Transcript A pioneer of the scientific study of memory: Hermann

Memory experiments of Ebbinghaus
Examination of forgetting curve with savings method
• most forgetting
happens
immediately
after learning
-> approx. 50%
in first 40 min
Forgetting on Brown-Peterson task
with consonant trigrams
1. Repeat ‘HLM’
2. Distractor task:
Count in 3s from
492
3. What were the
letters?
many trials with
different delays
in same session
after 6 sec only
40 % correct
recall
Forgetting on Brown-Peterson task
with consonant trigrams
• findings suggest short-lived memory component different
from type of memory studied by Ebbinghaus
Something special about short lists
• Ebbinghaus: lists with up to 7 nonsense syllables are
forgotten at different rate than lists with more items
• G. Miller (1956): immediate memory span
‘Magical number seven, plus or minus two’
+ holds for digits, letters, words
(digit span, letter span, word span)
-> suggests that short lived memory
component has limited capacity
Further examination of immediate memory span
• integration of information through chunking possible
e.g.
letters in meaningful word
letters in acronyms (USA, IBM)
area code in phone number
• chunks are basic storage units in short-lived memory
component
Chunking produces similar forgetting curve on
Brown-Peterson task with words and trigrams
Further research on chunking
• optimal chunks can be unique for different individuals
(e.g. running times; dates)
• learning of new chunking methods possible but laborintensive (e.g. digit-letter mapping system)
• when chunking leads to larger absolute number of
digits being retained, capacity still remains unchanged:
-> 7+ 2 chunks
-> change in coding of info instead
Consequences of findings with Brown-Peterson
task and with memory span in 1960s
• new notion: memory is not unitary
• there may be at least two separate stores that work
with different cognitive mechanisms
• they seem to differ in terms of forgetting rates and
capacity
-> longer lasting component may not have any capacity
limitations
e.g.
+ Ebbinghaus could remember lists of 40
and more nonsense syllables
+ everyday observations suggest that
memory capacity for life events unlimited
W. James: Primary vs secondary memory
• Primary memory:
“ it was never lost; its date was never cut off in
consciousness from that of the immediately present
moment; in fact, it comes to us as belonging to the
rear-ward portion of the present space of time, and
not the genuine past”
-> extended present
• Secondary memory:
“ the knowledge of a former state of mind after it has
already once dropped from consciousness; or rather
it is the knowledge of an event, or fact, of which we
have not been thinking, with the additional
consciousness that we have thought or experienced it
before”
Different proposals for two-store models
• W. James (late 1800s):
primary memory vs secondary memory
‘still in consciousness’ vs ‘lost from consciousness’
• Atkinson & Shiffrin (1968):
short-term memory (STM) vs long-term memory (LTM)
• Baddeley (1980s)
further theoretical development of concept of STM
-> renamed: working memory (WM)
Consequences of findings with Brown-Peterson
task and with memory span in 1960s
• controversy over need to postulate two stores
• philosophy-of-science argument relevant:
more parsimonious theory better than more
complicated one if it can explain the same findings
(principle of Ocham’s razor)
-> is there sufficient number of critical findings that
single-store theory cannot explain??
Forgetting mechanisms in STM vs LTM
• predominant explanation of forgetting on list-learning
tasks (à la Ebbinghaus) and others used in
behaviourist verbal-learning research: interference
e.g. for paired-associate learning task
First list
Second list
car – ball
car – fridge
tree – screen
tree – foot
table – paper
table – ocean
…
…
door – sports
door – hair
Forgetting mechanisms in STM vs LTM
• retroactive interference on paired-associate learning
task:
What was the missing word from first list?
tree – ?
door – ?
learning of second list impairs recall of first list
• interpretation in behaviourist learning theory:
associations between pairs in first list weakened
• application in everyday life:
previous postal code after move
Forgetting mechanisms in STM vs LTM
• suggestion for forgetting on Brown-Peterson task:
trace decay (fading)
- occurs as soon as stimulus absent
- can be prevented through active rehearsal
- different from interference on LTM tasks
-> numbers in distractor task known to produce
minimal interference for letters
-> forgetting observed on Brown-Peterson task
reflects trace decay
- interpretation later challenged by Keppel &
Underwood
Other evidence to support distinction between
STM and LTM:
Serial-position curve in free recall task
recency
effect
primacy
effect
Effects of presentation rate and delay on serial
position curve (Glanzer & Cunitz, 1966)
Interpreting effect of presentation rate and delay in
Glanzer & Cunitz’s experiment
• Presentation rate:
more rehearsal allows for better transfer from STM into
LTM in primacy portion
• Delay:
trace-decay in STM eliminates recency
-> different experimental manipulations have different
effects on recency and primacy portion of curve
-> suggests that primacy and recency effects reflect
operation of different stores (LTM, STM)
Other evidence in support of two-store
interpretation of serial position curve
• detrimental effect of old age only on primacy portion
• beneficial effect of word familiarity only on primacy portion
(more familiar words recalled better than less familiar
ones)
What makes a list difficult for STM (Baddeley, 1966)?
unrelated
semantically
similar
acoustically
similar
detrimental effect of acoustic but not semantic similarity on
immediate recall of short word lists
What makes a list difficult for LTM (Baddeley, 1966)?
detrimental effect of semantic but not acoustic similarity on
learning of long word lists
Interpretation of ‘difficulty findings’ in Baddeley’s
experiment
• detrimental effect of acoustic similarity on recall of brief
lists suggests that info is coded phonologically (in terms of
sound) in STM
• detrimental effect of semantic similarity on recall of longer
lists suggests that info is coded semantically (in terms of
meaning) in LTM
Other evidence suggesting semantic code for LTM
• prose passage experiment by Sachs (1967)
e.g. sentence heard as part of brief story:
‘she watched the kids on their way home’
Was the following sentence part of the story?
- she watched the birds on their way home
(no; easy)
- the kids were watched by her on their way home
(no; difficult)
- she observed the kids on their way home
(no; difficult)
-> good recognition of semantic but poor recognition of
verbatim information after delay
What kind of memory is impaired in neurological
patients suffering from amnesia?
• amnesia can occur as result of sudden brain injury (e.g car
accident) or stroke
- patients do not remember anything from hours / days
before incident
- patients have difficulty keeping track of daily events
- patients have difficulty learning names of new people
(e.g. physicians and nurses)
- patients have difficulty learning way around in new
environment (e.g. hospital)
• amnesic patients have memory deficits but otherwise normal
intelligence -> can be tested on experimental tasks used in
memory research
• critical question: does amnesia affect STM, LTM or both?
What kind of memory is impaired in neurological
patients suffering from amnesia?
• general findings in amnesic patients:
+ normal STM capacity on digit-span task
+ normal forgetting curve on Brown-Peterson task
could suggest that only LTM affected by amnesia
Serial position curve in amnesic patients
(Baddeley & Warrington, 1970)
• most aspects of list learning
impaired but recency portion
in immediate recall normal in
patients
-> further support for idea
that amnesia affects LTM but
not STM
Additional evidence showing that memory deficit is
specific to LTM in amnesia:
Findings with ‘span + 1’ task (Drachman & Arbit, 1966)
lists larger than STM span extremely difficult to learn for
patients
Can other types of brain damage lead to impaired
STM?
• patient K.F. studied by Shallice and Warrington (1970)
+ brain damage different from that in amnesic
patients
+ immediate memory span limited to 2-3 digits
+ poor performance on Brown-Peterson task,
especially with auditory presentations
+ normal performance on list learning tasks with
long lists
+ normal primacy but impaired recency effect
-> patient’s memory deficits seem to be limited to
STM; LTM unaffected by damage
What do patient studies tell us about distinction
between STM and LTM?
• observed pattern of deficits across studies:
+ one type of neurological condition leads to
impairments in LTM but not STM
+ other type of neurological condition leads to
impairments in STM but not LTM
-> evidence for double-dissociation of STM and
LTM deficits
-> suggests that LTM and STM require normal
functioning of different brain structures
-> suggest that LTM and STM have distinct neural
basis