What experimental evidence is there for the existence of multiple memory systems?

A major property of short-term memory is said to be the level of processing.  Craik and Tulving (1975) conducted a study from which they concluded that the level of processing during input affected recall.  They found that subjects remembered the words which had been presented with more complex sentences twice as well as the ones associated with less complex sentences.  In all cases semantic processing had been carried out, therefore there should have, in theory, been no difference in retrieval.  That is until you consider the nature of the elaborate rehearsal and the larger cognitive structure into which the elaborated code was being fed.  The more complex sentences must have accessed other cognitive codes whose nature was more elaborate than those accessed by the simpler sentences.

Warren and Warren (1976) studied semantic coding in Short term memory.  This experiment implied that interference is a cause of forgetting in the short-term memory, but also that coding is in terms of meaning and not just sounds. This supports the other studies which showed semantic processing is performed in the short term memory.

Some psychologists attempted to modify the multi store model in order to fit in with recent findings.  Baddley and Hitch (1974) are a good example.  They argued that the concept of short term store should be replaced with that of working memory system.  Their system had three components: A modality-free central executive resembling attention, an articulatory loop (known now as a phonological loop) holding information in a speech based form, and a visuo-spatial scratch pad (known as a visuo-spatial sketchpad) for spatial and visual coding.  Later versions of the model also included an episodic buffer, which lays down time markers, and also some versions split the articulatory loop and the visuo-spatial sketch pad onto two dual slave systems.

The last few studies that have been discussed suggest the existence of a short-term memory store.  Therefore, this suggests the existence of an opposing system, the long-term store. Also, the nature of the memory code seems to change as a function of its time in the memory system, further evidence of a long-term store.

Our long-term memories contain lots of varied information.  Consequently it is easy to assume that there are various long-term memory systems, each of which is specialised for certain types of information.  Tulving (1972, 1983) proposed a distinction between episodic and semantic memory. Episodic memories were autobiographical, personal and sensitive to the effects of context.  According to Tulving, episodic memory refers to the storage (and retrieval) of specific events or episodes occurring in a particular place at a particular time.  Semantic memory on the other hand, contains information about our knowledge of the world, which could be organised hierarchically.  Semantic memory was composed of knowledge that had no specific temporal or spatial referent, and was therefore not sensitive to the effects of context.  Although Tulving believed these two systems interact with each other to an extent, he believed that each system probably had its own encoding, storage, and retrieval laws.  

Kihlstrom’s (1980)  study, supports Tulving’s proposal to a degree.  His subjects were hypnotised and then memorised a list of unrelated words.  Following this, they were given a posthypnotic suggestion, telling them they would not be able to remember the memorised list until a specific cue was given.  The subjects were told that they would then have to supply free associates to words given by the experimenter.  The priming words were chosen because they had a high probability of eliciting the list words that the subjects had memorised.  Subjects performed well on this task, indicating that the hypnotic state had not affected their semantic knowledge of the words’ meanings.  But, even after they had recited many of the words on the list, the subjects maintained that they were unable to retrieve the studied items.  Yet, when the retrieval cue mentioned in the posthypnotic suggestion was finally given, the subjects’ retrieval was almost perfect.

A key theory of episodic memory was proposed by Wheeler et al (1997).  He proposed episodic memory depends on various cortical and sub cortical networks in which the prefrontal cortex plays a central role. Evidence from brain damaged patients and from PET scans were obtained to test this theory.  Wheeler et al (1997) argued that there are two main differences between semantic and episodic memory: it involves the subjective experience of consciously recollecting personal events from the past, where as semantic memory does not.  They also argued that the prefrontal cortex was much more involved in episodic than semantic memory.  Saying this, Wheeler et al (1997) themselves pointed out the finding that subjects with damage to the prefrontal lobes show impaired episodic memory is open to interpretation.  

Collins and Quillian (1969) proposed a network model of semantic memory.  They supposed that semantic memory was organised in a network with a hierarchical structure. In their initial experiment they compared questions requiring the comparison of features stored at different levels, to see if the speed in which a person could answer questions depended on how far down the network the answer was.  The problems with this theory were that the level at which the information was stored in the list did not predict how quickly the subjects answered a question.  In the subsequent experiment they studied if the use of a retrieval path helped ensuing retrievals.  They discovered that the retrieval is quicker if the same retrieval path is used.  They also revealed that typical instances gained faster decisions, and concluded that hierarchical organisation may be no more than a re-ordering of associative strength between features.  

The feature list model proposed by Smith, Shoben & Rips (1974) took over from Collin and Quillian’s network model.  They proposed that the meaning of an item is represented by a list of its features.  Some features are essential to the item and others are merely characteristic. Defining features are those essential to the concept, Characteristic Features are those which are commonly observed but not essential.  They proposed that questions of knowledge are answered by consulting the lists of features, looking for overlap between lists. One problem with this concept is that there can be a problem when handling awkward questions.

Along with distinctions between episodic and semantic memory, it is also possible to make distinctions between implicit and explicit memories.  Traditional measures of memory, for example free recall and recognition, involve use of direct instructions to retrieve information about specific experiences, therefore can be regarded as measures of explicit memory.  Recently, researchers have become interested in understanding implicit memory, which is revealed when “performance on a task is facilitated in the absence of conscious recollection” (Graf & Schacter 1985).  Tulving, Schacter & Stark (1982) showed evidence of implicit memory, through the repetition-priming effect.  

It is possible to look at explicit memory versus implicit memory.  The notion that memory performance always depends on conscious awareness has been disproved in studies on people with normal brain functioning, and there is compelling evidence from amnesic patients that conscious recollection is often not needed to produce good memory performance.  Schacter (1987) argued that amnesic patients are at a severe disadvantage when tests of explicit memory are used, but that they perform at normal levels on tests of implicit memory.  Overall though, it is thought that implicit and explicit memory distinctions are “descriptive concepts that are primarily concerned with a person’s psychological experience at the time of retrieval”(Schacter, 1987).

In this essay many studies have been discussed in reference to the existence of a multi-store model of memory.  The multi store model provided a systematic account of the structures and processes involved in memory, and the distinction between the three different stores makes sense.  In order to qualify their existence, it is necessary to look at the differences between them, and this has been done through various psychologist’s studies.  Saying this, the multi store model is possibly oversimplified.  An example of this would be the way that the short term and long term stores are assumed to be completely separate entities, when in fact there are probably more overlaps than shown.  Also, the long-term memory itself is oversimplified.  It is improbable that everything we store in our long-term memory is stored in a single long-term memory store.  Logie (1999) pointed out a major problem with the multi store model, in that in the model the short-term store acts as a gateway between the sensory stores and the long term memory, however the information processed in the short term store has already made contact with information stored in long term memory, therefore access to long term memory occurs before information is processed in short term memory.  Another criticism of the multi store model is its reliance on rehearsal as a process of transferring information into the long-term store.  In reality, rehearsal is not used as much as assumed in everyday life.

In conclusion, although there is a great deal of evidence supporting the theory of the multi store memory system, there is also evidence against the theory.  The multi store model provides a good basis upon which psychologists can base their studies, but the individual components of the memory system need to be studied more extensively, for example the long-term store.  When the separate components are better understood, the multi store memory system with gain greater validity as a model of memory.

Word Count: 2,274

Bibliography

Best, J,B.  Cognitive Psychology, 4th Edition.West Publishing Company, Eastern Illinois University, 1995.

Eysenck, M, W.  Cognitive Psychology, A student’s Handbook, 4th Edition.  Psychology Press: Essex, 2000.