Introduction

Cognitive resources are sometimes limited; as even very early studies of memory and attention have demonstrated. ‘Memory span’ was famously estimated as 7 +/- 2 as the result of experiments that looked at recall for digits or words, but one question was left unanswered by these studies: is the resource that limits short-term memory only related to verbal material, or do other cognitive processes draw on the same resource? In a famous study by Turner and Engle (1989), ‘operation span’ was estimated by alternating between the presentation of words to be remembered and the completion of arithmetic problems, and then testing for word recall (in the order that the words appeared). Turner and Engle found that their measure of operation span was highly correlated with reading skills and comprehension, even though mathematical processing was involved in developing the estimate. Based on these findings, the researchers concluded that our working memory capacity is an ability that is used for multiple cognitive processing, not just verbal memory.