Short Term Memory

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Short term memory (STM)
Memory has been of interest since antiquity.
For centuries memory was seen just as a storage system, however experimental research in the last century has identified several functions for memory such as: * Encoding
* Storage
* Retrieval
* Processing information.

Short term memory is also called
* Primary memory (William James 1890)
* Immediate memory
* Working memory

Free recall task (Murdock 1962)
Participants had to remember lists of 10, 15, 20, 30 and 40 words. He also manipulated the presentation rate, the items were presented either every second or every 2 seconds. Participants recalled the words immediately after hearing it. Participants tend to recall the first and last items on the list better than the middle. The better recall for the first items on the list is called the primacy effect, and for the last items this is called the recency effect. These 2 effects suggest the presence of 2 different memory systems. One related to long term storage and the other related to short term storage.

Postman and Phillips (1965)
Also tested the hypothesis of two memory systems
Used a free recall paradigm with lists of 10, 20 and 30 words Two main conditions:
* No retention interval
* Recall after a delay of 15 or 30 seconds, filled in with a counting task The presence of a retention interval led to the elimination of the recency effect.

Waugh and Norman (1965)
Participants are shown a list of digits
They are then given a probe, which is one of the digits in the list Their task is to retrieve the digit following the probe
List = (9 0 8 5 3)
Probe = 8
Answer = 5
Participants have to focus on one digit rather than the entire list This technique essentially suppresses rehearsal

The results showed:
Weak memory for the last items presented so it can be concluded that suppressing rehearsal eliminates the recency effect Waugh and Norman’s (1965) explanation for this was:
Information is put into short-term memory, where it is constantly rehearsed. If information is rehearsed long enough, it is put in long-term memory, Otherwise it is forgotten

If we assume short term and long term memory are separate systems we could expect that one for of memory can be damaged while the other remains intact. For example the case of HM- Neuropsychology (Scoville & Milner, 1957): HM, a patient suffering from epilepsy, underwent surgery

Removal of parts of his right and left medial temporal lobe, including about two thirds of the hippocampus and amygdala. After surgery, HM was unable to store new memories into long-term memory But could perform tasks requiring short-term memory e.g. holding a conversation HM had an intact short-term memory but a damaged long-term memory.

Kandel (2001) research in neurobiology stated that long-term memory and short-term memory use different biological mechanisms. The atoms of cognition: The chunking hypothesis
The cognitive system fuses together functionally-related bits of information. This is called Chunking (Miller 1956) A chunk is “a collection of elements having strong associations with one another, but weak associations with elements within other chunks” (Gobet et al 2001) Chunks are long term memory structures and as such are the result of learning. Word are good examples of chunks. Chunking enables the accretion of simple units into groups, which later can be used as units themselves. Chunks are long-term memory structures that are learnt. Learning one chunk takes between 5 and 10 seconds in ling term memory (Simon 1974) Miller states that the capacity of short term memory (Memory span) should be measure in chunks. Miller estimated that the capacity of short-term memory (memory span) is 7±2 chunks.

Support for chunking – Murdock (1961)
Recall task with four conditions
* Trigrams - Sequences of consonant-vowel-consonant, such as XUW and BAJ * Three-letter words e.g....
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