short-term memory
Short-term memory (sometimes referred to as "primary memory" or "active memory") refers to the capacity for holding a small amount of information in mind in an active, readily available state for a short period of time. The duration of short-term memory (when rehearsal is prevented) is about 20 seconds. Estimates of the capacity of short-term memory vary – from about 3 or 4 elements (i.e., words, digits, or letters) to about 9 elements. A commonly-cited capacity is 7±2 elements. In contrast, long-term memory indefinitely stores a seemingly unlimited 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.
Existence of a separate store
A classical model of memory developed in the 1960s assumed that all memories pass from a short-term to a long-term store after a small period of time. This model is referred to as the "modal model" and has been most famously detailed by Atkinson and Shiffrin.[Atkinson and Shiffrin, 1968] The exact mechanisms 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 controversial topics among experts.
One form of evidence, cited in favor of the separate existence of a short-term store comes from anterograde amnesia, the inability to learn new facts and episodes. Patients with this form of amnesia, typically caused by damage to the hippocampus, have intact ability to retain small amounts of information over short time scales (up to 30 seconds) but are dramatically impaired in their ability to form longer-term memories (a famous example is patient HM). This is interpreted as showing that the short-term store is spared from amnesia.
Other evidence comes from experimental studies showing that some manipulations (e.g., a distractor task, such as repeatedly subtracting a single-digit number from a larger number following learning) impair memory for the 3 to 5 most recently learned words of a list (presumably still held in short-term memory), while leaving recall for words from earlier in the list (presumably stored in long-term memory) unaffected; other manipulations (e.g., semantic similarity of the words) affect only memory for earlier list words,[Davelaar, Goshen-Gottstein, Haarmann & Usher, 2005] but do not affect memory for the last few words in a list. These results show that different factors affect short term recall (disruption of rehearsal) and long-term recall (semantic similarity). Together, these findings show that long-term memory and short-term memory can vary independently of each other.
Not all researchers agree that short-term and long-term memory are separate systems. Some theoriests propose that memory is unitary over all time scales, from milliseconds to years[Brown, G. D. A., Neath, I., & Chater, N. (2007). A ratio model of scale-invariant memory and identification. Psychological Review, 114, 539-576.]. Support for the unitary memory hypothesis comes from the fact that it has been difficult to demarcate a clear boundary between short-term and long-term memory. For instance, Tarnow shows that the recall probability vs. latency curve is a straight line from 6 to 600 seconds, with the probability of failure to recall only saturating after 600 seconds [Tarnow, 2007]. If there were really two different memory stores operating in this time frame, one could expect a discontinuity in this curve. Other research has shown that the detailed pattern of recall errors looks remarkably similar for recall of a list immediately after learning (presumably from short-term memory) and recall after 24 hours (necessarily from long-term memory)[Nairne, J. S., & Dutta, A. (1992). Spatial and temporal uncertainty in long-term memory. Journal of Memory and Language, 31, 396-407.]
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 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 Baddeley's influential 1986 model of working memory 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, because most of the work done on short-term memory has used verbal material. In recent years, however, there has been a surge in research on visual short term memory[Luck, S. J., & Vogel, E. K. (1997). The capacity of visual working memory for features and conjunctions. Nature, 390, 279-281.]
, and also increasing work on spatial short term memory[Parmentier, F. B. R., Elford, G., & Maybery, M. (2005). Transitional information in spatial serial memory: path characteristics affect recall performance. Journal of Experimental Psychology: Learning, Memory, and Cognition, 31, 412-427.]
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. Once information makes it past our sensory buffers, it passes into our short term memory, a second system for retaining information on our memories for brief periods of time. 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 seconds. Short term memory is the second factory worker in our memory assembly line. Memory that exceeds short-term memory duration limits is known as long-term memory.
The limited duration of short-term memory immediately suggests that its contents spontaneously decay over time. The decay assumption is part of many theories of short-term memory, most notably Baddeley's model of working memory. The decay assumption is usually paired with the idea of rapid covert rehearsal: In order to overcome the limitation of short-term memory, 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.
Several researchers, however, dispute that spontaneous decay plays any significant role in forgetting over the short term [Lewandowsky, S., Duncan, M., & Brown, G. D. A. (2004). Time does not cause forgetting in short-term serial recall. Psychonomic Bulletin & Review, 11, 771-790.][Nairne, J. S. (2002). Remembering over the short-term: The case against the standard model. Annual Review of Psychology, 53, 53-81.], and the evidence is far from conclusive[Jonides, J., Lewis, R. L., Nee, D. E., Lustig, C. A., Berman, M. G., & Moore, K. S. (2008). The mind and brain of short-term memory. Annual Review of Psychology, 59, 193-224.].
Authors doubting that decay causes forgetting from short-term memory often offer as an alternative some form of interference: When several elements (such as digits, words, or pictures) are held in short term memory simultaneously, their representations compete with each other for recall, or degrade each other. Thereby, new content gradually pushes out older content, unless the older content is actively protected against interference by rehearsal or by directing attention to it[Oberauer, K., & Kliegl, R. (2006). A formal model of capacity limits in working memory. Journal of Memory and Language, 55, 601-626.].
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: The experimenter presents lists of items (e.g., digits or words) of increasing length; the tested person's span is determined as the longest list length that she or he can recall correctly in the given order on at least half of all trials.
Prior to the creation of current memory models, George Miller argued that human short-term memory has a forward memory span of approximately seven items plus or minus two.[Miller, 1956] 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. 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,[Baddeley, Thomson & Buchanan, 1975] or when their speech sounds are similar to each other; this is called the phonological similarity effect.[Conrad & Hull, 1964 ] More words can be recalled when the words are highly familiar and/or occur frequently in the language;[Poirier & Saint-Aubin, 1996] 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.[Poirier & Saint-Aubin, 1995]
Chunking
Chunking is the process in which we can expand our ability to remember things in short term. Chunking is also a process by which a person organize material into meaningful groups.Though the average person may only retain about 7±2 different units in his or her short term memory, chunking can greatly increase a person's recall ability. Through putting each unit into a meaningful word or phrase, a person's recall ability can skyrocket through practice. For example, in recalling a phone number, the person usually chunks the digits into three groups: first, the area code (such as 814), 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. In one testing session, an All-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.)[Ericsson, Chase & Faloon, 1980]
See also
 Related to the concept of Working memory
Contrast Long-term memory
Iconic memory
Visual short-term memory
Attention versus memory in prefrontal cortex
Memento, a film
50 First Dates, a film
Patient HM
The Last Hippie in An Anthropologist on Mars, a medical case history
Bibliography
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.
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, pp. 575–589.
Conrad, R., & Hull, A. J. (1964). Information, acoustic confusion and memory span. British Journal of Pychology, 55, pp. 429–432.
Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24, pp. 1–185.
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, pp. 3–42.
Ericsson, K. A., Chase, W. G., & Faloon, S. (1980). Acquisition of a memory skill. Science, 208, pp. 1181–1182
Lehrl, S., & Fischer, B. (1988): The basic parameters of human information processing: their role in the determination of intelligence. Personality and individual Differences, 9, pp. 883–896. ( )
Miller, G. (1956): "", Psychological Review, vol. 63 pp. 81–97 ()
Poirier, M., & Saint-Aubin, J. (1996). Immediate serial recall, word frequency, item identity and item position. Canadian Journal of Experimental Psychology, 50, pp. 408–412.
Poirier, M., & Saint-Aubin, J. (1995). Memory for related and unrelated words: Further evidence on the influence of semantic factors in immediate serial recall. Quarterly Journal of Experimental Psychology, 48A, pp. 384–404.
Schacter, D. L. (1997): Searching for Memory: The Brain, the Mind, and the Past. ISBN 0-465-07552-5.
Tarnow, Eugen (2005): The Short Term Memory Structure In State-Of-The Art Recall/Recognition Experiments of Rubin, Hinton and Wentzel. ()
External links
Short term memory in educational psychology
An ERP Index of Visual Short Term Memory Capacity
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