Working memory

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1. A memory "system that holds information in temporary storage during the *planning and execution of a task.

2. The process in which newly "perceived information is combined with "retrieved information during the planning and execution of a task, to form and maintain short-lived "internal representations that guide the behavioural response.

'Working memory' is one of the most important and exciting concepts in modern neuroscience, and rightly so. It refers to a cognitive faculty that is essential for mentation and complex behaviour. This faculty subserves much of our ability to interact with the world in a flexible and intelligent manner, and is essential for thought, planning, and language. For example, reading these lines and combining them into a meaningful message, requires working memory. It is doubtful whether without working memory there would have been a Homer, a Shakespeare, a Mozart, or a Newton, and an audience to appreciate them. A mind without working memory is thus expected to be a rather dull place.

The idea that there should be a cognitive faculty that 'holds things in mind' temporarily, probably occurred long ago to thoughtful individuals while practising their own working memory. The term 'working mem-ory'itself was introduced by Miller et al. (1960) in referring to a postulated quick-access brain space where plans can be retained temporarily while they are being formed, manipulated, and executed. Working memory is hence some type of 'short-term memory' (Baddeley 1986; "phase). However, despite the overlap, 'short-term' and 'working' memory are not the same. Generally speaking, 'short-term memory' is a more comprehensive term, which refers to all internal representations that last for only a short while. It is a universal faculty of nervous systems that can learn. In contrast, 'working memory' combines "attention, short- and long-term memory, retrieval, computations over representations, and planning and decision making, to yield goal-directed short-lived internal representations. It is engaged in on-line processing of data from sensory channels ("percepts) as well as from long-term stores, and maintains the selected representations in a limited "capacity store only until the task is completed. The faculty of working memory is considered to have reached its pinnacle in primates and especially in humans, where it takes years to mature (Luciana and Nelson 1998). Species other than primates display rudimentary capabilities of working memory, e.g. rats while navigating in a "maze or solving olfactory riddles (Olton 1979; Staubli et al. 1995; Mumby 1995).

An influential cognitive "model of working memory considers three types of components: a 'central executive', 'phonological loop', and 'visuospatial sketchpad' (Baddeley and Hitch 1974; Baddeley 1986). The 'central executive' is an attentional control system, the 'phonological loop' deals with speech-based information, and the 'visuospatial sketchpad' with visual and spatial information.1 In recent years, many efforts have been devoted in an attempt to map in the brain the postulated central executive and its subordinate functions. In the process, much has been learned about candidate brain substrates of working memory in primates. In the "monkey, the data are based on circumscribed brain lesions, cellular recordings, and their correlation with performance on "delay tasks that are considered to tax working memory (e.g. Goldman-Rakic 1992). In humans, the data are based on the study of the behaviour of selected brain-damaged patients, as well as on the "functional neuroimaging of patients or healthy volunteers, using tests that tax visuospatial or verbal working memory (e.g. Paulesu et al. 1993; Bechara et al. 1998; E.E. Smith et al. 1998; Ungerleider et al. 1998; Prabhakaran et al. 2000).

In a nutshell, the findings indicate that working memory is subserved by multiple distributed systems, which vary in their identity from one type of working memory task to another. In all cases, however, the frontal lobe plays a central part (Miller et al. 1960; Fuster 2000b). Within the frontal lobe, there is division of labour, which appears more intricate as one increases the resolution of the experimental techniques and the sophistication of their use. The dorsolateral and ventro-lateral prefrontal cortex differ in their contribution to various types of working memory tasks, but the functional determinants of the specialization and subspecialization are not yet clear. These determinants may relate to the type of information processed (e.g. visual vs. verbal); to the role in maintaining internal representation over the task as opposed to selecting information from other brain areas; and to other attributes of the computations performed over multiple types of information (Petrides 1995; Goldman-Rakic 1996; 0 Scalaidhe et al. 1997; Owen 1997; Rushworth et al. 1997; Rowe et al. 2000). Some of the neurobiological findings so far can be construed within the framework of the aforementioned Baddeley-Hitch model (Badde-ley 1998). In addition to the dorsoventral prefrontal dissociations, at least in humans, laterality also counts: the left hemisphere plays a more prominent part in the proposed 'phonological loop', whereas the proposed 'visuospatial sketchpad' is subserved primarily by the right hemisphere (Baddeley 1998; E.E. Smith et al. 1998). The location of the hypothetical 'central executive' (Goldman-Rakic 1996; Roberts et al. 1996; Baddeley 1998; Carpenter et al. 2000) is also not yet established. It is probably embodied in the operation of parallel, distributed polymodal circuits ("homunculus).

To the student of memory, the cellular basis of working memory offers a conceptual challenge (Goldman-Rakic 1995, 1996; "dopamine). Working memory is designed specifically to hold information only transiently. In other memory systems, often the trick is to retain information over an extended period of time, whereas here, it is to prevent the information from lingering too long and interfering with subsequent thought and action. Are the cellular mechanisms

Central executive

Fig. 64 The influential *model of Hitch and Baddeley depicts working memory in the human brain as two 'slave' systems controlled by a limited *capacity central executive. One system, the phonological loop, is specialized for processing language material, whereas the other, the visuospatial sketch pad, is concerned with visuospatial memory. A new version of the model adds a capacity to *bind temporary episodic representations in an 'episodic buffer' (Baddeley 2000; not shown). Attempts to map working memory into the brain, which are facilitated in recent years by the introduction of Afunctional neuroimaging, implicate the prefrontal cortex and its interconnections with other cortices in multiple working memory functions. (After Baddeley 1986.)

Phonological Visuospatial loop sketch pad

Fig. 64 The influential *model of Hitch and Baddeley depicts working memory in the human brain as two 'slave' systems controlled by a limited *capacity central executive. One system, the phonological loop, is specialized for processing language material, whereas the other, the visuospatial sketch pad, is concerned with visuospatial memory. A new version of the model adds a capacity to *bind temporary episodic representations in an 'episodic buffer' (Baddeley 2000; not shown). Attempts to map working memory into the brain, which are facilitated in recent years by the introduction of Afunctional neuroimaging, implicate the prefrontal cortex and its interconnections with other cortices in multiple working memory functions. (After Baddeley 1986.)

of working memory different from those in other memory circuits? For example, do incoming signals in working memory circuits activate "immediate early and "late response genes, culminating in cellular remodelling? If so, what is the role of these changes, provided that working memory is not stored in the long term? Or is some type of memory stored even here in the long term? Are fast molecular 'memory erasures' involved (e.g. see protein phosphatases in "protein kinase)? Or is the gimmick in the unique mode of operation of the circuit? Studies of working memory may eventually affect current conceptual "paradigms concerning the role of cellular change in the retention of internal representations.

Finally, a note on terminology. In discussions of human memory, the concept of working memory is occasionally extended to include those situations in which information is being held temporarily for periods much longer than just the few seconds it takes to execute an ongoing cognitive task. For example, suppose I travel to a scientific meeting out of town; I remember the number of my hotel room as long as I am there, say a day or two, and then get rid of this information as it becomes useless. Is this 'working memory'? By some accounts it is, because it is a piece of temporary information that is usable only for the purpose of a transient task, in this case, getting back to my room. But on a second thought, it is not. Essential to the original concept of working memory is the active use of the memory ("taxonomy) under attentional control throughout the execution of the task.2 Clearly, my central executive, wherever it is, is not busy with my hotel room number throughout the meeting. Therefore, remembering the number of a hotel room number, or the position of a car in a parking lot, is a type of temporary memory, which deserves special attention from dedicated investigators, but is definitely of a different kind than the bona fide working memory.

Selected associations: Attention, Internal representation, Performance, Prospective memory, Retrieval

1A revised version of the model proposes a fourth distinct component, or alternatively a subdivision of the central executive, termed the 'episodic buffer', which *binds on-line and off-line information into transient episodes, i.e. events integrated across space and time (Baddeley 2000).

2Which should remind us that in animal studies as well, 'working memory' should be reserved to those situations in which the subject can be convincingly assumed to *attend the task information over the trial, or over a closely packed series of brief trials. This is unlikely to apply to protocols that last several hours.

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