Procedural + Priming
✅ Self-Assessment
- What is Miller's magic number? What is chunking?
- How does encoding differ between STM and LTM?
- What is the difference between episodic and semantic memory?
- What is sensory memory? How did Sperling demonstrate iconic memory?
- What causes forgetting in STM vs LTM?