Well, I remember Richard Feynman came up with an interesting experiment. He found he could not count objects when he read aloud some text at the same time. He had to name the numbers, and it was impossible if he was already engaging his speech.

He thought this was universal, but doing this experiment with friends, he discovered a guy who could count while reading aloud. So when Feynman asked him, how he does this, turned out that the guy instead of "pronouncing" numbers was "seeing" colored numbers in his imagination, so his speech was not involved.

I supposed this experiment can be modified and generalized, and at least to shed some light on this problem.