Conrad (1964) suggested that short-term memory codes all information acoustically, that is, according to sound. Visual information is encoded (transformed) to its acoustic (sound/language) codes.
Shulman (1970) disagreed and thought that short-term memory also coded information visually and according to semantics (meaning).
Heyer and Barrett (1974) suggested that visual images that are difficult to acoustically code may also be stored briefly in short term memory.
Research into encoding in short term memory - Conrad (1964)
Participants were presented with a list of consonants.
For example: P J N R Z D for about ¾ of a second.
Participants were then asked to recall what they had seen.
Conrad found that errors of recall were linked to letters which had a similar sound.
Bs were mistaken for Ps 62 times, Vs were mistaken for Ps 83 times but Ss were mistaken for Ps only 2 times. This suggests that visually presented information is encoded according to acoustics/sounds. Conrad referred to these errors as Acoustic Confusion.
Research into encoding in short term memory - Shulman (1970)
This research suggests that Conrad was incorrect in proposing that all encoding in short term memory was acoustic.
Shulman presented participants visually with lists of 10 words. Recall was then tested using cue or probe words which were one of three types.
Firstly, some of the probe words used were homonyms (words which sound the same but have different meanings, for example: ball and bawl).
Secondly, some probe words were synonyms (different words with same/similar meaning, for example: talk and speak).
Thirdly some of the probe words used were identical to the ones on the original stimulus list.
Similar numbers of errors of recall from the stimulus list was made for homonym and synonym probes. This suggests that the semantic encoding (meaning) as well as acoustic encoding occurs in the short term memory.
Both the Conrad and Shulman research were laboratory experiments. They therefore lack ecological validity due to controlled artificial environments. Participants were undergraduate students and therefore unrepresentative of the general population. They may have exhibited demand characteristics and experimenter bias may have occurred as the experiment did not employ blind conditions.
The results may also have been influenced by individual differences or participant variables. The research has good reliability.
Capacity refers to the amount of information that can be stored in the short-term memory.
Miller (1956) suggested that most people store about seven independent or discrete items in short term memory. These items may be numbers, letters or words etc. Miller referred to each of these items as chunks.
For example: 7 6 5 4 3 2 1 = Seven discrete chunks
Miller further suggested that the capacity of the short term memory may be enlarged by grouping items together by associations/links they have with each other.
Eg; 1+1 2+2 3+3 4+4 5+5 6+6 7+7 = Seven discrete chunks but combined according to same numbers therefore increasing capacity of short term memory. Items are chunked according to the meanings they have in long-term memory.
Miller therefore suggested that about seven chunks of information may be stored in short term memory whether in single or combined forms give or take one or two chunks, "The magical number seven plus or minus two"____________ 7 +/- 2.
Research into capacity in short term memory Miller (1956)
Participants were given 'sentences' of varying lengths that approximated 'true' English. They were asked to recall words in the correct order given in the sentence.
The more sense the sentence made, in terms of grammar, the better the recall. This suggests that the semantic (meaning) and grammatical structure, which is probably stored in LTM, is used to help increase amount of information stored in STM by combining items to create larger chunks. Participants still recalled about seven pieces of information.
Criticisms of this laboratory experiment include ecological validity, demand characteristics, experimenter bias, participant variables/individual differences. The experiment has good reliability. The research is dated.
Research into capacity in short term memory Bower & Springton (1970)
Participants were presented with one of two letter sequences. The first sequence was made up of well-known groups of letters for example; mfi, plc, aeb. The second sequence contained the same letters but not in the well known order: imf, lcp, eba. The first sequence was better recalled suggesting that chunking according to meaning increases the capacity of the short-term memory.
Criticisms of this laboratory experiment are as above for Millers research.
Brown & Peterson & Peterson (1959) devised a technique that prevents information from being continually repeated in the STM in order to test how long information will be retained. This continual repetition of information in order to hold on to it is referred to as Maintenance Rehearsal. Brown & Peterson suggested that the short-term memory can store information for approximately 15 to 30 seconds if maintenance rehearsal is prevented.
Reitman (1974) suggested that this short duration is due to displacement; as new information is coming into the short-term memory it is kicking out the previous information due to its limited capacity (7 +/- 2 chunks).
Peterson & Peterson suggest that information decays (fades away) rapidly in short term memory unless rehearsal of that information occurs.
How many did you recall?
This illustrates what Brown & Peterson referred to as a Distractor Task or Interpolated Task and should have prevented you from rehearsing the information - the trigrams.
This experiment shows that in the absence of rehearsal the short-term memory can only hold on to information for about 15 to 30 seconds.
Brown & Peterson suggested that where information is continually rehearsed it can be stored in the short-term memory indefinitely but is lost as soon as interference blocks rehearsal.
Ever been given a telephone number and had to keep repeating it avoiding all distractions until you wrote it down to prevent for- getting it then you were experiencing Maintenance Rehearsal.
Brown & Peterson employed a laboratory experiment and therefore may be subject to the criticisms above.