Brain dopamine levels, which are associated with interest, desire, and the expectation of reward, seem to correlate with blink rate. More blinks = more interest. The same is true for enlarged pupils, of course, but this is going to be easier to notice and won't change with the lighting.

Quote: If dopamine receptor 1 activity is enhanced (with D1 agonists), eye blink rate increases, and if it is blocked, eye blink rate decreases.

http://develintel.blogspot.com/2006/12/e...amine.html