Transcript 2 Memories or 1? - California State University, Fullerton

STM
2 Memories or 1?
LTM
The Issue
For decades. researchers have debated the question
of whether memory is divided into a short-term store
and a long-term store – two memories that function in
different ways and operate on different principles.
Dual-store theorists make this distinction. They say
that a given independent variable may have different
effects on recall depending on which memory the
individual uses.
This is called “dissociation”: The same variable has
different effects in different tasks.
STM
2 Memories or 1?
LTM
The Issue
Single-store theorists say that we have only one
memory. A single set of principles can be used to
explain performance in any memory task.
It’s like having a formula you can use to predict how
many items a subject will recall. For each task you
enter numbers for such variables as the number of
practice trials, the number of items in the list, and the
length of the retention interval.
Dual-store theorists would say you need two formulas,
not just one.
STM
2 Memories or 1?
LTM
Classic Experiment
An experiment by Peterson and Peterson (1959)
aroused a lot of controversy because it seemed to
demonstrate the existence of a short-term memory.
Procedure
1. You are read a single nonsense syllable consisting of three
consonants, like CHJ.
2. Immediately afterwards, a 3-digit number is stated, like 506.
You count backwards from that number continuously until a signal
light flashes 3 to 18 seconds later.
3. You then try to recall the three letters in their original order.
STM
LTM
2 Memories or 1?
Classic Experiment
Let’s Try It!
When the top (green)
light comes on, you
will hear 3 letters and
then a number. You
should immediately
repeat that number
to yourself and then
start counting
BACKWARDS from it
by 3’s.
s
When the bottom
(red) light comes on,
stop counting
immediately and try
to write down the
letters that were
given in their original
order.
STM
LTM
2 Memories or 1?
Classic Experiment
Let’s Try It!
The interval between
the last letter
pronounced and the
red light was 18
seconds. At this
retention interval,
participants were
correct only about
10% of the time.
s
The answer...
K S G
However, the percent
of the class who got
KSG right is probably
higher than that.
That’s because of a
flaw in the original
experiment that
undermined the
Petersons’
conclusions.
Before discussing the flaw, let’s look at the overall results.
STM
LTM
Probability of Correct Recall
2 Memories or 1?
1.0
At a retention interval of 3 seconds, recall
was down to a probability of about .80.
At an interval of 18 seconds,
recall was down to .10.
.5
.0
Classic Experiment
0
3
6
9
12
15
Retention Interval (seconds)
18
STM
LTM
2 Memories or 1?
Classic Experiment
Importance
Such rapid forgetting of a single nonsense syllable
over just 18 seconds was surprising.
Most studies used retention intervals of hours or days
and found less forgetting of whole lists of items.
The prevailing theoretical framework was the
“interference theory of forgetting.” It said that we
forget things because of other things we have
learned, not simply because of the passage of time.
STM
LTM
2 Memories or 1?
Classic Experiment
Importance
It appeared that memory simply “decayed” as time
passed. If it wasn’t decay, then what was the source
of the interference?
Could it be the numbers subjects recited during the
retention interval to prevent them from rehearsing the
nonsense syllable?
Target Information
Letters
Interfering Information
Numbers
STM
LTM
2 Memories or 1?
Classic Experiment
Importance
This would be _______________
retroactive
interference
(retroactive or proactive).
RI from the numbers to the letters was unlikely.
Studies showed that interference was strongest when
the interfering and target information were similar.
Target Information
Letters
Interfering Information
Numbers
STM
LTM
2 Memories or 1?
Classic Experiment
The Flaw
The Petersons acknowledged another possible source
of interference: Each participant got 48 different
nonsense syllables during the experiment, 8 at each
of 6 retention intervals. What if they remembered
some of them? Then we would have...
Interfering Information
Letters on
Previous Trials
Target Information
Letters on
Current Trial
STM
LTM
2 Memories or 1?
Classic Experiment
The Flaw
There could be proactive interference from letters on
previous trials to letters on the current trial. This
should not happen if those letters disappeared from
short-term memory and did not get into long-term
memory (since they weren’t rehearsed).
Interfering Information
Letters on
Previous Trials
Target Information
Letters on
Current Trial
STM
LTM
2 Memories or 1?
Classic Experiment
The Flaw
The Petersons tested this by looking at correct recalls
in blocks of 12 trials to see if recall went down as the
number of preceding trials went up. There was no
decrease, no evidence for proactive interference.
Interfering Information
Letters on
Previous Trials
Target Information
Letters on
Current Trial
STM
LTM
2 Memories or 1?
Classic Experiment
The Flaw
But then Keppel and Underwood (1962) looked at
recall on every trial, starting with Trial 1. Now there
was clear evidence for proactive interference.
Here is what they found for subjects’ very first
nonsense syllable.
Interfering Information
Letters on
Previous Trials
Target Information
Letters on
Current Trial
STM
LTM
Probability of Correct Recall
2 Memories or 1?
1.0
Classic Experiment
Recall on Trial 1 (Keppel &
Underwood, 1962)
.5
Caused by PI
from
previous
items
Peterson &
Peterson
.0
0
3
6
9
12
15
Retention Interval (seconds)
18
STM
2 Memories or 1?
LTM
Classic Experiment
Interference Effects
Keppel and Underwood (1962) found that as the
number of trials increased, recall of the current item
decreased. The previous trials were a source of PI.
An additional source of interference is the letters
within the nonsense syllable (“intraitem interference”.)
Recall of a single consonant is almost 100% at an
interval of 12 seconds, and it gets lower and lower as
you go from 1 to 5 consonants (Melton, 1963).
STM
2 Memories or 1?
LTM
Classic Experiment
Interference Effects
To reduce intraitem interference, you can present a
simple word instead of unrelated letters.
Murdock (1961) found that after 18 seconds, recall of
a single, monosyllabic word was almost 100%. Recall
of 3 words was as low as 3 consonants.
Such interference effects should not occur if subjects
recalled the items from short-term memory.
STM
2 Memories or 1?
LTM
Classic Experiment
Dissociation or Continuity?
Short-term memory implies information disappears
rapidly as time passes. That’s why it’s called “shortterm”. In contrast, the passage of time is considered
to be a minor factor in long-term memory...
Recall Jenkins & Dallenbach’s experiment on sleep and
memory: Subjects forgot relatively few nonsense
syllables when they were asleep during an 8-hour
retention interval. They forgot a lot more if they were
awake and susceptible to interference.
STM
LTM
2 Memories or 1?
Classic Experiment
Dissociation or Continuity?
Did Peterson & Peterson demonstrate that time was
an important factor in forgetting? This would
represent a dissociation of the effects of time in
different kinds of memory tasks and support the dualstore approach.
STM
2 Memories or 1?
LTM
Classic Experiment
Dissociation or Continuity?
Characteristics of Long-Term Tasks:
1. Multiple practice trials
2. Long lists — at least 10 items.
3. Long retention intervals — hours or days
Characteristics of Short-Term Tasks:
1. 1 presentation of the item(s)
2. Short lists — less than 10 (within memory span)
3. Brief retention interval — less than 30 seconds
STM
LTM
2 Memories or 1?
Classic Experiment
Dissociation or Continuity?
Interference variables, not time, caused forgetting, so
the Petersons’ task was not fundamentally different
from conventional long-term tasks.
This famous experiment on short-term memory did
not even support the dual-store theory!
STM
2 Memories or 1?
LTM
Classic Experiment
Short-Term Memory vs. Short-Term Store
Some researchers (like Atkinson and Shiffrin)
distinguish between these terms. “Short-Term Store”
refers to a theoretical concept. “Short-Term Memory”
refers to performance in short-term tasks, like the
Petersons’. This performance may or may not be
explainable in terms of a short-term store.
It’s important to distinguish between observation and
interpretation. However, short-term memory usually
refers to a theoretical construct.
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Some of the strongest evidence for the dual-store
theory comes from the free recall procedure.
This is where you are presented with a series of words
just once and immediately afterwards you try to recall
the words in any order.
You are most likely to remember the beginning words
(primacy effect) and the end words (recency effect).
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Repetition
According to the Atkinson-Shiffrin model, the more
times you repeat something, the more likely it is you
will transfer it from short-term to long-term memory.
In free-recall experiments, subjects usually repeat
words as the list is presented. Rundus (1971) asked
subjects to rehearse aloud so he could tape record
them and count how many times each word was said.
LTM
Probability of Correct Recall
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Repetition
1.0
Recall showed a
typical serial
position curve...
.5
.0
1.0
7
Earlier items were repeated
more times than later items.
1
5
10
15
Serial Position in List
1
20
Mean Number of Repetitions
STM
LTM
Probability of Correct Recall
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Repetition
1.0
Repetition went in the
same direction as
recall for beginning
and middle items.
.5
.0
1.0
7
It went opposite to recall
for the end items.
1
5
10
15
Serial Position in List
1
20
Mean Number of Repetitions
STM
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Repetition
Dual-store theorists would say that subjects had to
use LTM to recall the beginning and middle items.
These words wouldn’t last long enough in STM until
the recall test was given.
Repetition helped put these words into LTM: the more
repetitions, the higher the recall.
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Repetition
What about the end items? Recall went up as
repetitions went down, so recall of end items did not
depend on repetition.
Dual-store theorists would say that it depended on
time: The closer a word was to the test, the less it
would decay, and the better were the chances of
recalling it.
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Repetition
The bottom line?
The effects of repetition were dissociated according
to a word’s position in the list. This is evidence that
the words came from different memories that
functioned in different ways.
D I S S O C I A T I O N
s
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Retention Interval
The retention interval is the time between
presentation of the last word and the signal to recall
the list.
Suppose you had to wait 30 seconds before recalling
the list. Which words would be most affected?
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Retention Interval
Dual-store theorists say the beginning and middle
words are recalled from LTM. This holds information
for years; 30 seconds shouldn’t matter.
End items come from STM, which only holds
information up to 30 seconds without rehearsal. So
waiting 30 seconds should erase these words.
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Retention Interval
Glanzer and Cunitz (1966) presented a 15-word list
and gave a free-recall test either immediately after
the last word or after a 30-second delay.
During the delay, subjects counted backwards from a
number so they couldn’t rehearse the words.
The results...
STM
LTM
Probability of Correct Recall
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Retention Interval
1.0
Primacy
Effect
unaffected
by delay
.5
.0
Recency
Effect
eliminated
by delay
Immediate Test
30-second delay
1
5
10
Serial Position in List
15
STM
LTM
2 Memories or 1?
Free Recall
Evidence for Short-Term Memory
Effects of Retention Interval
The fact that retention interval had different effects on
different parts of the list is another example of...?
D I S S O C I A T I O N
It’s another reason to dissociate
short-term from long-term memory.
s