http://develintel.blogspot.com/2006/...-dopamine.html

The ancient proverb "the eyes are the window to the soul" may in some ways be validated by cognitive neuroscience. Pupil diameter is gaining currency as an index of mental effort ("cognitive workload") as well as arousal. In the most compelling finding from this literature, pupil diameter has been observed to increase with each successive item maintained in memory, up until each subject's working memory capacity - and then to contract incrementally as each item is reported back to the experimenter. Some recent work suggests that spontaneous eye blink rate - how quickly the eyes blink in normal, everyday situations - may also be an index of prefrontal or executive processes.