Short-term memory

Short-term memory (STM) (or "primary" or "active memory") is the capacity for holding a small amount of information in mind in an active, readily available state for a short period of time. The duration of short-term memory (when rehearsal or active maintenance is prevented) is believed to be in the order of seconds. Estimates of short-term memory capacity are 7 plus or minus 2 units, depending upon the experimental design used to estimate capacity. A commonly-cited capacity is 7±2 elements. In contrast, long-term memory indefinitely stores a seemingly unlimited amount of information.

Short-term memory should be distinguished from working memory which refers to structures and processes used for temporarily storing and manipulating information (see more details below). Newton's third law of motion, I before E, always.

Existence of a separate store

The idea of a division of memory into short term and long term dates back to the 19th century. A classical model of memory developed in the 1960s, now known to be flawed, 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 Shiffrin. 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, 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, 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.

Not all researchers agree that short-term and long-term memory are separate systems. Some theorists propose that memory is unitary over all time scales, from milliseconds to years. Support for the unitary memory hypothesis comes from the fact that it has been difficult to demarcate a clear boundary between short-term and long-term memory. For instance, Tarnow shows that the recall probability vs. latency curve is a straight line from 6 to 600 seconds (ten minutes), with the probability of failure to recall only saturating after 600 seconds . If there were really two different memory stores operating in this time frame, one could expect a discontinuity in this curve. Other research has shown that the detailed pattern of recall errors looks remarkably similar for recall of a list immediately after learning (presumably from short-term memory) and recall after 24 hours (necessarily from long-term memory).

Biological basis

It is proposed by Tarnow that short term memory involves the firing of neurons which depletes the Readily Releasable Pool (RRP) of neurotransmitter vesicles at presynaptic terminals. The pattern of depleted presynaptic terminals represents the long term memory trace and the depletion itself is the short term memory. After the firing has slowed down, endocytosis causes short term memory to decay. If the endocytosis is allowed to finish (the memory is not activated again), the pattern of exhausted postsynaptic terminals becomes invisible and the short term memory disappears. The long term memory remains as the metastable pattern of the neuronal excitations.

Relationship with working memory

The relationship between short-term memory and working memory is described differently by various theories, 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 be referred to as working attention. Short-term memory generally refers, in a theory-neutral manner, to the short-term storage of information, and it does not entail the manipulation or organization of material held in memory. 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 Baddeley's influential 1986 model of working memory 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. In recent years, however, there has been a surge in research on visual short term memory, and also increasing work on spatial short term memory.

Duration of short-term memory

The limited duration of short-term memory immediately suggests that its contents spontaneously decays over time. The decay assumption is part of many theories of short-term memory, most notably Baddeley's model of working memory. The decay assumption is usually paired with the idea of rapid covert rehearsal: 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.

Several researchers, however, dispute that spontaneous decay plays any significant role in forgetting over the short term , and the evidence is far from conclusive.

Authors doubting that decay causes forgetting from short-term memory often offer as an alternative some form of interference: When several elements (such as digits, words, or pictures) are held in short term memory simultaneously, their representations compete with each other for recall, or degrade each other. Thereby, new content gradually pushes out older content, unless the older content is actively protected against interference by rehearsal or by directing attention to it.

Capacity of short-term memory

Whatever the cause or causes of forgetting over the short term may be, there is consensus that it severely limits the amount of new information that we can retain over brief periods of time. This limit is referred to as the finite capacity of short-term memory. The capacity of short-term memory is often called memory span, in reference to a common procedure of measuring it. In a memory span test, the experimenter presents lists of items (e.g. digits or words) of increasing length. An individual's span is determined as the longest list length that he or she can recall correctly in the given order on at least half of all trials.

In an early and highly influential article, The Magical Number Seven, Plus or Minus Two, the psychologist George Miller suggested that human short-term memory has a forward memory span of approximately seven items plus or minus two and that that was well known at the time (it seems to go back to the 19th century researcher Wundt). 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 used. 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, or when their speech sounds are similar to each other; this is called the phonological similarity effect. More words can be recalled when the words are highly familiar or occur frequently in the language. 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. According to the available evidence, the best overall estimate of short-term memory is about four pieces or "chunks" of information.. In free recall it has been shown, to the contrary, that there is no such "quantized" limit, rather it is a function of memory decaying with time.

Chunking

Chunking is the process with which we can expand our ability to remember things in the short term. Chunking is also a process by which a person organizes material into meaningful groups. Although the average person may only retain about four different units in short-term memory, chunking can greatly increase a person's recall capacity. For example, in recalling a phone number, the person could chunk the digits into three groups: first, the area code (such as 215), 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 10 digits.

Practice and the usage of existing information in long-term memory can lead to additional improvements in one's ability to use chunking. In one testing session, an American cross-country runner was able to recall a string of 79 digits after hearing them only once by chunking them into different running times (e.g. the first four numbers were 1518, a three-mile time.)

Factors Affecting Short Term Memory

It is very difficult to demonstrate the exact capacity of STM because it will vary depending on the nature of the material to be recalled. There is currently no way of defining the basic unit of information to be stored in the STM store. It is also possible that STM is not the store described by Atkinson and Shiffrin. In that case, the task of defining the task of STM becomes even more difficult.

See also

• Related to the concept of Working memory
• Contrast Long-term memory
• Iconic memory
• Visual short-term memory
• Attention versus memory in prefrontal cortex
• Memento, a film
• 50 First Dates, a film
• Ghajini, a film
• Patient HM
• The Last Hippie in An Anthropologist on Mars, a medical case history

References

Bibliography

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• Conrad, R.; Hull, A. J. (1964). "Information, acoustic confusion and memory span". British Journal of Pychology. 55: 429–432.
• Cowan, N. (2001). "The magical number 4 in short-term memory: A reconsideration of mental storage capacity". Behavioral and Brain Sciences. 24: 1–185.
• Davelaar, E. J.; Goshen-Gottstein, Y.; Haarmann, H. J.; Usher, M.; Usher, M (2005). "The demise of short-term memory revisited: empirical and computational investigation of recency effects". Psychological Review. 112 (1): 3–42. doi:10.1037/0033-295X.112.1.3. PMID 15631586.
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• Lehrl, S., & Fischer, B. (1988): The basic parameters of human information processing: their role in the determination of intelligence. Personality and individual Differences, 9, pp. 883–896. ([2])
• Miller, G. A. (1956). "The magical number seven, plus or minus two: Some limits on our capacity for processing information". Psychological Review. 63 (2): 81–97. doi:10.1037/h0043158. PMID 13310704. {{cite journal}}: Invalid |ref=harv (help) (pdf)
• Poirier, M.; Saint-Aubin, J. (1996). "Immediate serial recall, word frequency, item identity and item position". Canadian Journal of Experimental Psychology. 50 (4): 408–412. PMID 9025332.
• Poirier, M.; Saint-Aubin, J. (1995). "Memory for related and unrelated words: Further evidence on the influence of semantic factors in immediate serial recall". Quarterly Journal of Experimental Psychology. 48A: 384–404.
• Schacter, D. L. (1997): Searching for Memory: The Brain, the Mind, and the Past. ISBN 0-465-07552-5.
• Tarnow, Eugen (2005) The Short Term Memory Structure In State-Of-The Art Recall/Recognition Experiments of Rubin, Hinton and Wentzel, Cognitive Neurodynamics.
• Tarnow, Eugen (2008): Short Term Memory May Be the Depletion of the Readily Releasable Pool of Presynaptic Neurotransmitter Vesicles, Cognitive Neurodynamics.
• Tarnow, Eugen (2009): The Atkinson-Shiffrin model is wrong. ([3])

External links

• Short term memory in educational psychology
• An ERP Index of Visual Short Term Memory Capacity