Short-term memory
From Wikinfo
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.
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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 & 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 slightly 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 minut: 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 limited capacity. In a very famous and influential paper, George A. Miller showed that a normal human short-term memory had a capacity of between 5 and 9 numbers - which he referred to as "The Magical Number, 7±2" (Miller, 1956). Although the exact numbers vary from study to study, this is generally considered a good approximation of the number of items which can be stored at any one time.
It is important to note the distinction between an item and, say, a single number or word. While a number can indeed be a single digit or letter, so can a whole number, word, or abstract concept. For instance, memorising the 9 digits 684804791 individually presents a challenge to most people; memorising the three numbers 684, 804, and 791 - for example, by assigning a mental image or pronunciation to each - is much easier. This is because the first approach requires storage of 9 separate items, whereas the second combines them into only 3 items - a process referred to as chunking. Similarly, an entire sentence may be represented as one or more abstract concepts, and thus held in a short-term store despite consisting of more than 9 words, and many more than 9 letters.
References
- Miller (1956): The Magical Number Seven, Plus or Minus Two (The Psychological Review, vol. 63 pp. 81-97) [1]
References
- Adapted from the Wikipedia article, "Short-term_memory" http://en.wikipedia.org/wiki/Short-term_memory, used under the GNU Free Documentation License
