Pupilometry

"Princeton University psychologist Daniel Kahneman showed several decades ago that pupil size increases in proportion to the difficulty of a task at hand. Calculate nine times 13 and your pupils will dilate slightly. Try 29 times 13 and they will widen further and remain dilated until you reach the answer or stop trying."

http://www.scientificamerican.com/article.cfm?id=eye-opener-why-do-pupils-dialate

More fuel for my belief that I need to get off my ass and write some serious gaze tracking/pupil tracking software for smartphones.