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STM. LTM. 2 Memories or 1?. 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.

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STM

LTM

2 Memories or 1?

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.


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STM

LTM

2 Memories or 1?

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.


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STM

LTM

2 Memories or 1?

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.


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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.

When the bottom (red) light comes on, stop counting immediately and try to write down the letters that were given in their original order.

s


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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.

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.

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The answer...

K S G

Before discussing the flaw, let’s look at the overall results.


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STM

LTM

2 Memories or 1?

Classic Experiment

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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.

Probability of Correct Recall

0 3 6 9 12 15 18

Retention Interval (seconds)


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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.


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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

Interfering Information

Letters

Numbers


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STM

LTM

2 Memories or 1?

Classic Experiment

Importance

This would be _______________ interference (retroactive or proactive).

retroactive

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

Interfering Information

Letters

Numbers


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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

Target Information

Letters on Previous Trials

Letters on Current Trial


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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

Target Information

Letters on Previous Trials

Letters on Current Trial


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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

Target Information

Letters on Previous Trials

Letters on Current Trial


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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

Target Information

Letters on Previous Trials

Letters on Current Trial


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STM

LTM

2 Memories or 1?

Classic Experiment

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.5

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Recall on Trial 1 (Keppel & Underwood, 1962)

Probability of Correct Recall

Caused by PI from previous items

Peterson & Peterson

0 3 6 9 12 15 18

Retention Interval (seconds)


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STM

LTM

2 Memories or 1?

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).


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STM

LTM

2 Memories or 1?

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.


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STM

LTM

2 Memories or 1?

Classic Experiment

Dissociation or Continuity?

Short-term memory implies information disappears rapidly as time passes. That’s why it’s called “short-term”. 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.


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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 dual-store approach.


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STM

LTM

2 Memories or 1?

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


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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!


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STM

LTM

2 Memories or 1?

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.


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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).


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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.


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STM

LTM

2 Memories or 1?

Free Recall

Evidence for Short-Term Memory

Effects of Repetition

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Recall showed a typical serial position curve...

Probability of Correct Recall

Mean Number of Repetitions

Earlier items were repeated more times than later items.

1 5 10 15 20

Serial Position in List


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STM

LTM

2 Memories or 1?

Free Recall

Evidence for Short-Term Memory

Effects of Repetition

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7

1

1.0

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Repetition went in the same direction as recall for beginning and middle items.

Probability of Correct Recall

Mean Number of Repetitions

It went opposite to recall for the end items.

1 5 10 15 20

Serial Position in List


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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.


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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.


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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.

s

D I S S O C I A T I O N


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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?


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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.


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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...


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STM

LTM

2 Memories or 1?

Free Recall

Evidence for Short-Term Memory

Effects of Retention Interval

1.0

.5

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Primacy Effect unaffected by delay

Probability of Correct Recall

Recency Effect eliminated by delay

Immediate Test

30-second delay

1 5 10 15

Serial Position in List


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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.

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