Short-term memory

Properties of STM• Capacity• Duration/Forgetting• Retrieval• Code type

• Alternatives to Modal’s view of STM

Measuring STM Capacity: Digit-Span Task

• Instructions: Recall the digits in the order presented.

• Span Defined – list length that produces accurate performance on 50% of trials

numbers

• Digit Span Defined: # of digits accurately recalled 50% of the time• Standard Span: 7±2 digits• Modal Model Interpretation (Miller, 1956):

– STM Capacity: ≈ 7 “chunks”

Measuring STM Capacity: Digit-Span Task

Chunking Demo

CDHAOETGN

C D HA O ET G N

Chunking Demo

CDHAOETGN

C D HA O ET G N

Chunking Demo

CDHAOETGN

C D HA O ET G N

Chunking• Chunking – “the process of combining information

so that it takes up as little as possible of the limited space in STM”

• Chunking capacity

• Why not a limitless STM?– “Chunk chunked chunks?

• Required:– Chunking scheme– Time to apply scheme

Extraordinary Digit-Span: SF

• Materials:– random digits

– auditory presentation

• Results:– After 45 days of practice: span = 83

SF: Digit Span

How did he do it?

Chunk (and elaborated) groups of digits into running times (or historical dates)

Duration & Forgetting in STM

• Brown-Peterson (1958/1959)Task:Initial attempt to measure duration of STM

• Procedure:– Hear target set: 3 letters

– count backwards for X s

– recall target set

• Manipulation – length of retention interval• Assumption:

– Counting task eliminates/minimizes rehearsal

– Measure of the rate of decay

Brown-Peterson: Main Finding

• Initial interpretation: information rapidly decays from STM

• Note: w/ 0-delay, only 80% accuracy.

A Test of Decay Hypothesis

• Waugh & Norman (1965) – Decay or interference?• Method:

– Auditory digit list (two speeds), followed by probe digit– TASK: name the digit that followed the probe in the list

• Decay prediction: – Recall better at faster presentation rate

• Interference prediction: – Recall equal for both presentation rates

Slow fast Slow fast

Waugh & Norman (1965)

Results:• Recall w/ # of

intervening items– consistent w/ both

decay & interference

• Recall (more or less) unaffected by presentation rate– Consistent mainly w/

interference

That was “retroactive” interference:

Items that follow the to-be-remembered information interfere with remembering the earlier stuff:

That was “retroactive” interference:

Items that follow the to-be-remembered information interfere with remembering the earlier stuff:e.g. if you have a psych and an english test in one week, if you study psych, then study english, all the english info will interfere with your memory for the psyc info.

But there is a second kind of interference as well:”proactive”:

Information that was previously studied will ALSO interfere with subsequent information you study.

Evidence for PI in Brown-Peterson Task

-Keppel & Underwood (1962): re-analyzed Brown & Peterson’s data.Took a look at recall trial-by-trial, at various retention intervals

Notice: on trial 1, 100% accuracy at ALL RI’s, only after 3 trials do you see substantial drops, which is more pronounced at longer RI’s

Wickens, Born & Allen (1963): release from PI by varying target stimuli

-first three trials, target consists of letters, fourth is letters in control, numbers in “changed category” group

Side track: This issue of decay versus interference can be seen in other areas: e.g. Ebbinghaus’ serial position curve

Side track: This issue of decay versus interference can be seen in other areas: e.g. Ebbinghaus’ serial position curve

What might a “decay-based” perspective say about primacy/recency? (e.g. Atkinson & Shiffrin’s Multistore/modal model)

What would the “interference” camp have to say?

So who’s right?

So who’s right?

These days, more evidence of forgetting based on interference than decay, although decay isn’t ruled out entirely, since it does have supporting evidence that an interference hypothesis wouldn’t anticipate (anterograde amnesics fail to show a primacy effect)

STM Retrieval/searching: 3 Possiblities

• Issue: – How do we access

information in STM?

– Is Item X in STM?

• Three possibilities:– Parallel –

simultaneous access to all items.

– Serial – consider 1 item at a time.

Retrieval Models

Parallel Serial

ExhaustiveSelf- Terminating

Testing between these alternatives: The Sternberg Task

– Set Size: 1 to 6 letters (well within limit for accurate memory in most people

– Probe Type:• positive (in memory set)• negative (not it set)

Competing Retrieval Model Predictions

Sternberg Task: Results

• RT w/ set size

Implication: serial

• Negative = PositiveImplication: exhaustive

Codes of short-term memory

Codes of short-term memory

-semantic code

Codes of short-term memory

Wickens & Morisano (1972): release from PI by varying category of studied items

-semantic code

Wickens & Morisano (1972): release from PI by varying category of studied items

-first three trials, target consists of fruit, fourth is fruit in control, other things in other conditions

-varied the similarity of the new target to the old category

Codes of short-term memory

-semantic code

Codes of short-term memory

-semantic code: Wickens & Morisano (1972)

-visual code: Shepard and associates

Shepard’s rotational stimuli

A.

A.

45

B.

B.

N/A

B.

B.

110

D.

D.

180

Codes of short-term memory

-semantic code: Wickens & Morisano (1972):

-visual code: Shepard and associates

Codes of short-term memory

-semantic code: Wickens & Morisano (1972):

-visual code: Shepard and associates

-other codes (e.g. Shand, 1982)

Working Memory: somewhat of an elaboration of the traditional STM description