Working memory ability matures late in life, in adolescence or early adulthood, and may be enhanced even in adulthood through cognitive training. The mechanisms through which working memory is mediated, and how it is modified so as to mediate such cognitive changes have been a matter of debate in recent years. To address this question, my laboratory has performed a series of experiments recording neuronal activity in adolescence and adulthood, as well as before and after training on working memory tasks. Neural activity comparisons between adolescent and adulthood revealed unchanged representation of visual stimuli, increased activation during working memory, but decreased representation of distracting stimuli. After task training, more prefrontal neurons were activated by the stimuli, and increased activity was present during working memory maintenance. These results reveal the nature of changes in neural activity that underlie cognitive enhancement in development and as a result of task training.