All people differ somewhat in their ability to remember information. However, some individuals have remarkable memories and perform feats that normal individuals could never hope to achieve. These individuals, sometimes called mnemonists , are considered to have exceptional memory.
Psychologists have described several cases of exceptional memory. Aleksandr R. Luria, a Russian neuropsychologist, described one of the most famous cases in his book The Mind of a Mnemonist (1968). Luria recounted the abilities of S. V. Shereshevskii, a man he called S. Luria studied Shereshevskii over many years and watched him perform remarkable memory feats. However, until Luria began studying these feats, Shereshevskii was unaware of how extraordinary his talents were. For example, Shereshevskii could study a blackboard full of nonsense material and then reproduce it at will years later. He could also memorize long lists of nonsense syllables, extremely complex scientific formulas, and numbers more than 100 digits long. In each case, Shereshevskii could recall the information flawlessly, even if asked to produce it in reverse order. Luria reported one instance in which Shereshevskii was able to recall a 50-word list when the test was given without warning 15 years after presentation of the list! He recalled all 50 words without a single error.
The primary technique Shereshevskii used was mental imagery. He generated very rich mental images to represent information. In addition, part of his ability might have been due to his astonishing capacity for synesthesia. Synesthesia occurs when information coming into one sensory modality, such as a sound, evokes a sensation in another sensory modality, such as a sight, taste, smell, feel, or touch. All people have synesthesia to a slight degree. For example, certain colors may “feel” warm or cool. However, Shereshevskii’s synesthesia was extremely vivid and unusual. For example, Shereshevskii once told a colleague of Luria’s, “What a crumbly yellow voice you have.” He also associated numbers with shapes, colors, and even people. Synesthetic reactions probably improved Shereshevskii’s memory because he could encode events in a very elaborate way. But they often caused him confusion, too. For example, reading was difficult because each word in a sentence evoked its own mental image, interfering with comprehension of the sentence as a whole.
A second case of exceptional memory illustrates the talent some people display for remembering certain types of material. In a series of tests in the 1980s and 1990s, Rajan Srinavasen Mahadevan (known as Rajan) demonstrated a remarkable talent for remembering numbers, but for other types of material, his memory ability tested in the normal range. Rajan memorized the mathematical ratio pi, which begins 3.14159 and continues indefinitely with no known pattern, to nearly 32,000 decimal places! If given a string of digits, within a few seconds he could accurately say whether or not the string appears in the first 32,000 digits of pi. He could also rapidly identify any of the first 10,000 digits of pi when given a specific decimal place. For example, he could tell what digit is in decimal place 6,243 in about 12 seconds, and he rarely made errors on this task. Rajan demonstrated great skill at learning new numerical information.
Shereshevskii and Rajan scored in the normal range on standard intelligence tests. Another group of people, those with savant syndrome (formerly called idiot savants), usually score low on intelligence tests but have one “island” of outstanding cognitive ability. Many children and adults who are deemed savants have extraordinary memory. Psychologists have studied many cases of savant syndrome, but its nature remains a mystery.
Cases of exceptional memory stand as remarkable puzzles whose implications for normal memory functioning are unclear. In some cases the remarkable talents exemplify techniques (such as mental imagery) that are known to magnify normal memory ability. These striking cases have not been integrated well into the scientific study of memory, but generally stand apart as curiosities that cannot yet be explained in any meaningful way.