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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 30-45 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. The last of these 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 controversial within cognitive psychology.
One form of evidence cited in favour of the separate existence of a short-term store is that a variety of conditions, including simple aging and various forms of amnesia, seem to diminish or destroy short-term memory, while leaving long-term memory intact.
Relationship to working memory
The relationship between short-term memory and working memory varies between authors, but it is generally acknowledged that the two concepts are distinct. Essentially, working memory is defined as the structures and processes used for temporarily storing and manipulating information, while short-term memory generally refers only to the storage structures involved. Thus, short-term memory can be considered a subset of working memory, and indeed most of the findings detailed here can be seen as occurring within the phonological loop component of the standard Working Memory Model.
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
The second key concept associated with a short-term memory is that it has a finite capacity. 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, or more accurately within the information theoretic context in which his article was written 2.3 bits (log27) (Miller, 1956). It is important to note the distinction between an item stored in memory and, say, a single digit or letter. While an item can indeed be a single digit or letter, it can also be a whole number, word, or abstract concept. Memorising 12 digits (e.g., 1, 9, 6, 6, 1, 7, 8, 8, 1, 0, 6, 6) is a difficult task; memorising them as three dates (i.e., 1966, 1788, and 1066) is comparatively easier. This is because the first approach requires storage of 12 separate items, whereas the second only requires the storage of 3. This process was referrered to by Miller as "chunking".
General cognitive ability, underlying IQ, is limited by the channel capacity of short-term memory. Mental power, that means the capacity C of short-term memory (measured in bits of information) is the product of the individual mental speed Ck of information processing (in bit/s), see below the external link to the paper by Lehrl and Fischer (1990), and the duration time D (in s) of information in short term working memory, that means the duration of memory span. Hence:
- C(bit) = Ck(bit / s)xD(s)
- 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.
- Miller, G. (1956): "The Magical Number Seven, Plus or Minus Two", The Psychological Review, vol. 63 pp. 81-97 
- Schacter, D. L. (1997): Searching for Memory: The Brain, the Mind, and the Past. ISBN 0465075525.
- The Basic Period of Individual Mental Speed, Underlying IQ
- Short term memory in educational psychology
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