Memory is not a unitary system." Discuss the types of evidence that have been used to buttress this

What is a memory? Is it a concrete and tangible atom, which, once stored, can be retrieved by a methodical search or can it get lost in the murky recesses of our minds? Are our memories stored in separate areas or as part of one whole? In everyday conversation we regularly refer to memory in ways which suggest it to contain selective processes - 'I have a terrible memory for names', or 'I never forget a face' but is it really the case that our memory has specialised systems for the recall of certain information, or is it that memory is in fact a single system? Any discussion of the way in which we store information that can be accessed at a later date must address the issues raised here.

To look first at an overview of the architecture of our memories, we are probably all familiar with the idea of a short term and a long term memory. These two oft-used terms, along with the sensory memory, combine to make up the modal model. Several theorists such as Waugh & Newman (1965) have proposed a model in which we have these three separate systems, arranged in a hierarchy. Sensory stores feed into a phonetically coded short term memory via selective attention, and information then moves to a semantically coded long term memory. We will now look at each of these modules in turn and examine the ideas and evidence underlying each.

The sensory memory is proposed to be the interface with the outside world. Information is received from each of our senses (a separate, modality specific store is suggested for each sense) and held in a buffer which very quickly decays and is updated. There is good evidence for the existence of the brief storage of a verbatim copy of sensory input in several sensory modalities. The two most studied are in the visual and auditory modalities, dubbed by Neisser 'iconic' and 'echoic' memory respectively. Sperling (1960) provided some of the most striking evidence to date when testing recall of a briefly shown (50 msec) 3*4 grid of letters. The results were that typically, 4 letters were recalled. However, if given an auditory cue to focus on one row, 3 of the 4 letters in that row were recalled, suggesting that the amount of information available a fraction of a second after the stimulus had been shown was a great deal more than was available a second or so later. Indeed, further similar experiments have suggested that the iconic store decays within about 0.5 sec. Treisman (1964) and Darwin, Turvey and Crowder (1972) have investigated echoic memory using similar experiments and have found a similar decay effect, though in the auditory modality this appears to be of the order of around two seconds.

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Such experiments as outlined above seem to provide good evidence for the existence of a sensory store which is quite seperate from short term and long term memories and suggests that the human memory is modular at least in this respect. The existence of a sensory store also has great intuitive appeal when we consider the possible uses of such a system. Sensory memory appears to be preattentive and this seems a useful facility which could be held briefly to provide a steady input for later, conscious processing. Using this line of reasoning it could then be argued that sensory memory is in fact a by-product of sensory processing rather than a memory system in it's own right. Whilst this part of memory seems intuitively and experimentally to fit within a modular framework the dichotomy between short term and long term memory is a more contentious area.

The arguments forwarded by advocates of the stores models suggest that some information from the sensory store will be particularly useful to the organism and will therefore be attended to for further processing, and will thus find it's way into the short term memory. This system is thought to be of limited capacity and is fragile - the information can easily be lost if distracted. Experiments have been conducted to investigate the capacity of the short term store, Miller (1956) concluding that it's span (calculated from the rote recall of a series of random digits) is 7 2. If information is to be stored for a longer period of time it is passed to the long term store, of which there are no known limits. Atkinson & Shiffrin (1971) have suggested that the transfer between the two stores is facilitated by rehearsal of the information and that the greater the rehearsal, the stronger the storage of the memory.

Several phenomena have been proposed as evidence for separate systems of long term and short term memory, the main two categories can be summarised as follows: pathological evidence from brain-damaged subjects who exhibit memory disorders and psychological evidence from normal subjects. To consider first the evidence from normal subjects, one observation which seems to lend support to the modularity argument comes from the serial recall test. Subjects repeating a list of words tend to exhibit primacy and recency effects, that is they remember best the first and last few words. It is argued that the recency effect is a product of the short term memory and that the primacy effect is a consequence of long term memory. Further support comes from the fact that these two effects can be manipulated independently, by for example making the subjects count backwards for 10 sec after hearing a list of digit strings, which eliminates the recency effect, e.g. (Glanzer & Cunitz 1966). This also shows the theorised fragility of short term memory and it's susceptibility to distraction.

Evidence for STM/LTM distinctions come from neuropsychology in the form of brain damaged subjects who exhibit differing levels of amnesia. Brain damage lends itself to two explanations for poor performance of certain tasks. Mass action or equipotentiality suggests that removing/damaging equal amounts of two brains would lead to equal impairment of performance, and this view gains some support from Lashley's studies of lesions in rat's brains. Contrarily, a modularity theory suggesting domain specificity of the brain would anticipate that damaging different parts of the brains would lead to impairments of different tasks. Whilst it could be argued that these particular tasks are simply very demanding on resources and are thus performed badly, if a double dissociation can be found between two subjects, that is, if one amnesic subject performs well on task A but poorly on task B whilst another subject exhibits the opposite then it would seem a fair assumption that there were indeed two processes for the two tasks. In practice, just such double dissociations have been found and purported to be evidence of modularity.

Patients suffering from Korsakoff's syndrome (amnesia through chronic alcohol abuse) exhibit poor long term memory whilst having a seemingly good short term memory. For example they are well able to hold normal conversations (where it is essential to remember what the other person has just said) and experiments have shown them to have normal intelligence on the WAIS, normal digit spans and also to exhibit normal recency effects but impaired primacy effects (Baddeley & Warrington 1970). These findings suggest a normally functioning short term memory in the face of poor long term memory. Less common are patients who have normal long term recall but impaired short term recall, although cases have been found. Shallice & Warrington (1970) report the case of KF who had no difficulty with long term learning but whose digit span was grossly impaired and who only had a very small recency effect.

Whilst the evidence here does seem to back up the modularity of STM and LTM, many criticisms have been levelled at these theories, often highlighting the over-simplification of their explanations of a unitary and uniform storage in each store. Experimental information has suggested at least two different long term stores, e.g. episodic and semantic memory (Tulving, 1972). Problems also arise when we consider that patients have been found with normal LTM but impaired STM, which seems in conflict with the idea that long term store is achieved via short term store. These problems have lead researchers such as Baddeley & Hitch (1974) and revised and updated by Baddeley (1986) to propose a modular theory in which the internal structure of the STM is revised and elaborated upon. Baddeley proposed that the STM consists of a central executive (a processor which is not expanded on in much depth) which utilises a key element of the STM; the 'phonological loop'. This can be used as a store for visually or phonetically coded information while the STM is working and was hypothesised as a direct result of neuropsychological evidence (Shallice & Warrington 1974) which refuted Baddeley's earlier attempts to revise the modal theory. This revised working memory model accounts for and is supported by psychological and pathological findings.

With a system of such complexity as memory it is very difficult to isolate individual functions and phenomena. Whilst the human brain does seem to exhibit some physical modularity, definite proof is very hard to obtain, though progress is being made through the study of neuropsychology in brains which function abnormally. The commonality of memory dysfunction from similar damaged brain regions does strongly suggest that the human memory is to some extent organised in a modular fashion. Specific memory impairments, for example in face recognition, suggest that the brain has specialised functions for remembering certain classes of stimuli, and it does not seem intuitively unreasonable that it should also have a physically modular structure. (approx 2,000 words)

Bibliography

Baddeley, A.D. Human Memory: Theory and Practice. Lawrence Erlbaum, 1990.

Eysenck, M.W. & Keane M.T. Cognitive Psychology. Lawrence Erlbaum, 1991.

Mayes, A.R. Human Organic Memory Disorders. Cambridge, 1988.

Parkin, A.J. Memory and Amnesia. Basil Blackwell, 1987.