Types of memory and models of memory

1. Introduction to short-term memory

Short-term memory (abbreviated as STM) is responsible for storing small amounts of material over short periods of time

• The amount of information varies with each individual person’s memory span
• A short time = up to several seconds
• Anything remembered for longer than this time is classified as long-term memory

STM has often been tested with verbal or visual information,

• but applies to other types of stimuli as well

Short-term memory and working memory have different functions and are not interchangeable terms

2. The magical number seven, plus or minus two

Classic research by George Miller (1956) described the apparent limits of short-term memory span

• Memory span is the longest number of items (such as digits) that can be immediately repeated back correctly
  • in the correct order

Most people appear able to have a memory span of 5-9 items (average of 7)

• The magical number 7 also turns up in absolute judgment tasks
  • Here, people asked to indentify a set of stimuli varying along only one dimension
  • For example, tones that vary only in their pitch
  • "Listen to a sequence of isolated tones, give each a number"
  • "Report the number for each subsequent tone"
• Performance is good up to six tones, up to eleven for some visual tasks
• These results have been widely confirmed and replicated

3. Memory span continued

Short-term memory span can be ‘stretched’ by meaningfully grouping or chunking information

• For example, it is easier to remember one year, 1918, than four numbers 1, 9, 1, and 8
• We tend to naturally recode information into chunks as an aid to memory
  • Remembering postcodes, phone numbers, and words as units rather than lists of individual pieces

Memory span for words in a sentence is significantly longer than for unrelated, context-less words

• About 15 words (per Brener, 1940), compared to the magical number 7 otherwise
• Constraints imposed by the rules of grammar, context, are not sufficient to explain this
• We return to this finding as a part of the Baddeley-Hitch model of memory

4. Introduction to working memory

Working memory (abbreviated to WM) is a ‘mental workspace’

This concept that assumes some part of the memory system does the following:

• Temporarily maintains and manipulates information
• This intermediate maintenance and manipulation is necessary for many complex tasks

Different theories and models of memory have more or less complex versions of this concept

• that cooperate with the rest of the memory system in different ways.

An example of working memory: adding two multi-digit numerals in your head

Short term memory is one component of the larger working memory system

5. What about long-term memory?

Long-term memory (abbreviated LTM) is a system which stores information over long periods of time

• Probably more than one system

A “long period of time” can be a few minutes or an entire lifetime

• This sounds surprising, but the same system appears to cover both of those situations

LTM can be divided into explicit (declarative) memory and implicit (non-declarative) memory

Explicit memory includes

• Semantic memory (definition memory and general knowledge)
• And episodic memory (memory of specific events)

Implicit memory includes information that is retrieved by performance,

• rather than by conscious recall
  • Motor skills
  • Party pieces

6. What good is a model?

Models can test theories of memory (or other phenomena)

• By compactly describing existing data
• By making testable predictions

How good is a model?

• How much of the existing data can the model explain?
• How well does a model explain current data?
• What new data does the model suggest be collected?
  • And is the model confirmed or disconfirmed by new data?

Moreover, creating a model forces precision and computational completeness for a theory

• Assumptions must be explained, and ambiguity must be resolved
• All elements and operations must be specified in the type of detail a computer program would need

This class will focus on the Baddeley-Hitch model of memory.

This is not the only model of memory, but a good place to start

7. The Baddeley-Hitch model of memory

This Baddeley-Hitch model(B-H) is also more informatively called the multicomponent theory

Originally proposed in 1974

• it remains a widespread and useful explanatory tool to this day

Involves the following pieces:

1. A phonological loop for verbal and auditory information
2. A visuo-spatial sketchpad for visual and spatial information
3. A central executive as an “attentional controller”
4. A multi-dimensional episodic buffer and episodic LTM
   • The episodic buffer is a more recent addition to the B-H model, about a decade ago

Diagram from Baddeley (2003), in refs on last slide

8. The phonological loop

Component of the B-H model which temporarily stores speech or verbally encodable information

The phonological loop is composed of two subcomponents:

• A short-term store of limited capacity
• An articulatory rehearsal process which repeats (rehearses) information to maintain it in memory

Information goes into this store if

• It is from speech that we physically hear
• It is "read out" from a short-term memory trace
  • For instance, sub-vocally rehearsing an item. This is essentially re-entering it in the loop again
• You see visual information but remember by "saying it to yourself" (digits, letters, nameable objects)

9. The phonological loop, cont'd

The phonological loop accounts for the phonological similarity effect

• People show poorer recall for a list of similar sounding items
• It's been suggested that these individual items may have few distinguishing features and are easier to confuse

The word length effect

• Verbal memory span decreases as participants are asked to recall longer words (shorter words are easier)
• Maybe because each word takes more time to rehearse in the phonological loop and to recall
• A robust and well-replicated finding

10. The visuo-spatial sketchpad

This store is responsible for image-based information, and is also subject to memory span limits.

Appears to consist of two subsystems which work closely together to hold and manipulate information

• Memory for objects and their features (what)
• Memory for spatial locations (where)
• Note the similarity of this distinction to the organisation of the visual system
• Special tasks have been devised to selectively study one aspect or the other, see textbook

One illustration of a span limit on the sketchpad is visual pattern span (Della Sala et al. 1999)

• Participants are shown matrices with some cells shaded in, required to recall them
• Matrices start small (2x2) and are increased in size until participants’ recall span is reached

11. A new addition: the episodic buffer

The original B-H model struggled to explain interactions between working and long-term memory.

The episodic buffer has been suggested as an additional component to link memory subsystems with perception and LTM input/output

• A multi-dimensional (multi-modal) space
• Streams of information from various modalities such as sound, imagery are bound into objects, scenes, and other meaningful units

Binding is the process of linking individual object features into complete objects

• For example blue-ness and square-ness into a blue square
• Also can apply to creating a meaningful sentence out of words
• This module could potentially explain the memory span finding that we can remember twice as many words in a sentence as we can individual, unrelated words.

12. Putting it all together: The central executive

These subsystems for different types of information do not operate autonomously

The central executive is like the air-traffic control tower of the memory system

• Selects which possible pieces of information to manipulate (the "work" in working memory)
• Switches between tasks (not necessarily a function specific to the memory system)
• Its functions and the information it manipulates are limited by attention

The central executive is a very important concept in cognition

• With other roles
• and far-reaching implications outside of memory

We will return to this when studying attention

• As described, it's not obviously testable

13. Baddeley-Hitch model recap

Phonological loop for speech or any information that can be verbally encoded and rehearsed

Visuo-spatial sketchpad for image information, objects and features, spatial locations/relationships

Central executive for selecting and controlling information flow

Episodic buffer for additional storage and binding information

Long-term memory for prolonged storage of declarative and non-declarative information

14. References

Course texts

• Memory (Baddeley, Eysenck, & Anderson, 2009)

Optional readings

• The magical number seven, plus or minus two (Miller, 1956)

Other resources, available through the library or Google Scholar

• Working memory: looking backward and forward (Baddeley, 2003) NB: This is a useful review paper
• An experimental investigation of memory span (Brener, 1940)
• Pattern span: a tool for unwelding visuo-spatial memory (Della Sala et al., 1999)