The Study of Memory

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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?

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Why 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

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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 HMs 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."

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HM 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 lives
• Who cares for him
• What he ate at his last meal
• What year it is
• Who the President of the United States is
• or 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.

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Tower of Hanoi Puzzle
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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.

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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

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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.

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Duration 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.

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Correct as function of delay in Brown/Peterson
task
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Brown/Peterson paradigm

• Result for interval gt 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 dont have to rehearse things in LTM so
  there must be a second memory system STM.

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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.

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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.

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correct
Position in list
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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

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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?

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The 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 Metzlers 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., Wickens 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 HMs 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.