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?