In psychology and neuroscience, memory span is the longest list of items that a person can repeat back in correct order immediately after presentation on 50% of all trials. Items may include words, numbers, or letters. The task is known as digit span when numbers are used. Memory span is a common measure of short-term memory. It is also a component of cognitive ability tests such as the WAIS. Backward memory span is a more challenging variation which involves recalling items in reverse order.
As functional aspect
Functionally, memory span appears to measure the number of discrete units over which the individual can successively distribute his attention and still organize them into a working unit. To generalize, memory span refers to the ability of an individual to reproduce immediately, after one presentation, a series of discrete stimuli in their original order. Practically any sort of material may be presented, such as digits, letters, words, and sounds, and almost any sense organ or combination of sense organs may be used to receive the impressions.
As structural aspect
A structural definition of memory span is difficult to give, for one immediately is faced by the distinctions between the prerequisites for memory span, and the actual processes involved. "Associability” is required in memory span. This term refers to the ability of the subject to group the series of elements together: to perceive relationships among the series in order to better reproduce them. Still another process involved in memory span is that of imagery. The subject, in order to be able to reproduce the series presented, must be able to image the series. The actual reproducing of the series of stimuli involves the process of memory. If the individual possessed no memory at all, reproduction of the series would be impossible. It is also known that memory span and memory are different in the length of time over which reproduction is possible. Memory span is transitory; memory is fairly permanent. In addition, the amount of material involved in memory span is ordinarily much less than the amount of material involved in memory. Reproduction of the series also involves certain other "reproduction factors," such as language ability and arithmetical proficiency.
There are a number of factors which definitely affect memory span; the effects of practically all of these factors have been investigated in statistical and experimental studies. Some of the factors are extrinsic, or present in the testing situation itself. These factors, if not carefully controlled, cause the memory span test to be statistically unreliable. Other factors are intrinsic in the individual, and it is these factors which are the basis of "true" memory span. Though numerous factors affect memory span, the test is one that shows surprisingly high reliability. Results obtained by different investigators show that the reliability coefficients for memory span are quite high.
Digit-span task is used to measure working memory's number storage capacity. Participants are presented with a series of digits (e.g., '8, 2, 4') and must immediately repeat them back. If they do this successfully, they are given a longer list (e.g., '9, 2, 4, 1'). The length of the longest list a person can remember is that person's digit span. While the participant is asked to enter the digits in the given order in the forward digit-span task, in the backward digit-span task the participant needs to reverse the order of the numbers.
This is a graphical representation of typical results that might be obtained from performing a forward/backward digit span recall task on participants in several different age groups. The numbers on the y-axis indicate number of digits successfully recalled.
The digit span task exercises verbal working memory. Scientists refer to working memory as the cognitive system that allows the temporary storage and manipulation of information. According to one influential cognitive theory, this system has specialised components, one of which, the 'Phonological loop', underlies verbal working memory abilities (Baddeley & Hitch 1974). The phonological loop consists of a verbal storage system and a rehearsal system. Participants may find themselves mentally rehearsing the string of digits as they appear on screen; this is the rehearsal system in action. It allows the visual inputs to be recoded so that they can enter short term verbal store and it also refreshes decaying representations (that is, any item that is about to be forgotten).
Verbal working memory is involved in many everyday tasks, from remembering a friend's telephone number while entering it into a phone, to understanding long and difficult sentences. For example, it is difficult to understand a whole sentence without remembering the words at the beginning long enough to connect them with the words at the end. Verbal working memory is also thought to be one of the elements underlying intelligence (often referred to as 'IQ,' meaning "intelligence quotient"); thus, the digit span task is a common component of many IQ tests, including the widely used Wechsler Adult Intelligence Scale (WAIS). Performance on the digit span task is also closely linked to language learning abilities; improving verbal memory capacities may therefore aid mastery of a new language.
(1) Characteristics of the material that is used: The characteristics of the material used will definitely affect the memory span score. If, for example, the material is all closely related, it will be much more easily reproduced. This relationship of the material is called the "coefficient of associability." Subjects also display an increased memory span for concrete words like hammer over abstract ones like justice.
(2) Rhythm of the presentation of the material: Closely related to the problem of presenting the stimuli in groups, is the presentation of the stimuli in rhythmic fashion. Most investigators point out that the stimuli used in testing memory span should be presented with as little rhythm as possible. The effect of rhythm is to group the units in the series, again enabling the individual to secure a span higher than his "true" one.
(3) Rate of presentation of the stimuli: The speed with which the stimuli are presented has an effect on the memory span score attained. Actual experimental investigation also indicates that the speed of presenting the stimuli affects the score.
(4) Modality of presentation: Studies have shown a consistent increase in memory span for lists presented auditorally over ones presented visually.
(5) Time required to vocalize responses: Memory span is consistently higher for short words than for long words. This increase is due to the decreased amount of time needed to pronounce the shorter words, and studies have shown that memory span is approximately equal to the number of items which a subject can articulate in two seconds. A study of Welsh-English bilinguals confirmed this effect as it showed these subjects had larger digit spans for English numbers than for Welsh numbers, which take longer to pronounce. The spans were equivalent when corrected for the time taken to articulate the digits.
(6) The method of scoring the responses: The method of scoring the responses also has an effect upon the apparent memory span of the individual. Variations in scoring are common; scarcely two investigators have scored alike. Most investigators take the point of view that an incorrect series should not be scored at all.
(7) Distraction: Naturally enough, one would expect that the greater the distraction present in the situation, the poorer would be the performance of the individual, and this is actually the case. The reason for this effect is apparent. Inasmuch as attention is one of the processes involved in the successful functioning of memory span, if the processes of attention are directed towards some other stimulus, they cannot operate effectively in the memory span function.
The intrinsic factors are those within the individual which influence the individual's permanent memory span.
The age of the individual is a factor which appears to affect memory span. Memory span has been found to increase with age, up to a given point. It should be pointed out that if the mental age of the individual does not increase, the memory span will not, either. So far as is known, memory span increases along with intelligence, and is generally found to level off at a similar point. Some investigators claim that memory span increases to a point somewhere between the sixteen- and twenty-six-year-old level, though a large number of researchers believe that memory span remains constant after the individual reaches a point somewhere between 12 and 16 years. A study conducted by Gregoire and Van der Linden in 1997 observed gradual declines in both forward and backward memory spans between the ages of 20 and 70, with backward memory span capacity being marginally less than forward memory span capacity, but declining at an equal rate; these declines did not become statistically significant, however, until after the age of 70, when the declines in both forward and backward memory spans became more pronounced. Because the backward digit span task (measuring backward memory span) is thought to involve not only short term memory stores but also the involvement of the central executive (which is believed to be responsible for moderating short term memory stores), equal rates of decline in forward and backward memory spans may indicate that forward memory span relies on the moderating effects of central executive resources as well 
- Permanent pathological conditions
When the physical condition of the individual becomes permanently modified, the memory span has been found to be lower than that for a normal individual. With other words, permanent medical conditions can lead to a deterioration of memory.
The memory span procedure
In a typical test of memory span, a list of random numbers or letters is read out loud or presented on a computer screen at the rate of one per second. The test begins with two to three numbers, increasing until the person commits errors. Recognizable patterns (for example 2, 4, 6, 8) should be avoided. At the end of a sequence, the person being tested is asked to recall the items in order. The average digit span for normal adults without error is seven plus or minus two. However, memory span can be expanded dramatically - in one case to 80 digits - by learning a sophisticated system of recoding rules by which substrings of 5 to 10 digits are translated into one new chunk. In December 2015, Lance Tschirhart entered the Guinness Book of World Records for memorizing a sequence of 456 digits spoken aloud at the rate of one per second at the World Memory Championship in Chengdu, China.
In a backward digit span task, the procedure is largely the same, except that subjects being tested are asked to recall the digits in backward order (e.g., if presented with the following string of numbers "1 5 9 2 3," the subject would be asked to recall the digits in reverse order; in the case, the correct response would be "3 2 9 5 1").
From simple span to complex span
Research in the 1970s has shown that memory span with digits and words is only weakly related to performance in complex cognitive tasks such as text comprehension, which are assumed to depend on short-term memory. This questioned the interpretation of memory span as a measure of the capacity of a central short-term memory or working memory. Daneman and Carpenter introduced an extended version of the memory span task which they called reading span.
The reading span task was the first instance of the family of complex span tasks, which differ from the traditional simple span tasks by adding a processing demand to the requirement to remember a list of items. In complex span tasks encoding of the memory items (e.g., words) alternates with brief processing episodes (e.g., reading sentences). For example, the operation span task combines verification of brief mathematical equations such as "2+6/2 = 5?" with memory for a word or a letter that follows immediately after each equation. Complex-span tasks have also been shown to be closely related to many other aspects of complex cognitive performance besides language comprehension, among other things to measures of fluid intelligence.
The role of interference
There is the possibility that susceptibility to proactive interference (PI) affects performance on memory span measures. For older adults, span estimates increased with each PI-reducing manipulation[clarification needed]; for younger adults, scores increased when multiple PI manipulations were combined or when PI-reducing manipulations were used in paradigms in which within-task PI was especially high. It is suggested that PI critically influences span performance. There might be the possibility that interference-proneness may influence cognitive behaviors previously thought to be governed by capacity.
PI-reducing procedures did act to improve span scores in many instances. The impact of PI is greater for older adults than for younger adults. Older adults showed relatively poor span performance when PI was maximal. By contrast, younger adults improved only when PI reductions were combined, suggesting that they are relatively resistant to PI. The fact that PI contributes to span performance raises a number of interesting possibilities with respect to previously held assumptions based on memory span performance. Working memory span tasks may measure interference-proneness in addition to capacity for both older and younger adults, suggest that resistance to interference may also affect performance on many cognitive tasks. Indeed, other studies show that individual differences in susceptibility to PI are predictive of scores on standard achievement tests.
- Albert B. Blankenship(1938). The psychological bulletin, Vol. 35, No. 1, 2-3.
- Humstone, H. J.(1919). Memory Span Tests. Psychol. Clin., 12, 196-200.
- Cambridge Brain Science. About this test: Improve your digit-span performance by 'chunking'. Medical Research Council. http://www.cambridgebrainsciences.com/browse/memory/test/digit-span
- Sage Journals. Reliable Digit Span A Systematic Review and Cross-Validation Study. Ryan W. Schroeder, Philip Twumasi-Ankrah, Lyle E. Baade and Paul S. Marshall. 6 December 2011. http://asm.sagepub.com/content/19/1/21.abstract
- Sage Journals. WAIS Digit Span-Based Indicators of Malingered Neurocognitive Dysfunction Classification Accuracy in Traumatic Brain Injury. Matthew T. Heinly, Kevin W. Greve, Kevin J. Bianchini, Jeffery M. Love and Adrianne Brennan. http://asm.sagepub.com/content/12/4/429.short
- Walker, I.; Hulme, C. (1999). "Concrete words are easier to recall than abstract words: Evidence for a semantic contribution to short term serial recall". Journal of Experimental Psychology: Learning Memory, and Cognition. 25 (5).
- Drewnowski, A.; Murdock, B. B. (1980). "The role of auditory features in memory span for words". Journal of Experimental Psychology: Human Learning and Memory. 6: 319–332. doi:10.1037/0278-73220.127.116.119.
- Baddeley, A. D.; Thomson, N.; Buchanan, M. (1975). "Word length and the structure of short-term memory". Journal of Verbal Learning and Verbal Behavior. 14: 575–589. doi:10.1016/S0022-5371(75)80045-4.
- Ellis, N. C.; Hennelly, R. A. (February 1980). "A bilingual word-length effect: Implications for intelligence testing and the relative ease of mental calculation in Welsh and English". British Journal of Psychology. 71 (1): 43–51. doi:10.1111/j.2044-8295.1980.tb02728.x.
- Lumiley, F. H., and Calhoon, S. W.(1934). Memory Span for Words Presented Auditorially. Appl. Psychol., 18, 773-784.
- Gregoire, J., and Van der Linden, M.(1997). "Effect of age on forward and backward digit spans". Aging, Neuropsychology, and Cognition. 4(2), 140-149.
- Meyer, D., Glass, J., Mueller, S., Seymour, T. & Kieras, D. (2001). “Executive-process interactive control: A unified computational theory for answering 20 questions (and more) about cognitive ageing." European Journal of Cognitive Psychology, 13(1/2), 123-164.
- Hester, R., Kinsella, G., & Ong, B. (2004). “Effect of age on forward and backward span tasks." Journal of the International Neuropsychological Society, 10(4), 475-481
- Miller, G. (1956). The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information, Psychological Review, 63, 81–97.
- Ericsson, K. A., Delaney, P. F., Weaver, G., & Mahadevan, R. (2004). Uncovering the structure of a memorist's superior "basic" memory capacity. Cognitive Psychology, 49, 191-237
- Perfetti, C. A., & Goldman, S. R. (1976). Discourse memory and reading comprehension skill. Journal of Verbal Learning & Verbal Behavior, 15, 33-42
- Daneman, M., & Carpenter, P. A. (1980). Individual differences in working memory and reading. Journal of Verbal Learning and Verbal Behavior, 19, 450-466
- Turner, M. L., & Engle, R. W. (1989). Is working memory capacity task dependent? Journal of Memory and Language, 28, 127-154
- Kane, M. J., Hambrick, D. Z., Tuholski, S. W., Wilhelm, O., Payne, T. W., & Engle, R. W. (2004). The generality of working-memory capacity: A latent-variable approach to verbal and visuo-spatial memory span and reasoning. Journal of Experimental Psychology: General, 133, 189-217
- Conway, A. R. A., Kane, M. J., Bunting, M. F., Hambrick, D. Z., Wilhelm, O., & Engle, R. W. (2005). Working memory span tasks: A methodological review and user’s guide. Psychonomic Bulletin & Review, 12, 769-786
- May, C.P., Hasher, L., & Kane, M.J. (1999). The role of interference in memory span. Memory & Cognition, 27, 759-767.