Working memory

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Working memory is a theoretical framework within cognitive psychology that refers to the structures and processes used for temporarily storing and manipulating information. There are numerous theories as to both the theoretical structure of working memory (i.e., the "organizational map" it follows) as well as to the specific parts of the brain responsible for working memory. However, it is of the accepted view that the frontal cortex, parietal cortex, anterior cingulate, and parts of the basal ganglia are crucial for functioning.

Much of our understanding of working memory has come from lesion experiments in animals and imaging experiments in humans. Today there are hundreds of research laboratories around the studying various aspects of working memory. There are numerous applications of working memory in the field, ranging from increasing our understanding of autism^[1] and ADHD^[2] to helping improve teaching methods^[3] to creating artificial intelligence based on the human brain^[4]^[5].

History

The term was first used in the 1960s in the context of theories that likened the mind to a computer. Before then, what we now call working memory was referred to as short-term memory, operant memory, or provisional memory^[6]. Most theorists today use the concept of working memory to replace or include the older concept of short-term memory, thereby marking a stronger emphasis on the notion of manipulation of information instead of passive maintenance.

The earliest mention of working memory-like experiments can be traced back to over 100 years ago, when Hitzig^{[citation needed]}and Ferrier^{[citation needed]} described ablation experiments of the prefrontal cortex. They concluded that the frontal cortex was important for cognitive processes rather than sensory ones. In 1935 and 1936, Jacobsen and colleagues ^{[citation needed]} were the first to conclude that the cognitive processes in the PFC were notable in delay-dependent tasks; in other words, they suffered from short-term memory loss.

Modern working memory research

There have been numerous models proposed regarding how working memory functions, both anatomically and cognitively. Of those, three have received the distinct notice of wide acceptance.

The Baddeley and Hitch model

Main article: Baddeley's Model of Working Memory

Baddeley and Hitch (1974)^[7] introduced and made popular the multicomponent model of working memory. This theory proposes that two "slave systems" are responsible for short-term maintenance of information, and a "central executive" is responsible for the supervision of information integration and for coordinating the slave systems. One slave system, the articulatory loop, stores phonological information and prevents its decay by silently articulating its contents, thereby refreshing the information in a rehearsal loop. It can, for example, maintain a seven-digit telephone number for as long as one repeats the number to oneself again and again. The other slave system, the visuo-spatial sketch pad, stores visual and spatial information. It can be used, for example, for constructing and manipulating visual images, and for the representation of mental maps. The sketch pad can be further broken down into a visual subsystem (dealing with, for instance, shape, colour, and texture), and a spatial subsystem (dealing with location). The central executive (see executive system) is, among other things, responsible for directing attention to relevant information, suppressing irrelevant information and inappropriate actions, and for coordinating cognitive processes when more than one task must be done at the same time. Baddeley (2000) extended the model by adding a fourth component, the episodic buffer, which holds representations that integrate phonological, visual, and spatial information, and possibly information not covered by the slave systems (e.g., semantic information, musical information). The component is episodic because it is assumed to bind information into a unitary episodic representation. The episodic buffer resembles Tulving's concept of episodic memory, but it differs in that the episodic buffer is a temporary store.

The theory of Cowan

Cowan (2005)^[8] regards working memory not as a separate system, but as a part of long-term memory. Representations in working memory are a subset of the representations in long-term memory. Working memory is organized in two embedded levels. The first level consists of long-term memory representations that are activated. There can be many of these, there is no limit to activation of representations in long-term memory. The second level is called the focus of attention. The focus is regarded as capacity limited and holds up to four of the activated representations. Oberauer (2002)^[9] has extended the Cowan model by adding a third component, a more narrow focus of attention that holds only one chunk at a time. The one-element focus is embedded in the four-element focus and serves to select a single chunk for processing. For example, you can hold four digits in mind at the same time in Cowan's "focus of attention". Now imagine that you wish to perform some process on each of these digits, for example, adding the number two to each digit. Separate processing is required for each digit, as most individuals can not perform several mathematical processes in parallel. Oberauer's attentional component selects one of the digits for processing, and then shifts the attentional focus to the next digit, continuing on until all of the digits have been processed.

The theory of Ericsson and Kintsch

Whereas most adults can repeat about seven digits in correct order, some individuals have shown impressive enlargements of their digit span - up to 80 digits! This feat is possible by extensive training on an encoding strategy by which the digits in a list are grouped (usually in groups of three to five) and these groups are encoded as a single unit (a chunk). To do so one must be able to recognize the groups as some known string of digit. One person studied by K. Anders Ericsson and his colleagues, for example, used his extensive knowledge of racing times from the history of sports. Several such chunks can then be combined into a higher-order chunk, thereby forming a hierarchy of chunks. In this way, only a small number of chunks at the highest level of the hierarchy must be retained in working memory. At retrieval, the chunks are unpacked again. That is, the chunks in working memory act as retrieval cues that point to the digits that they contain. It is important to note that practicing memory skills such as these do not expand working memory capacity proper. This can be shown by using different materials - the person who could recall 80 digits was not exceptional when it came to recall words. Ericsson and Kintsch (1995) have argued that we use skilled memory in most everyday tasks. Tasks such as reading, for instance, require to maintain in memory much more than seven chunks - with a capacity of only seven chunks our working memory would be full after a few sentences, and we would never be able to understand the complex relations between thoughts expressed in a novel or a scientific text. We accomplish this by storing most of what we read in long-term memory, linking them together through retrieval structures. We need to hold only a few concepts in working memory, which serve as cues to retrieve everything associated to them from by the retrieval structures. Ericsson and Kintsch refer to this set of processes as "long-term working memory".

Working memory capacity

Working memory is generally considered to have limited capacity. The earliest quantification of the capacity limit associated with short-term memory was the "magical number seven" introduced by Miller (1956)^[10]. He noticed that the memory span of young adults was around seven elements, called chunks, regardless whether the elements were digits, letters, words, or other units. Later research revealed that span does depend on the category of chunks used (e.g., span is around seven for digits, around six for letters, and around 5 for words), and even on features of the chunks within a category. For instance, span is lower for long than for short words. In general, memory span for verbal contents (digits, letters, words, etc.) strongly depends on the time it takes to speak the contents aloud, and on the lexical status of the contents (i.e., whether the contents are words known to the person or not)^[11]. Several other factors also affect a person's measured span, and therefore it is difficult to pin down the capacity of short-term or working memory to a number of chunks. Nonetheless, Cowan (2001)^[12] has proposed that working memory has a capacity of about four chunks in young adults (and less in children and old adults).

Measures of working-memory capacity and their correlates

Working memory capacity can be tested by a variety of tasks. A commonly used measure is a dual-task paradigm combining a memory span measure with a concurrent processing task. For example, (Daneman & Carpenter, 1980) used "reading span". Subjects read a number of sentences (usually between 2 and 6) and try to remember the last word of each sentence. At the end of the list of sentences, they repeat back the words in their correct order. Other tasks that don't have this dual-task nature have also been shown to be good measures of working memory capacity^[13]. The question what features a task must have to qualify as a good measure of working memory capacity is a topic of ongoing research.

Measures of working-memory capacity are strongly related to performance in other complex cognitive tasks such as reading comprehension, problem solving, learning a programming language, and with any measures of the intelligence quotient^[14]. Some researchers have argued^[15] that working memory capacity reflects the efficiency of executive functions, most notably the ability to maintain a few task-relevant representations in the face of distracting irrelevant information. The tasks seem to reflect individual differences in ability to focus and maintain attention, particularly when other events are serving to capture attention. These effects seem to be a function of frontal brain areas^[16].

Others have argued that the capacity of working memory is better characterized as the ability to mentally form relations between elements, or to grasp relations in given information. This idea has been advanced, among others, by Graeme Halford, who illustrated it by our limited ability to understand statistical interactions between variables^[17]. These authors asked people to compare written statements about the relations between several variables to graphs illustrating the same or a different relation, for example "If the cake is from France then it has more sugar if it is made with chocolate than when it is made with cream but if the cake is from Italy then it has more sugar if it is made with cream than if it is made of chocolate". This statement describes a relation between three variables (country, ingredient, and amount of sugar), which is the maximum most of us can understand. The capacity limit apparent here is obviously not a memory limit - all relevant information can be seen continuously - but a limit on how many relationships we can discern simultaneously.

It has been suggested that working memory capacity can be measured as the capacity C of short-term memory (measured in bits of information), defined as 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 working memory, meaning the duration of memory span. Hence:

C (bit) = Ck(bit/s) × D (s).

Lehrl and Fischer measured speed by reading rate. They claimed that C is closely related to general intelligence. Roberts, Pallier, and Stankov (1996) have shown, however, that C measures little more than reading speed. Moreover, the idea that working memory capacity can be measured in terms of bits has long been discredited by the work of Miller (1956)^[10], who demonstrated that working memory capacity depends on the number of chunks, not the number of bits (chunks can vary dramatically in how many bits they carry: a sequence like "1 0 0 1 0 1 1" consists of seven chunks worth seven bits - less than a single word, which is just one chunk).

Training of working memory

Recent studies suggest that working memory can be improved by training (Klingberg et al., 2002). After training, measured brain activity related to working memory increased in the prefrontal cortex, an area that many researchers have associated with working memory functions. Perhaps of greater importance, another study has found a period of working memory training increases a range of cognitive abilities and increases IQ test scores approximately 8%.^{[citation needed]} Consequently, this study supports previous findings suggesting that working memory underlies general intelligence. Improving or augmenting the brain's working memory ability may prove to be a reliable method for increasing a person's IQ.

References

^ Hill, E. L. (2004). Executive dysfunction in autism. Trends Cogn Sci, 8(1), 26-32
^ Levy, F., & Farrow, M. (2001). Working memory in ADHD: prefrontal/parietal connections. Curr Drug Targets, 2(4), 347-352
^ Postle, B. R. (2006). Working memory as an emergent property of the mind and brain. Neuroscience, 139(1), 23-38
^ Constantinidis, C., & Wang, X. J. (2004). A neural circuit basis for spatial working memory. Neuroscientist, 10(6), 553-565.
^ Vogels, T. P., Rajan, K., & Abbott, L. F. (2005). Neural network dynamics. Annu Rev Neurosci, 28, 357-376
^ Fuster, J. M. (1997). The Prefrontal Cortex: Anatomy, physiology, and neuropsychology of the frontal lobe (2 ed.): Lippincott, Williams & Wilkins
^ Baddeley, A.D., Hitch, G.J. (1974). Working Memory, In G.A. Bower (Ed.), Recent advances in learning and motivation (Vol. 8, pp. 47-90), New York: Academic Press
^ Working memory capacity. New York, NY: Psychology Press
^ Oberauer, K. (2002). Access to information in working memory: Exploring the focus of attention. Journal of Experimental Psychology: Learning, Memory, and Cognition, 28, 411-421.
^ ^a ^b Miller, G. A. (1956). The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychological Review, 63, 81-97
^ Hulme, C., Roodenrys, S., Brown, G., & Mercer, R. (1995). The role of long-term memory mechanisms in memory span. British Journal of Psychology, 86, 527-536.
^ Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24, 87-185
^ Oberauer, K., Süß, H.-M., Schulze, R., Wilhelm, O., & Wittmann, W. W. (2000). Working memory capacity - facets of a cognitive ability construct. Personality and individual Differences, 29, 1017-1045.
^ Conway, A. R. A., Kane, M. J., & Engle, R. W. (2003). Working memory capacity and its relation to general intelligence. Trends in Cognitive Sciences, 7, 547-552.
^ Engle, R. W., Tuholski, S. W., Laughlin, J. E., & Conway, A. R. A. (1999). Working memory, short term memory and general fluid intelligence: A latent variable approach. Journal of Experimental Psychology: General, 128, 309-331.
^ Kane, M. J., & Engle, R. W. (2002). The role of prefrontal cortex in working-memory capacity, executive attention, and general fluid intelligence: An individual differences perspective. Psychonomic Bulletin & Review, 9, 637-671.
^ Halford, G. S., Baker, R., McCredden, J. E., & Bain, J. D. (2004). How many variables can humans process? Psychological Science, 16, 70-76.