2.1.2 Short Term Memory

What is short-term memory?

• Short-term memory (STM) has the ability to hold a small amount of information for a relatively short period of time
• The amount (capacity) and duration of STM is greater than sensory memory but smaller than long-term memory
• It is seen more as a holding device before memory is forgotten/lost or moved to long-term memory

What is the capacity of STM?

• Experiments conducted by Miller (1956) indicate the capacity of STM is 7 items or chunks plus or minus 2 (5-9 items)
• Miller demonstrated this in an experiment where he asked participants to recall information, adding an extra bit as he moved on: A bit like the game ‘I went to the shops’
• Starting participants with two or three words to recall, he gradually built it up until they made an error
• It was found that most participants struggled with between 5-9 words
• This is known as ‘Miller’s Magic 7’

How can we evaluate research into capacity and STM?

• Jacobs (1887) had completed a similar experiment using digits with 443 female students
• He called this a digit span experiment, using numbers instead of words
• His results were similar to Miller's, with 7.3 being the average recall
• This supports Miller and suggests his study is valid
• Miller’s study is also reliable as it is easy to copy and it is especially reliable as the results are more often than not the same
• As it is a lab study extraneous variables would have been controlled
• However, it could be argued it lacks ecological validity as the task bears little resemblance to real life

Short-term memory duration

• Duration relates to how long a memory can be stored for
• As short-term memory (STM) is only a temporary device, then this is limited
• The duration of short-term memory is around 18-30s

Evidence for the duration in short-term memory

• The main study into duration and STM comes from Peterson and Peterson (1959)
• They gave participants non-sensical three-letter trigrams to learn eg. CGR or BHT
• These were presented visually to the participants, one at a time
• The participants had to recall the trigram in the correct order after a delay of either 3s, 6s, 9s, 12s, 15s or 18s
• During this delay they were asked to complete a distraction task: usually counting backward from 300 in 3s (300, 297,294 etc.)
• This was to prevent them from rehearsing during the delay
• A graph of the correctly recalled trigrams over time was plotted and was shown to be a decay curve
• This demonstrates that over time the memory seems to decay
• The graph was extrapolated to show that after the 30s recall in STM would be zero
• Therefore, Peterson and Peterson stated that the duration of STM was 18-30s

Evaluation of duration in short-term memory

• Peterson and Peterson conducted a well-controlled study and many extraneous variables would have been removed or controlled for
• But does it prove decay?
• Could it be that previous trigrams, maybe similar to the one being recalled, interfered with the memory and that is why the participant made the mistake?
• This challenges the validity of the study

What is encoding in short-term memory?

• Encoding is how the information is processed from the senses into the memory itself
• This is how the information will be stored and ultimately recalled
• Encoding is always in the form of a modality or one of the senses
• Short-term memory encodes acoustically or using sound

What is the evidence for acoustic encoding in short-term memory?

• We know this from a study by Baddeley (1966)
• Baddeley considered encoding in short-term memory and long-term memory but here we will look at STM only
• He gave participants four lists of words to learn:
  • Acoustically similar: words that sounded the same such as cat, bat, rat
  • Acoustically dissimilar: words that did not sound the same such as laugh, bear, pencil
  • Semantically similar: words that mean the same such as large, huge, enormous
  • Semantically dissimilar: words that do not mean the same such as police, computer, chair
• The words were all presented visually, on a screen as part of a slideshow
• They had to be recalled in the order presented: Free recall was not allowed
• For the short-term part of the experiment, he asked for a recall immediately
• He found the acoustically similar words had the worst recall
• There was no difference in STM for the semantically similar and dissimilar words
• From this, he concluded that STM relied on acoustic encoding to process information

How can we evaluate the research into encoding in STM?

• Once again, this is a lab experiment so is highly controlled, with extraneous variables taken care of
• However, this also means that it lacks ecological validity as the tasks do not relate to real life
• The words had little to no meaning for participants and so they were harder to recall
• Information we have to recall in reality often has meaning and significance so making recall easier