(emphasis mine)

Because our ability to maintain our attention also depends on our working memory— “we have to remember what it is we are to concentrate on,” as Torkel Klingberg says — a high cognitive load amplifies the distractedness we experience. When our brain is overtaxed, we find “distractions more distracting.” (Some studies link attention deficit disorder, or ADD, to the overloading of working memory.) Experiments indicate that as we reach the limits of our working memory, it becomes harder to distinguish relevant information from irrelevant information, signal from noise. We become mindless consumers of data. Difficulties in developing an understanding of a subject or a concept appear to be “heavily determined by working memory load,” writes Sweller, and the more complex the material we’re trying to learn, the greater the penalty exacted by an overloaded mind.