Short-term memory

Short-term memory, sometimes referred to as "primary," "working," or "active" memory, is said to hold a small amount of information for about 20 seconds.

Estimates of short-term memory capacity 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.

Short-term memory 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
A classical model of memory developed in the 1960's 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 (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 et al., 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. This is regarded as a double dissociation and constitutes evidence for separate systems underlying short-term and long-term memory.

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 one influential model of working memory (Baddeley, 1986) 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.

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 seconds. Memory that exceeds short-term memory duration limits is known as long-term memory.

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. The process of consolidation (transfer of short-term memory to long term memory) is enhanced by the relationship, if any, of an item of short-term memory to an item in long-term memory (for example, if a sensory short-term event is linked to a trauma already in long-term memory).

The time to find a short term memory is reversely proportional to the recognition probability (see Tarnow, 2005).

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 ). 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) ). Fewer words can be recalled when their speech sounds are similar to each other, this is called the phonological similarity effect ( Conrad, 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 ).

Some authors have argued that even 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.

Chunking
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 ten digits. In one testing session, an All-American cross country runner was able to recall a string of 73 digits after hearing them only once by chunking them into different running times (e.g. the first four number were 1518, a three mile time.) (Ericsson et al., 1980)

In contrast to chunking, there are also ways to lose information quickly. An example of this is interference: something that distracts a person from being able to rehearse information given to them to be remembered. This can cause a person to lose the given information much quicker than someone able to rehearse as much as they want.