Structures of Memory » Short-term Memory Store

What you'll learn this session

Study time: 30 minutes

The characteristics and functions of short-term memory (STM)
The capacity and duration of STM
The encoding processes in STM
Key studies on STM including Peterson & Peterson and Miller
How STM works within the multi-store model of memory
Real-life applications of understanding STM

Introduction to Short-term Memory Store

Short-term memory (STM) is like a mental workspace where we temporarily hold and manipulate information. Think of it as the brain's notepad – it holds information for a brief period while we're using it. Unlike long-term memory, which can store information for years, STM is limited in both capacity and duration.

Key Definitions:

Short-term memory: A memory system that holds limited information for approximately 15-30 seconds without rehearsal.
Capacity: The amount of information that can be held in memory at one time.
Duration: How long information remains in memory.
Encoding: The way information is changed to be stored in memory.
Rehearsal: The process of repeatedly going over information to keep it in STM or transfer it to long-term memory.

💡 Characteristics of STM

Capacity: STM can hold about 7±2 items (Miller's Magic Number).

Duration: Information lasts around 15-30 seconds without rehearsal.

Encoding: Information is primarily stored in acoustic (sound) form.

📖 STM in the Multi-Store Model

STM sits between sensory memory and long-term memory in Atkinson and Shiffrin's model. Information from sensory memory enters STM, where it can either be forgotten or transferred to long-term memory through rehearsal.

Capacity of Short-term Memory

One of the most famous findings in psychology is that our STM can hold approximately 7 items (plus or minus 2) at once. This was discovered by George Miller in 1956 and is often called "Miller's Magic Number".

Key Study: Miller's Magic Number (1956)

George Miller reviewed existing studies on memory span and found that people could typically remember between 5-9 items (7±2). This applied whether the items were digits, letters, words, or other units.

Miller introduced the concept of "chunking" - grouping information into meaningful units to increase STM capacity. For example, remembering the digits 1-9-9-7 as the year "1997" uses just one chunk instead of four separate digits.

Chunking explains how we can remember things like phone numbers. The number 07700900123 is 11 digits (beyond STM capacity), but we might chunk it as 07700-900-123, reducing it to just 3 chunks.

Duration of Short-term Memory

Without rehearsal (repeating the information to yourself), information in STM typically lasts only about 15-30 seconds before it fades away. This was demonstrated in a classic study by Peterson and Peterson.

Key Study: Peterson & Peterson (1959)

Method: Participants were shown a three-letter combination (like "GHK") and asked to remember it. To prevent rehearsal, they had to count backwards in threes from a given number until told to recall the letters.

Results: After just 3 seconds, recall was about 80%. After 18 seconds, it dropped to 10%.

Conclusion: Without rehearsal, information in STM disappears rapidly, typically within 15-30 seconds.

Encoding in Short-term Memory

While long-term memory primarily uses semantic encoding (based on meaning), STM mainly uses acoustic encoding (based on sound). This was demonstrated by Conrad in his 1964 study.

Types of Encoding in STM

🔈 Acoustic

Primary encoding in STM is based on how things sound. This explains why similar-sounding letters (like B, D, P, T) are often confused in STM tasks.

👁 Visual

Some information may be stored visually, especially when the task specifically involves visual elements like shapes or patterns.

🧠 Semantic

While less common in STM, meaning-based encoding can occur, especially when we actively try to understand the information.

Evidence for Acoustic Encoding: Conrad (1964)

Conrad showed participants sequences of letters briefly and asked them to recall them. When participants made errors, they typically confused letters that sounded similar (like B and P) rather than letters that looked similar (like F and P).

This suggests that even when information is presented visually, it's often converted to an acoustic code in STM.

The Role of Rehearsal in STM

Rehearsal is crucial for maintaining information in STM and potentially transferring it to long-term memory. There are two main types:

🔃 Maintenance Rehearsal

Simply repeating information over and over (like repeating a phone number until you dial it). This keeps information in STM but doesn't necessarily transfer it to long-term memory.

💭 Elaborative Rehearsal

Making connections between new information and existing knowledge (like creating a story or image). This helps transfer information to long-term memory.

The Serial Position Effect

When people try to remember a list of items, they typically recall items from the beginning (primacy effect) and end (recency effect) better than items from the middle. This is called the serial position effect.

The recency effect is linked to STM - items at the end of the list are still in STM when recall is requested. If recall is delayed (allowing STM to clear), the recency effect disappears, supporting the idea that recent items are held in STM.

Real-Life Applications of STM Research

How STM Knowledge Helps Us

Understanding the limitations of STM has important practical applications:

🎓 Education

Teachers use chunking to help students remember complex information. Breaking down phone numbers, postal codes and long numbers into chunks makes them easier to remember.

Understanding that STM is primarily acoustic explains why students might confuse similar-sounding terms when learning new vocabulary.

💻 Technology Design

User interfaces are designed with STM limitations in mind. For example, phone numbers are displayed in chunks (e.g., 07700 900 123).

PINs and passwords are typically 4-6 digits, staying within STM capacity.

Case Study Focus: Air Traffic Control

Air traffic controllers must maintain awareness of multiple aircraft positions simultaneously - a task that challenges STM capacity. To manage this, they:

Use visual displays to offload information from STM
Chunk information (treating a plane and its details as one unit)
Use both maintenance rehearsal (repeating crucial information) and elaborative rehearsal (connecting aircraft to familiar patterns)

Understanding STM limitations has led to improved training and systems that reduce cognitive load, enhancing safety in this critical profession.

Evaluating STM Research

Strengths:

Strong scientific support from laboratory studies like Peterson & Peterson and Miller
Practical applications in education, technology and workplace design
Fits well within broader memory models like the multi-store model

Limitations:

Many studies use artificial tasks that may not reflect how memory works in real-life situations
The exact capacity may vary depending on the type of information and individual differences
More recent models suggest STM is more complex than originally thought, with multiple components (as in Baddeley's working memory model)

Summary: Key Points About STM

STM has limited capacity (7±2 items) and duration (15-30 seconds without rehearsal)
Information in STM is primarily encoded acoustically (based on sound)
Chunking can effectively increase the functional capacity of STM
Rehearsal is necessary to maintain information in STM and potentially transfer it to long-term memory
Understanding STM limitations has important applications in education, technology design and workplace safety

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