Short-term memory (STM)

Chapter 5

Information-processing approach

Sensory memory

Short-term memory (STM)

Long-term memory (LTM)

Encoding

Storage

Retrieval

Short-term memory

Questions – Are STM and LTM distinct systems?

– How is information represented in STM?

– What is the capacity of STM?

– What is the duration of STM?

– How is information retrieved from STM?

Method – List learning

– Recall or recognition

– Immediate or delayed

You will see one number at a time. When I say “Go”, write down the numbers in the order you saw them.

5

2

9

1

4

8

7

3

6

Answers

5

2

9

1

4

8

7

3

6

Memory span

Measure of capacity of STM

General method – Present increasing number of stimuli to repeat

– 2 or 3 trials for each length

– “Span” is longest string of stimuli can successfully reproduce

Tasks – Digit span (Forward digit span)

– Alphabet span, Word span

– Picture span, Location span (“corsi blocks”)

Capacity of STM Gathercole (1999)

Examine development

of STM capacity

Method

– Memory span

– Stimuli: Digits, words,

spatial patterns, etc.

Results

– Steep increase to 8yrs

– Gradual improvement

until 12yrs

What is capacity of STM?

Task: memory span

– Recall

Capacity estimates

– Miller’s magical

number 7 +/- 2

– Cowan’s model: 4

items

Why is there lack of

agreement?

Luck & Vogel (1999)

What is capacity of STM?

It depends on…

Rehearsal rate – Naveh-Benjamin & Ayres

(1986): #s in 5 languages

Type of information – Verbal vs. visuo-spatial

Chunking – Create larger, meaningful

units

Knowledge (long-term memory)

Brown-Peterson task: Brown (1958); Peterson & Peterson (1959)

What is duration of STM?

Method – 3-letter stimulus to remember

– 3-digit number to count backwards (distractor)

Results – Forgetting curve

Conclusion – Information DECAYS from

memory

– Due to passage of time

Brown/Peterson CogLab results

Study trigram

2s

Count

backward by 3s

IV: distractor

duration

DV: accuracy

New task

version?

Surprise test? Fall 2010; N = 9

Keppel &Underwood (1962) Analysis of Brown-Peterson task

50

55

60

65

70

75

Block 1 Block 2 Block 3 Block 4

3s delay% accuracy

12 trials/block

Finding: More trials -> worse performance Conclusion: Effect due to proactive interference (PI) PI: Old learning leads to worse performance on new info

What is duration of STM?

Decay

– Info fades

Interference

– Proactive interference

Previously presented material interferes with new learning

– Retroactive interference

Recent material interferes with older learning

How do you separate influence of decay vs. interference?

– Blank interval = rehearsal

– Busy interval = interference

Waugh & Norman (1965)

Hyp: Forgetting curve due to interference

Method

– Probe digit task

– Heard 16 digits at rate of 1 or 4 per sec

Same amount of items to interfere

Differ amount of time (decay time)

– Repeat digit recall following digit in series

Waugh & Norman (1965)

Results # intervening items

reduces memory

Conclusion Forgetting due

interference not

decay

Talland (1967)

Examine nature of distractor task

Method

– Brown-peterson task

– Distractor task: subtraction vs read answers

Results

– Subtraction group did worse than reading group

Conclusion

– Forgetting depends on type of interference

Sternberg (1966)

Research question:

– How long does it take to retrieve info from STM?

– Measure speed of “search” process

Method:

– Additive-factor method (repeat stage multiple

times)

– STM scanning

– Memory set; probe y/n; DV: RT

Possible results:

Search predictions

Parallel

search

Serial

exhaustive

search

Serial self-

terminating

search

Sternberg’s (1966) findings

Sternberg: CogLab S09 data

Sternberg Results/Conclusions

STM: serial exhaustive search

– Entire set is scanned whether or not match is

found

Scan rate = 38ms per item

Size does matter!

Why would a serial exhaustive search be

more efficient than a serial self-terminating

search?

Unanswered research questions still…

Is there a separate STM system?

– Same as sensory memory with rehearsal?

– Same system as long-term memory?

What is the capacity of STM?

What is the duration of STM?

Are there other STM codes?

Is there a distinction between verbal and

visuo-spatial STM?

– Do they have different capacities?