Intact STM versus Impaired LTM

In amnesia Opens in new window, patients lose much of their long-term memory but still retain intact short-term memory. They are usually able to carry on a fairly normal conversation. The ability to keep up a conversation demonstrates that patients are able to remember what has been said in the last few seconds, although they may be unable to recall anything that took place any earlier than that.

Such observations can be explained by the fact that organic amnesics usually retain an intact short-term working memory.

Early observations of the preservation of STM (short-term memory) were largely anecdotal, but more recently they have been supplemented by objective measurements of STM function Opens in new window, such as digit span.

Talland (1965) carried out a study involving no less than twenty-nine Korsakoff patients, all of whom proved to be significantly impaired in a whole battery of long-term memory tests such as story recall, wordlist recall and picture recognition. However, their digit span scores were similar to those of normal subjects, averaging about seven items.

Baddeley and Warrington (1970) again reported apparently normal STM span in Korsakoff patients, and in addition they found a normal recency effect in a test of free recall. The recency effect Opens in new window is thought to reflect the STM component of free recall, so this provided further confirmation of the apparent preservation of STM.

HM, the patient whose amnesia was brought about inadvertently by temporal lobe surgery, has been found to retain a normal digit span despite his extremely dense anterograde amnesia Opens in new window (Wickelgren, 1968).

On the other hand, when presented with a list of digits which was just one item larger than his span, HM was unable to learn this list even after twenty-five repetitions (Drachman and Arbit, 1966). This task is known as the ‘extended digit span’, and most normal subjects have no difficulty in learning such a list within a few trials, and can even learn lists of fifteen or twenty digits if given a few repetitions of the list.

Patients suffering from HSE (Herpes Simplex Encephalitis) amnesia have also been found to have a normal STM in the face of a severe LTM impairment (Starr and Philips, 1970; Wilson and Wearing, 1995). Similar findings of preserved STM span have been obtained with patients in the early stages of Alzheimer’s disease (Miller, 1977), though in the later stages Alzheimer patients do show a deterioration of STM performance (Corkin, 1982), reflecting the general dementia which eventually pervades all aspects of their cognitive functioning.

From a consideration of all these studies it would appear that in virtually every type of organic amnesia there is severe LTM impairment but a relatively unimpaired STM or working memory.

The above findings suggest a dissociation between STM and LTM, since STM can remain intact despite severe LTM impairment. However, the opposite pattern (intact LTM but impaired STM) has also been found occasionally. Consider KF, who suffered brain damage after a motorcycle accident. He had no problem with long-term learning and recall, but his digit span was greatly impaired (Shallice & Washngton, 1970).

Patients with STM deficit have usually been found to be capable of functioning fairly well in normal life, because there are few everyday tasks which make major demands on STM and these individuals tend to retain normal LTM function. However, some recent studies have shown that individuals with STM deficit may have difficulty in fully understanding long sentences (Vallar and Baddeley, 1982), and in very severe cases may have difficulty in learning new vocabulary or a new foreign language (Baddeley et al., 1988). These findings may possibly provide a clue as to the function of the STM ‘working memory’, since it is apparently necessary to be able to hold new input briefly in the STM in order to process the information it contains and to transfer it to the LTM ‘storage memory’.