Superior Memory Ability

Much research on human memory has focused on its limitations (omissions; distortions). However, it is useful to study individuals with unusually good memories to understand the principles involved in efficient human learning. The best known mnemonist or memory expert is Shereshevskii, who is usually referred to as S. His amazing powers were studied by the Russian neuropsychologist Luria (1975). After only three minutes' study, S learned a matrix of 50 digits perfectly, and was then able to recall them effortlessly in any direction. More strikingly, he showed almost perfect retention for much of what he had learned several years later. The digits were encoded in the form of visual images. He used a variety of memory strategies in a flexible way. For example, he learned complex verbal information by linking each piece of information to a different, well known location. This is known as the method of loci.

S also made frequent use of synaesthesia, which is the tendency for one sense modality to evoke another. His usual strategy was to encode all kinds of material in vivid visual terms. For example, S once said to the psychologist Vygotsky, "What a crumbly yellow voice you have" (Luria, 1975, p. 24). Unfortunately, we do not know why S had such strong synaesthesia and such exceptional memory. He did not dedicate much time to improving his memory, which suggests that his abilities were innate. Wilding and Valentine (1991) suggested that S may have had more brain tissue than most people devoted to processing sensory information.

S was unusual among those with superior memory ability in two ways. First, his memory powers were much greater. Second, his superiority seemed to owe little to the use of highly practised memory techniques. More typical is the case of the young man (SF) studied by Ericsson and Chase (1982). He was a student at Carnegie-Mellon University who was paid to practise the digit-span task for one hour a day for two years. The digit span (the number of random digits that can be repeated back in the correct order) is typically about seven items, but this individual eventually attained a span of 80 items.

How did he do it? He reached a digit span of about 18 items by using his extensive knowledge of running times. For example, if the first few digits presented were "3594", he would note that this was Bannister's time for the mile, and so those four digits would be stored away as a single chunk or unit. He then increased his digit span to 80 by organising these chunks into a hierarchical structure.

SF's memory had outstanding digit span, but his letter and word spans were only average. A similar pattern was found with Rajan Mahadevan. He managed to produce the first 31,811 digits of pi (the ratio of a circle's radius to its circumference) in just under four hours, and this gained him a place in the Guinness Book of Records. His exceptional ability to remember digits was also found with digit span: his digit span was 59 for visually presented digits and 63 for heard digits. However, he was below average at remembering the position and orientation of images of various objects (Biederman, Cooper, Fox, & Mahadevan, 1992). The pattern of memory performance showed by individuals such as SF and Rajan led Groeger (1997, p. 242) to conclude: "There is very little evidence that exceptional abilities extend beyond the limits of the particular strategies which the mnemonist has learned to use effectively."

Theoretical views

Ericsson (1988) proposed that there are three requirements to achieve very high memory skills:

• Meaningful encoding: the information should be processed meaningfully, relating it to pre-existing knowledge; this resembles levels-of-processing theory (see Chapter 6).

• Retrieval structure: cues should be stored with the information to aid later retrieval; this resembles the encoding specificity principle (see Chapter 6).

• Speed-up: there is extensive practice so that the processes involved in encoding and retrieval function faster and faster; this produces automaticity (see Chapter 5).

This theoretical approach was developed by Ericsson and Kintsch (1995). They argued that exceptional memory depends on pre-existing knowledge rather than an enlarged working memory. According to Ericsson and Kintsch (1995, p. 216), the crucial requirements for exceptional memory are as follows: "Subjects must associate the encoded information with appropriate retrieval cues. This association allows them to activate a particular retrieval cue at a later time and thus partially reinstates the conditions of encoding to retrieve the desired information from long-term memory". The various mnemonic techniques discussed in the next section provide examples of these principles in action.

The theoretical approach of Ericsson (1988) and of Ericsson and Kintsch (1995) might lead one to conclude that those with exceptional memory rely on highly practised memory strategies. However, Wilding and Valentine (1994) found that matters are more complicated. They took advantage of the fact

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