I have seen time and time again, both in literature and in conversations with people I know, people describing someone's eyes. What I mean is that I have come across phrases like "Focused gaze", "Vacant gaze", "Fearful eyes", "Eyes lighting up", "Eyes glazed over",etc. What exactly is it that causes one to perceive another's gaze as focused, vacant, etc? Is it how often and how much the pupils contract? Is it the frequency of saccades? Is it something else? What is it about the eyes that causes us to perceive a person's gaze as having a certain meaning?