The number of items a person can hold in short-term memory is strongly correlated with their IQ. But short-term memory has been recently found to vary along another dimension as well: some people remember (‘see’) the items in short-term memory more clearly and precisely than other people. This discovery has lead to the hypothesis that both of these factors should be considered when measuring working memory capacity. But do both these aspects correlate with fluid intelligence?
A new study presented 79 students with screen displays fleetingly showing either four or eight items. After a one-second blank screen, one item was returned and the subject asked whether that object had been in a particular location previously. Their ability to detect large and small changes in the items provided an estimate of how many items the individual could hold in working memory, and how clearly they remembered them. These measures were compared with individuals’ performance on standard measures of fluid intelligence.
Analysis of data found that these two measures of working memory — number and clarity —are completely independent of each other, and that it was the number factor only that correlated with intelligence.
This is not to say that clarity is unimportant! Only that it is not related to intelligence.
 . Quantity, not quality: The relationship between fluid intelligence and working memory capacity. Psychonomic Bulletin & Review [Internet]. 2010 ;17(5):673 - 679. Available from: http://pbr.psychonomic-journals.org/content/17/5/673.abstract