Coding Capacity & duration

• Sensory memory- Immediate information from the environment first registers in the sensory store. Info is held for a fraction of a second. Can be visual or auditory
• Short term memory is a store which keeps small amounts of info for a brief amount of time, info is easily lost
• Long term memory can store limitless amounts of info for long periods of time
• Jacob (1887) aimed to investigate the capacity of STM for numbers and letters using a digit span test
• Jacob had 443 female students aged between 8-19 repeat back a string of numbers or letters in the same order. The number of digits increased until the participants could no longer recall them
• Jacobs found that students could recall, on average, 7.3 letters and 9.3 numbers
• Jacobs (1887) study suggests the capacity of short-term memory is very limited
• STRENGTHS of Jacobs (1887) study

  • high internal validity
• LIMITATIONS of Jacobs (1887) study

  • low external validity
  • unrepresentative sample
  • not generalisable
  • lacks mundane realism
• Miller investigated the capacity of short-term memory
• Miller aimed to investigate the capacity of STM
• Miller reviewed published investigations into perception and STM from the 1930s to 1950s
• Miller found that existing research suggested organising stimulus inputs into chunks allowed for STM to cope with 7 chunks of info
• Miller concluded that organisation (or encoding) can extend the capacity of STM and enable more info to be shared there
• One limitation of Miller's that he may have overestimated STM capacity. Cowan (2001) reviewed other research and found the capacity of STM is about 4
• LTM is the store for all our knowledge, so presumably has a larger capacity. However, it is impossible to test experimentally but it appears to be unlimited
• Peterson and Peterson (1959) tested the duration of STM
• Peterson and Peterson aimed to see if rehearsal was necessary to hold info in the STM
• Peterson and Peterson asked participants to remember 3 sets of letters but were immediately asked to count back in 3s out loud for 3, 6, 9, 12, 15 and 18 seconds. Participants were then asked to recall the letters in the correct order
• Peterson and Peterson found that the longer the interval, the less accurate the recall was
• Peterson and Peterson concluded that we cannot hold info in the STM unless we rehearse it. The STM has a capacity of about 18 seconds
• One limitation of Peterson and Peterson's study was that the stimulus material was artificial. Lacks mundane realism, lacks external validity
• Bahrick et al (1975) aimed to study the duration of LTM
• Bahrick et al studied 392 US high school graduates between the ages of 17 and 74. Yearbooks were obtained for all participants. Participants were asked to recall fellow students through photo-recognition tests consisting of 50 photos and free recall tests.
• Bahrick et al found that participants tested within 15 years of graduation were 90% accurate in photo recognition and 60% accurate in free recall. Those who graduated 48 years or more ago recognised 70% of photos and recalled 30% in free recall
• Bahrick et al concluded that the duration of LTM lasts a prolonged period of time
• One strength of Bahrick's et al study is that it has high external validity. They studied real-life meaningful memories. Shepard (1967) found that studies conducted with meaningless pictures has lower recall rates
• Iconic refers to visual memory
• Echoic refers to auditory memory
• Info arrives in the sensory store as sound or an image or a feeling
• There are 3 main ways of coding:

  • acoustic coding: the sound of a stimulus
  • Visual coding: the physical appearance of a stimulus
  • Semantic coding: the meaning of the stimulus
• In general, STM seems to use acoustic coding and LTM uses semantic coding
• Baddeley (1966a, 1966b) investigated what type of info is encoded by the STM and LTM
• Baddeley gave different lists of words to 4 groups of participants to remember:

  1. acoustically similar (cat, car, cab)
  2. acoustically dissimilar (pit, few, cow)
  3. Semantically similar (big, great, large)
  4. semantically dissimilar (good, huge, hot)
• Baddeley found that when participants had to recall lists immediately after hearing them, they performed worst with acoustically similar words
• Baddeley found that when recalling words after 20 minutes, participants performed worse with semantically similar words
• Baddeley concluded that info is encoded acoustically in the STM and semantically in LTM
• One strength of Baddeley's study is that it had identified a difference between two memory stores which shows there must be different memory stores with specific functions
• One limitation of Baddeley's study is that it used artificial stimuli which had no meaning to participants and therefore lacks external validity