Coding, capacity and duration of memory

Describe short-term memory (STM)

• The limited-capacity memory store

• Coding is mainly acoustic (sounds)

• Capacity is between 5 and 9 items on average, duration is about 18 seconds

Describe long-term memory (LTM)

• The permanent memory store

• Coding is mainly semantic (meaning)

• It has unlimited capacity and can store memories for up to a lifetime

Outline coding

The process of converting information between different forms

Outline capacity

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

Outline duration

The length of time information can be held in memory

Describe research into the duration of STM

• Peterson and Peterson (1959) tested 24 students in eight trials. On each trial the student was given a consonant syllable to remember. They were also given a 3 digit number.

• The student counted backwards from this number until told to stop, which was done to prevent any mental rehearsal of the consonant syllable

• On each trial they were told to stop after varying periods of time: 3, 6, 9, 12, 15 or 18 seconds. After 3 seconds, average recall was about 80%, after 18 seconds it was about 3%

• These findings suggest that STM duration may be about 18 seconds, unless we rehearse the information over and over (such as verbal rehearsal)

Describe research into the duration of LTM

• 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: 1) photo-recognition test consisting of 50 photos, 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 the LTM may last up to a lifetime for some material

Strengths of Baddeley's study

• Baddeley's study identified a clear difference between two memory stores.

• For example later research showed that there are some exceptions to Baddeley's research. But the idea that STM uses mostly acoustic coding and LTM mostly semantic has stood the test of time

• This was an important study in our understanding of the memory system, which led to the multi-store model

Weaknesses of Baddeley's study

• Baddeley's study used quite artificial study rather than meaningful material, for example the word lists had no personal meaning to the participants.

• So therefore, 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

Strengths of Jacob's study

• Jacob's study has been replicated.

• The study is very old and early research in psychology often lacked adequate controls. (such as the participant's digit spans being underestimated because of distractions during testing; confounding variables)

• Despite this, Jacobs' findings have been confirmed by other, better controlled studies (Bopp and Verhaeghen 2005)

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

Limitations of Miller's study

• Miller's research may have overestimated STM capacity.

• 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

Limitations of Peterson and Peterson's study

• In Peterson and Peterson's study the stimuli material was artificial.

• 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

Strengths of Bahrick et al's study

• High external validity:

• Researchers investigated meaningful memories (people's names and faces). When studies on LTM were conducted with meaningless pictures to be remembered, recall rates were lower

• This suggests that Bahrick et al's findings reflect a more 'real' estimate of the duration of LTM

Describe research into coding

• Baddeley (1966) gave different lists of words to four groups of participants to remember: Group 1 (acoustically similar): words sounded similar Group 2 (acoustically not similar): words sounded different Group 3 (semantically similar): words with similar meanings Group 4 (semantically not similar): words with different meanings

• Participants shown original words and were asked to recall them in correct order

• When they did this task immediately, recalling from STM, they tended to do worse with acoustically similar words. When they recalled the word list after 20 minutes, recalling from LTM, they did worse with semantically similar words

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

Describe two pieces of research into capacity

• Digit Span

• Jacobs (1887) found out how much information STM can hold at one time through measuring digit span:

• The researcher reads out four digits and the participant recalls them out loud in the correct order. They keep doing this until the participant cannot recall the order correctly, indicating the individual's digit span

• Jacobs found that the mean span for digits across all participants was 9.3 items, with the mean span for letters being 7.3

• Span of memory and chunking

• Miller (1956) thought that the span 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. This is done through chunking (grouping sets of digits or letters into units or chunks).

• He used everyday observation.