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. Age-related decline in working memory can be briefly reversed using low intensity transcranial stimulation to synchronize rhythms in prefrontal and temporal areas. Training 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