Coding Capacity & Duration of Memory
__Coding __- How the memory is stored.
__Capacity __- How much can be stored in memory.
__Duration __- How long the memory lasts before decaying/disappearing.
Research into coding: Baddeley (1966) gave participants one of four word lists to learn. The lists contained words that were either acoustically similar (sounded the same, e.g. hat, cat, bat); acoustically dissimilar (sounded different, e.g. hat, stage, ball); semantically similar (had the same meaning, e.g. big, large); or semantically dissimilar (had different meanings, e.g. gate, big). Participants either recalled the list immediately, testing the coding of short-term memory (STM) or after 20 minutes, testing the coding of long-term memory (LTM). Participants did worse with acoustically similar words in STM, suggesting that information in STM is coded according to sound, as similar-sounding information conflicted with each other. For LTM, they did worse with semantically similar words, suggesting that information in LTM is coded according to meaning, as information with similar meanings conflicted with each other.
Research into capacity: Jacobs (1887) developed the digit-span technique, where a participant has to immediately recall a sequence of letters or numbers which increased by one letter or number with each trial. The mean amount of letters that could be correctly recalled was 7.3, and for numbers it was 9.3. Miller (1956) concluded that the capacity of STM is 7 plus or minus 2 ‘bits’ of information. He also noted that in order to increase this capacity, people ‘chunk’ information together, for example remembering the area code of a phone number (5 digits) as one chunk. The capacity of LTM is thought to be potentially unlimited.
Research into duration: STM duration was investigated by Peterson and Peterson (1959). Participants were given a nonsense ‘trigram’ of three syllables, together with a three-digit number (for example, ‘TJF 374’). To prevent rehearsal, they had to count backwards in threes from the number until told to stop after either 3, 6, 9, 12, 15 or 18 seconds. After the interval, they had to recall the trigram. Recall was generally accurate after 3 seconds (over 80%), but by 18 seconds it declined to around 10%. It was concluded that the duration of STM is around 18-30 seconds, if rehearsal is prevented.
LTM duration was investigated by Bahrick_ et al_ (1975). Participants were aged between 17 and 74, and were required to identify schoolmates from their high school yearbook, name them in a free recall test, or match photos to names. Participants who had left school within 15 years were very accurate with photo recognition (90%), but even those who had left decades earlier were still 70% accurate. Older participants were less good at free recall, but nearly as good as younger participants in the matching task. The conclusion was that the duration of LTM is potentially lifelong.
- Many studies such as these use artificial and meaningless stimuli (for example, Baddeley). This is a weakness because they do not reflect how memory works in everyday life, so they lack external validity.
- Jacobs’s study was conducted a long time ago, so may not have been done to the same scientifically rigorous standard as research today, therefore the validity of the findings is in question.
- Bahrick’s study used more meaningful material, so is higher in external validity, however it is very hard to control variables in a study such as this, as it is unknown how many participants may have looked at their yearbook in the intervening years, for example.
The Multi-Store Model of Memory(MSM)
Sensory register: This is where sensory stimuli from the environment is stored, although it only lasts for a fraction of a second unless attention is paid to it. The store can be divided into different senses, for example iconic for images, echoic for sounds. The capacity for the store is quite large, but the duration is very short.
Short-term memory (STM): If attention is paid to a stimulus, it will enter STM. Information is coded acoustically, and there is a capacity of around 7 chunks of information. The duration is around 30 seconds unless the information is rehearsed. Maintenance rehearsal can be used to keep information in STM (saying something over and over again).
Long-term memory (LTM): If information is rehearsed for a longer period of time, it will transfer to LTM. Information is coded semantically (according to meaning) and LTM has a potentially unlimited capacity, and a lifelong duration. Information is retrieved from LTM into STM so that it can be recalled.
- Baddeley’s research into coding and the studies into capacity and duration support the assumptions of the MSM.
- The case of KF weakens the MSM. KF suffered from amnesia, and was able to recall visual information without difficulty, but had problems recalling verbally presented information. This suggests there is more than one type of STM store, so the MSM is too simplistic.
- The case of HM could be used to support the model. HM underwent an operation to remove his hippocampus in an attempt to relive symptoms of epilepsy. Afterwards, his STM seemed unaffected, but he was unable to make new long-term memories. His LTM from before the procedure was intact. This suggests that the link between STM and LTM had been broken, supporting the assumptions of the MSM.