Coding capacity and duration

Coding, capacity and duration of memory

The specification says…

Short-term memory and long-term memory.

Features of each store: coding, capacity and

Our everyday experience of memory is that there are two main types. some are brief and quickly forgotten, but others can last a very long time

investigated in great detail three main features of

What they call short term memory (oim) ana long. term memory (LTM).

Key terms

Short-term memory (STM) The limited-capacity

memory store. In STM, coding is mainly acoustic (sounds), capacity is between 5 and g items on average, duration is about 18 seconds.

memory store. In LTM, coding is mainly semantic (meaning), it has unlimited capacitv and can store memories for up to a lifetime.

Coding The format in which information is

storean me varlous memor stores.

Capacity The amount of information that can be held in a memory store.

Duration The length of time information can be held in memory.

Apply it Methods

Peterson and

Peterson

Some psychology students tried the technique

used oy relerson and reterson to assess the

are shown in the graph below.

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％ of corred responses

12

15

Retention interval (seconds)

Questions

1. Estimate the percentage of correct responses at each retention interval using the graph above.

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2. The results shown above are almost identical to Peterson and Peterson's results. What can you

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46 // Chapter 2 Memory

Research on coding

Information is stored in memory in different forms, depending on the memory store. The process of converting information between different forms is called coding.

Alan Baddeley (1966a, 1966b) gave different lists of words to four groups of participants to remember:

• Group 1 (acoustically similar): words sounded similar (e.g. cat, cab, can).

• Group 2 (acoustically dissimilar): words sounded different (eg. pit, few, cow).

• Group 3 (semantically similar): words with similar meanings (e.g. great, large, big).

• Group 4 (semantically dissimilar): words with different meanings (e.g. good, huge, hot).

Participants were shown the original words and asked to recall them in the correct order. When they did this task immediately, recalling from short-term memory (STM), they tended to do worse with acoustically similar words. When they recalled the word list after a time interval of 20 minutes,

recalling from long-term memory (LIM), they aid worse with the semantically similar words

These findings suggest that information is coded acoustically in STM and semantically in LTM.

Research on capacity

Digit span

How much information can STM hold at one time - what is its capacity? Joseph Jacobs (1887) found out by measuring digit span. For example, the researcher reads out four digits and the participant recalls these out loud in the correct order. If this is correct the researcher reads out five digits and so on until the participant cannot recall the order correctly. This indicates the individual's digit span.

Jacobs found that the mean span for digits across all participants was 9.3 items. The mean

span for letters was 7.3.

Span of memory and chunking

George Miller (1956) made observations of everyday practice. For example, he noted that things come in sevens: seven notes on the musical scale, seven days of the week, seven deadly sins, etc. Miller thought that the span (i.e. capacity) of STM is about 7 items, plus or minus 2. But he also noted that people can recall five words as easily as they can recall five letters. We do this by chunking - grouping sets of digits or letters into units or chunks.

Research on duration

Research on the duration of the sensory register is described on the next spread.

Duration of STM

How short is the duration of STM? Margaret and Lloyd Peterson (1959) tested 24 students in eight trials each (a 'trial' is one test). On each trial the student was given a consonant syllable (such as YCG) to remember. They were also given a 3-digit number. The student counted backwards from this number until told to stop. The counting backwards was to prevent any mental rehearsal of the consonant syllable (which would increase the duration of STM memory for the syllable).

On each trial they were told to stop after varying periods of time: 3, 6, 9, 12, 15 Or 18

seconds (the retention interval) The findings were similar to the student data on the left After 3 seconds, average recall was about 80%, after 18 seconds it was about 3%. Peterson and Peterson's findings suggested that STM duration may be about 18 seconds, unless we repeat the information over and over (i.e. verbal rehearsal).

Duration of LTM

Harry Bahrick et al. (1975) studied 392 American participants aged between 17 and 74. High school yearbooks were obtained from the participants or directly from some schools. Recall was

tested in various ways including- (1) nhoto-recognition test consisting of 5o nhotos some from the participants' high school yearbooks, (2) free recall test where participants recalled all the names of their graduating class.

Participants tested within 15 years of graduation were about 90% accurate in photo recognition. After 48 years, recall declined to about 70% for photo recognition. Free recall was less accurate than recognition - about 60% after 15 years, dropping to 30% after 48 years.

This shows that LTM may last up to a lifetime for some material.

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Evaluation

Separate memory stores

One strength of Baddeley's study is that it identified a clear difference between two memory stores.

Later research showed that there are some exceptions to Baddeley's

mostly semantic nas stood the test orume.

This was an important step in our understanding of the memory system, which led to the multi-store model (see next spread.

Artificial stimuli

One limitation of Baddeley's study was that it used quite artificial stimuli rather than meaningful material.

For example, the word lists had no personal meaning to participants.

So Baddeley's findings may not tell us much about coding in different kinds of memory tasks, especially in everyday life. When processing more meaningful information, people may use semantic coding even for STM tasks.

This suggests that the findings from this study have limited application.

Evaluation

A valid study

One strength of Jacobs' study is that it has been replicated.

The study is a very old one and early research in psychology often lacked adequate controls. For example, some participants' digit spans might have been underestimated because they were distracted during testing (confounding variable). Despite this, Jacobs' findings have been confirmed by other, better controlled studies since (e.g. Bopp and

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This suggests that Jacobs' study is a valid test of digit span in STM.

Not so many chunks

One limitation of Miller's research is that he may have overestimated

STM capacity.

Nelson Cowan (2001) reviewed other research and concluded that the

capacity of STM is only about 4 (plus or minus 1) chunks.

This suggests that the lower end of Miller's estimate (five items) is

more appropriate than seven items.

Evaluation

Meaningless stimuli in STM study

One limitation of Peterson and Peterson's study is that the stimulus

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The study is not completely irrelevant because we do sometimes try to remember fairly meaningless material (e.g. phone numbers). Even so, recalling consonant syllables does not reflect most everyday memory activities where what we are trying to remember is meaningful.

This means the study lacked external validity.

High external validity

One strength of Bahrick et al's study is that it has high external validity.

This is because the researchers investigated meaningful memories (i.e. of people's names and faces). When studies on LTM were conducted with meaningless pictures to be remembered, recall rates were lower (e.g. Shepard 1967).

This suggests that Bahrick et al's findings reflect a more 'real' estimate

Apply it

Concepts

Chunking in STM