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on January 4, 2007 at 2:55:15 pm

Human Memory: Short term and long term memory

Along with processes such as learning, perception and attention, memory is a crucial aspect of cognition. Indeed, when we ask precisely what ‘memory’ means it is impossible to define it independently from these other processes.

There is also a long history of scientific investigations into human memory, beginning with Ebbinghaus (1885), but the dominant view of memory has become the information processing approach. This suggests that memory can be best understood in terms of three essential stages of that involve the flow of information through memory: registration, storage and retrieval.


Different types of memory

Most psychologists make a distinction between different types of memory — particularly short-term and long-term memory. There is also a third type, the sensory register, which enables information to be stored very briefly so that feature and pattern recognition processes can operate.

Short-term memory and long-term memory are usually distinguished in terms of capacity, duration and coding (or encoding).


Capacity of STM

In a famous paper, George A Miller (1956) suggested that the capacity of STM was 7, plus or minus 2 items (7±2). This conclusion is typically supported by means of a digit-span experiment such as this Activity.


While investigations such as this support the idea that the capacity of STM is limited, determining exactly what the capacity of STM is has proved rather difficult. There are two basic reasons for this:

  • Different psychologists mean different things by the term capacity. For some it is how much the system can hold, storage capacity, while for others it is how much can be done on the information in storage, attentional capacity. If STM is seen as a kind of workbench then storage capacity refers to how many items can be placed on the workbench. Attentional capacity however is how many items can be worked on at any one time.
  • Determining what is meant by an item is difficult. Individual units can be grouped together to increase the capacity. For example, numbers can be grouped as dates, letters as words and so on. Each of these meaningful units is referred to as a ‘chunk’ and through chunking the capacity of STM can be increased significantly.


Coding in STM

The sensory register has no real organisation and so information is stored in more or less the same form as it arrived. However material in STM is highly transformed. Thus, no matter whether we read or hear a word, it will be stored in an acoustic form. Conrad (1964) demonstrated coding in STM.


Duration of STM


In a famous experiment Peterson & Peterson (1959)investigated the duration of STM. They asked participants to recall strings of consonants (e.g. FBK) selected so as to be difficult to pronounce. Recall delay was set to 3, 6, 9, 12, 15 & 18 seconds during which rehearsal was prevented by participants counting backwards in threes from a target number (e.g. 397). Each subject was tested a total of 8 times at each of the 6 delay intervals.

The findings of the study showed that while after a 3 second retention interval trigrams about 90% of trigrams were recalled, after 18secs only 10% were. The duration of STM without rehearsal is therefore very short.


The Peterson and Peterson technique has a number of positive features, including the very effective method used to prevent rehearsal. To see why, consider what the retention would be if this had not been used. However, the main criticism of the study is normally directed at their conclusion that forgetting from STM is due to decay. In fact, interference (retroactive) from the verbal counting task is just as likely an explanation of the forgetting.


Characteristics of LTM

Measurement of the capacity of STM memory is difficult enough, but estimating that of LTM is probably impossible. All we know for sure is that the capacity is very large and may have no upper limit. This huge capacity means that LTM must have a highly organised structure, otherwise retrieval would be much more difficult than it actually is.

In terms of coding in LTM, this is normally considered to be semantic. Baddeley’s (1966) study described earlier shows this, as performance over longer retention intervals on the semantically similar words (BIG, HUGE) is worse than that for acoustically confusable words.

Measuring the duration of LTM is not as straightforward as it might appear. It is no use just asking people what they can remember from the past as we can never be sure whether the information was coded accurately in the first place or whether it was revised later. The results of Ebbinghaus’s studies suggested that memories decayed fairly rapidly, but he used nonsense syllables and even after a month he could still recall some of them. A more realistic estimate of the duration of LTM is the study by Bahrick and his colleagues (1975).

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