that the contents of short-term or working memory decay over time, unless decay is prevented by rehearsal, goes back to the early days of experimental research on short-term memory. it is also an important assumption in the multi-component theory of working memory. the most elaborate decay-based theory of working memory to date is the "time-based resource sharing model". this theory assumes that representations in working memory decay unless they are refreshed. refreshing them requires an attentional mechanism that is also needed for any concurrent processing task. when there are small time intervals in which the processing task does not require attention, this time can be used to refresh memory traces. the theory therefore predicts that the amount of forgetting depends on the temporal density of attentional demands of the processing task—this density is called "cognitive load". the cognitive load depends on two variables, the rate at which the processing task requires individual steps to be carried out, and the duration of each step. for example, if the processing task consists of adding digits, then having to add another digit every half-second places a higher cognitive load on the system than having to add another digit every two seconds. in a series of experiments, barrouillet and colleagues have shown that memory for lists of letters depends neither on the number of processing steps nor the total time of processing but on cognitive load. resource theories resource theories assume that the capacity of working memory is a limited resource that must be shared between all representations that need to be maintained in working memory simultaneously. some resource theorists also assume that maintenance and concurrent processing share the same resource; this can explain why maintenance is typically impaired by a concurrent processing demand. resource theories have been very successful in explaining data from tests of working memory for simple visual features, such as colors or orientations of bars. an ongoing debate is whether the resource is a continuous quantity that can be subdivided among any number of items in working memory, or whether it consists of a small number of discrete "slots", each of which can be assigned to one memory item, so that only a limited number of about 3 items can be maintained in working memory at all. interference theories several forms of interference have been discussed by theorists. one of the oldest ideas is that new items simply replace older ones in working memory. another form of interference is retrieval competition. for example, when the task is to remember a list of 7 words in their order, we need to start recall with the first word. while trying to retrieve the first word, the second word, which is represented in proximity, is accidentally retrieved as well, and the two compete for being recalled. errors in serial recall tasks are often confusions of neighboring items on a memory list (so-called transpositions), showing that retrieval competition plays a role in limiting our ability to recall lists in order, and probably also in other working memory tasks. a third form of interference is the distortion of representations by superposition: when multiple representations are added on top of each other, each of them is blurred by the presence of all the others. a fourth form of interference assumed by some authors is feature overwriting. the idea is that each word, digit, or other item in working memory is represented as a bundle of features, and when two items share some features, one of them steals the features from the other. the more items are held in working memory, and the more their features overlap, the more each of them will be degraded by the loss of some features. limitations none of these hypotheses can explain the experimental data entirely. the resource hypothesis, for example, was meant to explain the trade-off between maintenance and processing: the more information must be maintained in working memory, the slower and more error prone concurrent processes become, and with a higher demand on concurrent processing memory suffers. this trade-off has been investigated by tasks like the reading-span task described above. it has been found that the amount of trade-off depends on the similarity of the information to be remembered and the information to be processed. for example, remembering numbers while processing spatial information, or remembering spatial information while processing numbers, impair each other much less than when material of the same kind must be remembered and processed. also, remembering words and processing digits, or remembering digits and processing words, is easier than remembering and processing materials of the same category. these findings are also difficult to explain for the decay hypothesis, because decay of memory representations should depend only on how long the processing task delays rehearsal or recall, not on the content of the processing task. a further problem for the decay hypothesis comes from experiments in which the recall of a list of letters was delayed, either by instructing participants to recall at a slower pace, or by instructing them to say an irrelevant word once or three times in between recall of each letter. delaying recall had virtually no effect on recall accuracy. the interference theory seems to fare best with explaining why the similarity between memory contents and the contents of concurrent processing tasks affects how much they impair each other. more similar materials are more likely to be confused, leading to retrieval competition. development the capacity of working memory increases gradually over childhood and declines gradually in old age. childhood measures of performance on tests of working memory increase continuously between early childhood and adolescence, while the structure of correlations between different tests remains largely constant. starting with work in the neo-piagetian tradition, theorists have argued that the growth of working-memory capacity is a major driving force of cognitive development. this hypothesis has received substantial empirical support from studies showing that the capacity of working memory is a strong predictor of cognitive abilities in childhood. particularly strong evidence for a role of working memory for development comes from a longitudinal study showing that working-memory capacity at one age predicts reasoning ability at a later age. studies in the neo-piagetian tradition have added to this picture by analyzing the complexity of cognitive tasks in terms of the number of items or relations that have to be considered simultaneously for a solution. across a broad range of tasks, children manage task versions of the same level of complexity at about the same age, consistent with the view that working memory capacity limits the complexity they can handle at a given age. an experiment has correlated that a decrease of complexity regarding capacity limits are articulated from the implications that research concerning language processes, have an effect on the capacity that the children who have developed language disorders, have performed lower then their age-matched peers. a correlation between memory storage deficits can be viewed as a contribution due to these language disorders or causing the language disorder, but hasn't fully suggested a deficit in being able to rehearse information. although neuroscience studies support the notion that children rely on prefrontal cortex for performing various working memory tasks, an fmri meta-analysis on children compared to adults performing the n back task revealed lack of consistent prefrontal cortex activation in children, while posterior regions including the insular cortex and cerebellum remain intact. aging working memory is among the cognitive functions most sensitive to decline in old age. several explanations for this decline have been offered. one is the processing speed theory of cognitive aging by tim salthouse. drawing on the finding that cognitive processes generally slow as people grow older, salthouse argues that slower processing leaves more time for working memory content to decay, thus reducing effective capacity. however, the decline of working memory capacity cannot be entirely attributed to slowing because capacity declines more in old age than speed. another proposal is the inhibition hypothesis advanced by lynn hasher and rose zacks. this theory assumes a general deficit in old age in the ability to inhibit irrelevant information. thus, working memory should tend to be cluttered with irrelevant content that reduces effective capacity for relevant content. the assumption of an inhibition deficit in old age has received much empirical support but, so far, it is not clear whether the decline in inhibitory ability fully explains the decline of working memory capacity. an explanation on the neural level of the decline of working memory and other cognitive functions in old age has been proposed by west. she argues that working memory depends to a large degree on the prefrontal cortex, which deteriorates more than other brain regions as we grow old. the prefrontal cortex hemodynamics also play an important role in the impairment of working memory through a prevalence of sleeping disorders that many older adults face but it is not the only region that is influenced since other brain regions have demonstrated an output of influence within neuroimaging studies. within the studies of fmri, a connection between sleep deprivation was observed through a reduction of performance on the prefrontal cortex and a overall decrease in working memory performance. age-related decline in working memory can be briefly reversed using low intensity transcranial stimulation to synchronize rhythms in prefrontal and temporal areas. training further information: working memory training and neurobiological effects of physical exercise § cognitive control and memory some studies in the effects of training on working memory, including the first by torkel klingberg, suggest that working memory in those with adhd can improve by training. this study found that a period of working memory training increases a range of cognitive abilities and increases iq test scores. another study by the same group has shown that, 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. one study has shown that working memory training increases the density of prefrontal and parietal dopamine receptors (specifically, drd1) in test subjects. however, subsequent experiments with the same training program have shown mixed results, with some successfully replicating, and others failing to replicate the beneficial effects of training on cognitive performance. in another influential study, training with a working memory task (the dual n-back task) improved performance on a fluid intelligence test in healthy young adults. the improvement of fluid intelligence by training with the n-back task was replicated in 2010, but two studies published in 2012 failed to reproduce the effect. the combined evidence from about 30 experimental studies on the effectiveness of working-memory training has been evaluated by several meta-analyses. tr the authors of these meta-analyses disagree in their conclusions as to whether or not working-memory training improves intelligence. yet these meta-analyses agree that, the more distant the outcome measure, the weaker is the causal link–training working memory almost always yields increases in working memory, often in attention, and sometimes in academic performance, but it is still an outstanding question what exact circumstances differs between cases of successful and unsuccessful transfer of effects. in the brain neural mechanisms of maintaining information the first insights into the neuronal and neurotransmitter basis of working memory came from animal research. the work of jacobsen and fulton in the 1930s first showed that lesions to the pfc impaired spatial working memory performance in monkeys. the later work of joaquin fuster recorded the electrical activity of neurons in the pfc of monkeys while they were doing a delayed matching task. in that task, the monkey sees how the experimenter places a bit of food under one of two identical-looking cups. a shutter is then lowered for a variable delay period, screening off the cups from the monkey's view. after the delay, the shutter opens and the monkey is allowed to retrieve the food from under the cups. successful retrieval in the first attempt – something the animal can achieve after some training on the task – requires holding the location of the food in memory over the delay period. fuster found neurons in the pfc that fired mostly during the delay period, suggesting that they were involved in representing the food location while it was invisible. later research has shown similar delay-active neurons also in the posterior parietal cortex, the thalamus, the caudate, and the globus pallidus. the work