Traditionally, the construct of memory has been divided into a number of different types, defined largely in terms of the length of time over which information is retained or stored. For example, memory is divided into a very brief (on the order of milliseconds) sensory store for visual or acoustic properties of a stimulus; short-term or working memory, in which information can be stored and manipulated for about twenty seconds; and long-term memory, in which information can be stored virtually permanently. Long-term memory can be further divided into storage of procedures or skills, such as how to tie a shoe, and storage of explicit or declarative memories, such as memories of personal events or of general knowledge about the world. The study of the development of each of these systems can aid in understanding the cognitive abilities of both children and adults. Because working memory has important implications for learning and education, it is the focus of this entry.
Defining and Measuring Working Memory
Whereas short-term memory refers to the storage of information over brief delays, working memory refers to the capacity to store information for brief periods and to manipulate it during storage. A prominent model of working memory suggests that it is a multicomponent resource consisting of a limited-capacity central executive or "work space," where processing takes place, and two storage components, one for verbal information and one for spatial information. Working memory underlies a variety of complex cognitive tasks, including reading comprehension and mental arithmetic. For example, mentally adding the numbers 12 and 49 requires that both numbers be held in mind as the operation of addition is performed. Because conscious manipulation of information depends on working memory, one must examine its development in order to understand the abilities of different aged children to comprehend, learn, and remember the information taught to them.
Unlike the capacity for long-term memory, which is considered to be virtually unlimited, the capacity for working memory is limited to a few items. Indeed, the increase in the number of items that can be stored and manipulated at a time (referred to as the working memory "span") is a major source of age-related change in working memory. Moreover, at any given age, there are differences among individuals in their working memory spans. Measures of working memory span thus are integral to the study of working memory. Methods of assessing working memory include the reading span task, the A-not-B task, and the imitation task.
The reading span task. A classic measure of working memory in adults is the reading span task. Reading span is assessed by having adults read a series of sentences and then recall the final word of each of the sentences in the order that they read them. The reading span task requires both the storage and manipulation of information: The reader must store the last word of each sentence while reading subsequent words and sentences. Measures have also been developed to assess working memory throughout childhood, and these measures reveal systematic increases in working memory capacity across age.
The A-not-B task. In the second half of the first year of life, working memory most frequently is assessed by the A-not-B task. In the A-not-B task, a small toy is hidden in one of two identical wells (Well A) in full view of the infant. After a brief delay, the infant is allowed to reach into Well A to find the toy. Following several "A" trials, the toy is hidden in the second well (Well B). Even though the infants watch as the toy is hidden in Well B, they often reach to Well A again, making the "A-not-B error." Overcoming the A-not-B error, and thus, successfully searching in Well B, requires that infants (1) remember where they saw the toy hidden (requiring storage of information) and (2) inhibit the learned tendency to reach to Well A (requiring processing of information). As working memory ability increases, infants are able to withstand longer delays without making the A-not-B error. The delay that infants are able to tolerate without making the error increases about two seconds per month between the ages of seven to twelve months.
The imitation task. In the second year of life, working memory can be assessed using imitation. In a standard imitation task designed to assess short-term or long-term memory, props are used to produce a sequence of actions (e.g., making a rattle by putting a ball into a nesting cup [step 1], covering it with another cup [step 2], and shaking the cups to make a rattle [step 3]). The child then is allowed to imitate the sequence either immediately (as a measure of short-term memory) or after a delay (as a measure of long-term memory). To assess working memory, the steps of several sequences are presented in interleaved order. That is, rather than the steps of a single event in sequence (A-1, A-2, A-3, with the alphabetic character referring to the sequence and the number referring to a step in the sequence), the child sees, for example, A-1, B-1, C-1, A-2, B-2, C-2, A-3, B-3, C-3. The child is then provided with the materials for each of the sequences in turn (e.g., all of the materials for sequence A) and is encouraged to produce the sequences. The interleaving of the sequences during presentation requires that the child not only store the information for each individual step but also attend to subsequent steps and integrate the steps into their respective sequences. Researchers have used the imitation task with seventeen- and twenty-month-old children, finding increases in performance, and therefore in working memory, with age.
Tasks for assessing older children. Working memory may be assessed in older children with an adaptation of the reading span task and with a similar task using numbers. Both tasks indicate increases in working memory across the age range of seven to thirteen years. In addition, children with reading disabilities perform at lower levels on both tasks than do their normal age-mates, and children with arithmetic disabilities have trouble with the number task. Thus, working memory plays an important role in the development of reading and number skills during middle childhood. Adult levels of performance on working memory tasks are reached by the high school years.
Factors Affecting Developmental Changes in Working Memory
Developmental changes in working memory may be due to several factors, including brain maturation, increases in the speed of information processing, increases in knowledge, better use of strategies, and more effective management of attention. For example, the processes involved in working memory are largely dependent on the prefrontal cortex of the brain. The prefrontal cortex matures late relative to other brain regions, such as those involved in sensory and motor processes, and does not reach full maturity until adolescence or even early adulthood. Thus, the time courses of development of the functions of working memory and of the brain regions thought to support them are closely linked. Brain maturation also involves a process called myelination, in which a fatty substance surrounds the nerve cells and aids in the conduction of brain impulses. Myelination may increase the speed of processing, thereby increasing working memory abilities as children mature: Faster processing allows for the storage of more information before it decays from working memory.
Other factors that may affect the development of working memory include increased knowledge, strategy use, and management of the focus of attention. Breadth of knowledge affects working memory to the extent that new information can be linked to existing knowledge. For example, it is easier to store nine letters that form three words that are already stored in long-term memory (e.g., p-e-n, d-o-g, ha-t) than to store a list of nine random letters in working memory (e.g., p-o-h-e-d-t-n-g-a). The learning of and increased efficiency in the use of strategies also aids working memory. For example, as children reach the late grade-school years, they begin to spontaneously use rehearsal (the strategy of repeating the information mentally) when they attempt to remember something new. Working memory also develops with age as children gain increasing control over the focus of their attention. This permits them to attend to more information, switch the focus of attention as needed, and inhibit attention to irrelevant information. All three of these factors–increased knowledge, strategy use, and management of attention–likely play a role in the development of working memory throughout childhood.
Working memory involves the conscious storage and manipulation of information that is integral to the performance of complex cognitive tasks. It is clear that working memory develops throughout childhood, as children are able to hold increasingly more information "online" even as they perform a greater number of mental manipulations on the information. Because working memory underlies so much of mental functioning, it is important to understand its development, as well as the sources and implications of individual differences in it.
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PATRICIA J. BAUER
REBECCA M. STARR