[fraboniface]

This could explain why we naturally perceive and talk about an empty gaze or intelligent eyes. We all have a sense that someone's eyes somehow give a clue about their intelligence. Perhaps we're just inconsciously looking at pupil size.

[colordrops]

My experience is it's more about how the eyes are used, e.g. what is being looked at, how focused they are on something, or when they intentionally unfocus their eyes, and the subtle rapid movements that indicate thought. Pupil size may factor in as well though.

[tdullien]

My father had a brain hemorrhage 2 years ago and now dementia. Watching the only video I have of him pre accident, there is a very marked difference in eye movements.