Short term memory

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 (Davelaar 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
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 plus or minus two ( Miller, 1956 ).

Even the effects of the general intelligence factor can be understood as the channel capacity of short-term memory. In the theoretical framework of information psychology mental power, or 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 the external link below to the paper by Lehrl and Fischer (1990) ), and the duration time D (in s) of information in short-term working memory, meaning the duration of memory span. Hence:


 * C (bit) = Ck(bit/s) &times; 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.