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Short-term memory, sometimes referred to as "primary" or "active" memory, is that part of memory which stores a limited amount of information for a limited amount of time (roughly 15-30 seconds). This can be contrasted to long-term memory, in which a seemingly unlimited amount of information is stored indefinitely. It can be described as the capacity (or capacities) for holding in mind, in an active, highly available state, a small amount of information.
The information held in short-term memory may be: recently processed sensory input; items recently retrieved from long-term memory; or the result of recent mental processing, although that is more generally related to the concept of working memory.
Existence of a separate store
It is generally considered that some or all memories pass from a short-term to a long-term store after a small period of time, a model referred to as the "modal model" and most famously detailed by Atkinson and Shiffrin (1968). The exact mechansims by which this transfer takes place, whether all or only some memories are retained permanently, and indeed the existence of a genuine distinction between the two stores, remain a controversial topic among experts.
One form of evidence cited in favor of the separate existence of a short-term store is that anterograde amnesia, the inability to learn new facts and episodes, affects long-term memory while leaving short-term memory intact. Other evidence comes from experimental studies showing that some manipulations (e.g., a distractor task following learning) affect only memory for the 3 to 5 most recently learned words of a list (presumably still held in short-term memory), whereas other manipulations (e.g., semantic similarity of the words) affect only memory for earlier list words (Lubna et al., 2005)
Relationship to working memory
The relationship between short-term memory and working memory is differently described by various theorists, but it is generally acknowledged that the two concepts are distinct. Working memory is a theoretical framework that refers to structures and processes used for temporarily storing and manipulating information. As such, working memory might also just as well be referred to as working attention. Short-term memory generally refers in a theory-neutral manner to the short term storage of information. Thus while there are short-term memory components to working memory models, the concept of short-term memory is distinct from these more hypothetical concepts. Within working memory models there are two short-term storage mechanisms -- the phonological loop and the visuospatial sketchpad. Most of the research referred to here involves the phonological loop.
Duration of short-term memory
The most important characteristic of a short-term store is, clearly, that it is short-term — that is, it retains information for a limited amount of time only. Most definitions of short-term memory limit the duration of storage to less than a minute: no more than about 30 seconds, and in some models as little as 2. In order to overcome this, and retain information for longer, information must be periodically repeated, or rehearsed — either by articulating it out loud, or by mentally simulating such articulation. In this way, the information will re-enter the short-term store and be retained for a further period.
Capacity of short-term memory
Whatever the cause or causes of forgetting over the short term may be, there is consensus that it severely limits the amount of new information that we can retain over brief periods of time. This limit is referred to as the finite capacity of short-term memory. The capacity of short-term memory is often called memory span, in reference to a common procedure of measuring it. In a memory span test, the experimenter presents lists of items (e.g. digits or words) of increasing length. An individual's span is determined as the longest list length that he or she can recall correctly in the given order on at least half of all trials.
In an early and highly influential article, The Magical Number Seven, Plus or Minus Two, the psychologist George Miller suggested that human short-term memory has a forward memory span of approximately seven items plus or minus two. More recent research has shown that this "magical number seven" is roughly accurate for college students recalling lists of digits, but memory span varies widely with populations tested and with material used. For example, the ability to recall words in order depends on a number of characteristics of these words: fewer words can be recalled when the words have longer spoken duration; this is known as the word-length effect, or when their speech sounds are similar to each other; this is called the phonological similarity effect. More words can be recalled when the words are highly familiar or occur frequently in the language. Recall performance is also better when all of the words in a list are taken from a single semantic category (such as sports) than when the words are taken from different categories. According to the available evidence, the best overall estimate of short-term memory is about four pieces or "chunks" of information.
Chunking is the process with which we can expand our ability to remember things in the short term. Chunking is also a process by which a person organizes material into meaningful groups. Although the average person may only retain about four different units in short-term memory, chunking can greatly increase a person's recall capacity. For example, in recalling a phone number, the person could chunk the digits into three groups: first, the area code (such as 215), then a three-digit chunk (123) and lastly a four-digit chunk (4567). This method of remembering phone numbers is far more effective than attempting to remember a string of 10 digits.
Practice and the usage of existing information in long-term memory can lead to additional improvements in one's ability to use chunking. In one testing session, an American cross-country runner was able to recall a string of 79 digits after hearing them only once by chunking them into different running times (e.g. the first four numbers were 1518, a three-mile time.)
Factors Affecting Short Term Memory
It is very difficult to demonstrate the exact capacity of STM due to the fact that it will vary depending on the nature of the material to be recalled. Until now, there is no way of defining the basic unit of information to be stored in the STM store. It is also possible that STM is not the store described by Atkinson and Shiffrin. In that case, the task of defining the task of STM becomes even more difficult.
Some other factors are listed below: Reading Aloud: Digital spans tend to increase if the digits are read aloud by participants instead of being read sub-vocally. Baddley (1999) suggests that the sounds are also stored in the echoic store which makes storing it easier.
- C (bit) = Ck(bit/s) × D (s).
However, against the trend of the 1950s to understand cognition in an information theoretic context, Miller himself was in doubt that the capacity of short-term memory could be measured in such a way in terms of a constant amount of information, as expressed in bits. Miller argued that the unit of measurement for short-term memory capacity is a chunk. A chunk can be a single digit or letter, it can also be a word, a multiple-digit number or even a whole phrase if the number or the phrase form a unit already learned in long-term memory before.
Modalities of short term memory
- Main article: Visual short-term memory
- Main article: Auditory short-term memory
Models of short-term memory
There are competing models of short term memory
- Main article: Models of short-term memory
The biological substrate of short-term memory
- Main article: The neurochemistry of short-term memory
- Main article: The neurophysiology of short-term memory
Key texts – Books
- Baddeley, A. D. (1986). Working memory. New York: Oxford University Press. ISBN 0198521332
- D. Deutsch and J. A. Deutsch (Eds.), Short Term Memory New York: Academic Press, 1975, 107-151.ISBN 0122133501
- Miyake, A & Shah, P (1999)Models of Working Memory: Mechanisms of Active Maintenance and Executive Control. Cambridge UP.ISBN 0521587212
Additional material – Books
- Neath, I. et al (2005))Special issue Short term/working memory. Memory, Vol 13, Issues 3&4, 2005, ISBN 1-84169-965-9
- Schacter, D. L. (1997): Searching for Memory: The Brain, the Mind, and the Past. ISBN 0465075525.
Reviews of the area
Key texts – Papers
- Davelaar, E. J., Goshen-Gottstein, Y., A., A., Haarmann, H. J., & Usher, M. (2005): The demise of short-term memory revisited: empirical and computational investigation of recency effects. Psychological Review, 112, 3-42.Fulltext
- Miller, G. (1956): "The Magical Number Seven, Plus or Minus Two", Psychological Review, vol. 63 pp. 81-97 Fulltext
- Warrington, E.K. and Weiskrantz, L. (1973) An analysis of short term and long term memory defects in man. In: J.A. Deutsch (ed.) The Physiological Basis of Memory, New York: Academic Press.
Additional material – Papers
- Google Scholar
- Atkinson, R. C. & Shiffrin, R.M. (1968): Human memory: A proposed system and its control processes - In K.W. Spence & J.T. Spence (Eds.), The Psychology of Learning and Motivation, Vol 2. London: Academic Press.
- Conrad, R. (1964) Acoustic confusions in immediate memory, British Journal of Psychology 55: 75-84
- Lehrl, S., & Fischer, B. (1988): The basic parameters of human information processing: their role in the determination of intelligence. Personality and Individual Differences., 9, 883 - 896. (Fulltext)
- Farnham-Diggory, S, and Gregg, L.W. (1975)Short-term memory function in young readers, Journal of Experimental Child Psychology 19: 279-98.
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- Miller, G. A. (1956). The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychological Review, 63, 81-97.
- Baddeley, Thomson & Buchanan, 1975
- Conrad & Hull, 1964
- Poirier & Saint-Aubin, 1996
- Poirier & Saint-Aubin, 1995
- Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24, 97-185.
- Ericsson, Chase & Faloon, 1980