The Study of Memory

Part 2 – Short Term Memory

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Three Questions for Today

1. Why did researchers come to believe in STM independent of LTM?

2. What do we think of those reasons now?

3. Do we need the STM construct?

3Why did people originally believe in STM independent of LTM?

Because of STM – LTM differences in:

* Loss following brain damage* Capacity* Duration* Type of code* Serial position effect* Mechanism of loss

Memory Loss Following Brain Damage

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Memory loss following brain damage

Issue: is there a patient who can get new information into STM but not into LTM?

If so, that selective impairment could be used in an argument for an independent STM.

The most famous of all memory patients is HM.

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HM (Scoville & Milner, 1957)

Surgery to relieve severe epilepsy, in 1953, at age 27.

* bilateral excision of medial temporal lobe

* after surgery, HM had profound anterograde amnesia. Capable of little if any new learning.

* some retrograde amnesia.

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Definitions (1)

Anterograde amnesia – inability to remember things that happened after brain damage. Implies inability to encode new memories

Retrograde amnesia – inability to remember things that happened before brain damage. Implies inability to retrieve existing memories.

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In HM’s words:

"At this moment everything looks clear to me, but what happened just before? That's what worries me. It's like waking from a dream; I just don't remember."

9HM – Psychological studies by Brenda Milner.

HM has:

* Good vocabulary and language; normal IQ

* No attention disorder.

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Things HM does not know

Where he livesWho cares for himWhat he ate at his last mealWhat year it isWho the President of the United States isor how old he is.

In 1982, HM failed to recognize a picture of himself that had been taken on his 40th birthday in 1966 (13 years post surgery).

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Definitions (2)

Declarative knowledge – knowledge that you (a) know you have, and (b) can can talk about. E.G., Your name.

Procedural knowledge – knowledge that you have but may not know you have and cannot articulate. E.G., How do you lean a bicycle into a corner?

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Declarative tasks – tests show:

HM cannot learn (and later recall) new

* photographs of people* verbal material* sequences of digits* complex geometric designs* nonsense patterns.

He also cannot expand his digit span.

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Procedural tasks – tests show:

Milner (1962) trained H. M. on a mirror-drawing task.

* HM, like normal people, improves with practice. But he denies having practice.

Cohen and Corkin (1981) showed a similar result on the Tower of Hanoi puzzle.

Tower of Hanoi Puzzle

Tower

Rings

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HM - Conclusion

Though HM can learn new procedures he cannot acquire new declarative learning.

* LTM impaired. But STM spared.

Argument in favor of view that STM and LTM are independent.

Capacity

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Capacity

If capacity of STM is different from that of LTM, that supports view that LTM and STM are independent.

Capacity of LTM is essentially infinite.

What is capacity of STM?

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Capacity

Shepard & Tehgtsoonian (1961)

* Presented 200 3-digit numbers in a row.

* E.g. … 492, 865, 931, 758… 865, …

* Task: report when you hear a repeated number

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Shepard & Teghtsoonian (1961)

I.V.: Interval before repetition

D.V.: Probability of noticing repetition

* Repetition can only be noticed if first occurrence is still in memory.* Forgetting function: how does probability of noticing repetition vary with interval?* Question: Are there separate forgetting functions for LTM and STM?

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Shepard & Teghtsoonian (1961)

Result:

* P(noticing repetition) fell dramatically at first

* Steep decline ended at interval = 7 items

* P(noticing) then fell more gradually, asymptoting at 60%

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S & T (1961) – Interpretation:

* Initial steep decline in P (noticing) occurs because response coming from STM.* Decline is steep because STM contents decay quickly.* More gradual decline occurs when response depends upon LTM.*Decline is gradual because LTM contents decay very slowly if at all.* Two forgetting functions – two memory stores, one large and one small.

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Question:

Why should STM have so small a capacity?

Sensory memory has large capacity. LTM has large capacity.

Why did we evolve a limited capacity store between two large capacity stores?

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Answer:

If STM was any larger, it would take too long to search through.

When we need information from STM, to choose or guide a response, we need it fast.

Things have to be processed fast in STM…

Duration

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Duration.

Issue: how long do STM traces last?

LTM traces last a long time – possibly your whole life.

If STM traces last less time, that supports the view that STM and LTM are independent.

26Duration – how long do STM contents last?

Brown (1958) and Peterson & Peterson (1959)

Task: subjects briefly see a stimulus (e.g., BRG) and have to recall it after an interval.

Rehearsal is prevented by having them count backwards during retention interval.

I.V. = length of interval in seconds.

% Correct as function of delay in Brown/Peterson task

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Brown/Peterson paradigm

Result: for interval > 18 seconds, subjects can no longer report stimulus.

Interpretation: there is a memory system in which things must be rehearsed, or they are lost.

But we don’t have to rehearse things in LTM – so there must be a second memory system – STM.

Type of code

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Type of Code

Issue: every stimulus has multiple aspects – e.g.

* color* brightness* shape* category* name

Information about all these aspects is found in LTM. Which are found in STM?

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Brown/Peterson paradigm – again…

* Many studies used this paradigm in the ’60s. Most of the errors subjects made were phonological – e.g., P for T.

* Errors based on shape were rare – e.g., C for O.

* No semantic errors observed (or possible).

* Conclusion: STM uses a phonological code.

Serial Position Effect

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Serial Position Effect:

In ordered recall, subjects recall a list of words in the order they were given.

Out-of-order responses are counted as errors.

Accuracy is higher for the beginning and end of the list, lower for the middle of the list.

Position in list

%correct

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Serial Position Effect

Better performance at beginning of list is called Primacy Effect.

Better performance at end of list is called Recency effect.

Theory:* Primacy due to transfer to LTM (rehearsal).* Recency reflects availability of items still in STM

Mechanism of Loss

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Mechanism of Loss from Memory

How are things lost from memory – if at all?

Decay?

Interference?

Retrieval failure?

* LTM loss was blamed on interference* STM loss was blamed on decay – as in Brown/Peterson paradigm.

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Three Questions for Today

1. Why did researchers come to believe in STM independent of LTM?

2. What do we think of those reasons now?

3. Do we need the STM construct?

39The argument for independence of STM

Differences between STM and LTM:

* Type of code* Serial position effect* Mechanism of loss* Patient data* Capacity* Duration

Do these reasons survive?

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Type of Code

Original argument – any kind of code in LTM, only phonological codes in STM.

We now know – that STM can contain any kind of code.

* See, for example, Brooks (1968), and Wickens ‘Release from Proactive Inhibition’ studies.

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Type of Code

Shepard & Metzler’s Mental Rotation Studies

* Pairs of abstract forms displayed* Subject asked whether one is a rotated version of the other.* Have to mentally rotate one to see if it ever matches the other.* Mental rotation requires a visual code in STM. (Why?)

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Serial Position Effect:

Original argument – Primacy effect produced by LTM, Recency effect produced by STM.

We now know – that both Primacy and Recency effects can be found in pure LTM studies (e.g., recalling U.S. Presidents).

Thus, recency effect cannot be taken as “empirical signature” of STM.

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Mechanism of Loss

Original argument – information lost from STM through decay, from LTM through interference.

We now know – that information can be lost from STM through interference.

* E.g., Wicken’s Release from Proactive Inhibition studies.

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Duration

Original argument:

* newly-acquired memories must be rehearsed to survive

* but older memories do not need to be

* therefore, new and old memories must be in separate stores.

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Duration

Alternative account:

* traces in LTM are vulnerable until they have been consolidated.

* new items in LTM are more vulnerable to loss than ‘established’’ items.

* so, vulnerable items could be in LTM

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HM

If traces in LTM are vulnerable until they have been consolidated, then HM’s problem is that he cannot consolidate.

* He has normal digit span because new items can be inserted in LTM.

* But he has anterograde amnesia because new items cannot be consolidated in LTM.

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Capacity

Many psychologists now say ‘Capacity’ means capacity of the Articulatory Loop (AL)

* AL is used for rehearsal of information and for planning articulation.

* AL is not a short-term memory.

* For one thing, you cannot search your articulatory loop, the way you can search memory.

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Articulatory loop.

Capacity is determined by rate of loss. You can rehearse about 7 items.

If you try to rehearse more than 7 items, the first ones will be lost before you finish one cycle through the list and go back to the beginning.

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Articulatory loop in action (1)

Memory load = r l z t c j a

Articulatory loop rehearses:

r l z t c j a .. r l z t c j a .. r l z t c j a ..

‘r’ is still in loop when you finish ‘a.’

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Articulatory loop in action (2)

Memory load = r l z t c j a m k s c p y

Articulatory loop rehearses:

r l z t c j a m k s c p y ..

‘r’ is no longer in loop by the time you finish ‘y,’ so cannot be rehearsed – ‘r’ is lost.

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Three Questions for Today

1. Why did researchers come to believe in STM independent of LTM?

2. What do we think of those reasons now?

3. Do we need the STM construct?

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Do We Need the STM Construct?

No. Many cognitive psychologists argue that we do not need STM in our memory theory.

We can explain all memory phenomena in terms of LTM and the articulatory loop.

All we need is two premises:

* Limited capacity in articulatory loop.* Items in LTM are vulnerable to loss until they have been consolidated.