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Short-term memory
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Short-term memory

Table of contents
1 Introduction
2 Existence of a separate store
3 Relationship to working memory
4 Duration of short-term memory
5 Capacity of short-term memory
6 See also
7 References


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. 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. George Miller has argued that human short-term memory can store approximately seven items, or more accurately within the information theoretic context in which his article was written 2 or 3 bits (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 referred to by Miller as "chunking.

See also