Structures of Memory » Capacity of Memory Stores

What you'll learn this session

Study time: 30 minutes

The three main memory stores: sensory, short-term and long-term memory
The capacity limitations of each memory store
Miller's "Magic Number 7 (±2)" theory
How chunking affects memory capacity
Peterson & Peterson's study on short-term memory duration
The virtually unlimited capacity of long-term memory

Capacity of Memory Stores

Our memory system works a bit like a computer, with different types of storage that can hold varying amounts of information. In this session, we'll explore how much information each of our memory stores can hold and for how long.

Key Definitions:

Capacity: The amount of information a memory store can hold at one time.
Duration: How long information can be held in a memory store.
Memory store: A component of memory that holds information for a specific period of time.

The Multi-Store Model of Memory

Before we dive into capacity, let's quickly review the three main memory stores according to Atkinson and Shiffrin's Multi-Store Model (1968):

👁 Sensory Memory

The first stage where sensory information enters our memory system.

📕 Short-Term Memory (STM)

Where we actively process and work with information.

💡 Long-Term Memory (LTM)

Our permanent storage system for knowledge and experiences.

Sensory Memory Capacity

Sensory memory is our first contact with information from the environment. It has some interesting capacity features:

⏱ Duration

Sensory memory is incredibly brief, lasting only about:

Visual information (iconic): 0.5 seconds
Auditory information (echoic): 2-4 seconds

📊 Capacity

Sensory memory has a very large capacity but for an extremely short time. It can hold almost everything you see or hear momentarily before most of it fades away.

Research Spotlight: Sperling's Iconic Memory Study (1960)

George Sperling showed participants a grid of 12 letters for just 50 milliseconds. When asked to recall all letters, participants could only remember about 4-5. However, when asked to recall just one specific row (indicated by a tone after the display disappeared), they could recall most letters in that row. This showed that more information was initially available in sensory memory than could be reported afterwards.

Short-Term Memory Capacity

Short-term memory (STM) is where we consciously process information. It has significant limitations:

📊 Capacity Limits

George Miller's famous 1956 research found that most people can hold 7±2 items (so between 5-9 items) in their short-term memory. This is often called "Miller's Magic Number."

Try this: Read these numbers once, then cover them and try to recall them in order:

4 7 1 9 3 8 6

⏱ Duration

Information in STM lasts approximately 18-30 seconds without rehearsal (mentally repeating the information).

Peterson & Peterson (1959) demonstrated this by asking participants to remember three-letter combinations (like "FQR") while counting backwards by threes from a given number. After just 18 seconds, most people forgot the letters.

Chunking: Expanding STM Capacity

While the 7±2 limit seems strict, we can use a technique called "chunking" to effectively increase our STM capacity.

What is Chunking?

Chunking involves grouping individual pieces of information into meaningful units or "chunks." Each chunk counts as just one item in STM, allowing us to remember more information.

Chunking Example

Try remembering these letters:

B B C I T V F B I C I A

Now try remembering them as familiar chunks:

BBC - ITV - FBI - CIA

Much easier, right? Instead of 12 individual items, you're remembering just 4 chunks!

Long-Term Memory Capacity

Long-term memory (LTM) is where we store information for extended periods, from days to decades.

📈 Unlimited Capacity

Unlike STM, LTM appears to have virtually unlimited capacity. There's no known upper limit to how much information we can store over a lifetime.

Think about it: You can remember thousands of words, faces, places, facts, skills and experiences from throughout your life.

⏱ Duration

Information in LTM can last a lifetime, though some memories may become harder to retrieve without regular use.

Some memories from early childhood or significant life events can remain accessible for decades, even without rehearsal.

Case Study Focus: H.M. (Henry Molaison)

H.M. was a patient who had parts of his brain (including the hippocampus) removed to treat severe epilepsy in 1953. After surgery, he could no longer form new long-term memories (anterograde amnesia). However, he retained most memories from before his surgery, demonstrating that LTM storage is distributed throughout the brain and has enormous capacity. H.M. could also still learn new procedural skills (like mirror drawing) even though he couldn't remember learning them, showing different types of memory are stored differently.

Comparing Memory Store Capacities

Memory Store	Capacity	Duration
Sensory Memory	Very large (all sensory input)	0.5 seconds (visual) 2-4 seconds (auditory)
Short-Term Memory	7±2 items (or chunks)	18-30 seconds without rehearsal
Long-Term Memory	Virtually unlimited	Potentially lifetime

Practical Applications

Understanding memory capacity has important real-world applications:

🎓 Study Techniques

Use chunking to remember more information when revising. For example, when memorising the periodic table, group elements by their properties rather than trying to remember each one separately.

Take regular breaks when studying to allow information to transfer from STM to LTM.

💻 Technology Design

Phone numbers are chunked (e.g., 07700 900123) to make them easier to remember.

Websites and apps limit the number of navigation options to avoid overwhelming users' STM capacity.

Summary: Key Points to Remember

Sensory memory has a large capacity but extremely brief duration (0.5-4 seconds)
Short-term memory has limited capacity (7±2 items) and brief duration (18-30 seconds)
Chunking can effectively increase STM capacity by grouping information
Long-term memory has virtually unlimited capacity and can last a lifetime
Each memory store plays a different but essential role in how we process and remember information

Exam Tip 💡

When discussing memory capacity in exams, always mention specific studies (like Miller's 7±2 or Peterson & Peterson's duration study) to support your points. Also, be ready to explain how chunking can overcome STM limitations, as this demonstrates deeper understanding of the topic.

🧠 Test Your Knowledge!