Did you know that your pupils will dilate when you are doing complex mental computation? Yes, this is according to Daniel Kahneman in his book Thinking, Fast and Slow. Examples of mental computation would be if you had to increment each of the digits of a 4-digit number by 1. Or if one is trying to hold number in one’s head.
Some psychology experiments would measure a subjects pupil dilation as an indication of the amount of thinking the subject is undergoing.