Tri girl is right! Talking will increase your heart rate. When I had fitness testing done with my personal trainer I couldn't talk while doing it, I had to point to a scale because he didn't want my heart rate to be thrown out of wack!

As for concrete evidence of why that is... Can't back it up with anything, Except maybe you'e using more oxygen so youre heart beats faster to accomodate. (breathing rates do increase when you talk)