In one of Feynman's books, he discusses and experiment that he did with people keeping time in their heads (count 30 seconds) while simultaneously reading a passage of a book. Some people had absolutely no issue with doing this, but many people found it impossible.

When he asked people how they kept track of how much time had passed, he found that people who pictured a number in their heads were able to keep time and read without issue, while those who "spoke" the time to themselves found it impossible to read.