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The nature of memory: STM, LTM and Duration

Duration of STM

One of the key differences between the concepts of STM and LTM is duration. “Duration” refers to how long a memory lasts before it is no longer available.

Short term memories don’t last very long. An example of STM in action would be trying to remember a seven-digit phone number that you have just been given. This is maintained in the short-term memory by REPETITION until the number is dialled, and then fades once the conversation starts.

The way most people keep information in their STM for more than a few seconds is to rehearse it. So rehearsal is one way of keeping a memory active. The result of verbal rehearsal is that STM are held in the STM store and eventually become long term.

Duration of LTM

LTM refers to memories that last anywhere for 2 hours to 100 years plus, i.e. anything that isn’t short term. Some memories are very long lasting. For example

Shepard (1967) tested duration of LTM. He showed participants 612 memorable pictures, one at a time. An hour later they were shown some of these pictures among a set of others and showed almost perfect recognition. Four months later they were still able to recognise 50% of photographs. The material to be remembered was more meaningful to the participants and therefore the duration of the LTM was better.

Key study on duration of STM

Lloyd and Margaret Peterson (1959) conducted a landmark study of the duration of STM. They enlisted the help of 24 students attending their university.

The experimenter said a consonant syllable to the participant followed by a three-digit number (e.g. WRT 303 or SCX 591). The consonant syllable was selected to have no meaning. Immediately after hearing the syllable and number, the participants had to count backwards from this number in 3s or 4s until told to stop. Then the participants were asked to recall the nonsense syllable. The reason for counting backwards was to stop the participants rehearsing the syllable because rehearsal would aid recall. Each participant was given two practice trials followed by eight trials. On each trial the retention interval (time spent counting backwards) was different.

They found that participants remembered about 90% when there was only a 3-second interval and about 2% when there was an 18-second interval. This suggests that, when rehearsal is prevented, STM lasts about 20 seconds at most.

Evaluation

The findings from the Peterson and Peterson study have been challenged. We might argue that, in this experiment, participants were relying on more than STM alone because they knew they were going to be asked to recall the items after an interval filled with a distracting activity.

Other research such as Marsh et al, (1997) has suggested that when participants do not expect to be tested after this interval, forgetting may occur after just 2 seconds. This suggests that our understanding of the duration of STM may not be as clear-cut as first thought.

In fact, more recent research even suggests that the duration of STM is not as short as Peterson and Peterson’s study would suggest. Nairne’s et al (1999) found that items could be recalled after as long as 96 seconds. In Nairne’s study, participants were asked to recall the same items across trials, whereas in the earlier study different items were used on each trial, which would have led to interference between items, decreasing recall.

Capacity and Encoding

Capacity is a measure of how much can be held in memory. It is measured in terms of bits of information such as number of digits. STM has a very limited capacity (less than 7 chunks of information) whereas LTM has potentially unlimited capacity.

Increasing the capacity of STM

The magic number 7+/-2

George Miller (1956) wrote a memorable article called “The magic number seven plus or minus two”. He reviewed psychological research and concluded that the span...
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