Faces, particularly eyes, naturally attract human attention. One aspect of the biological machine of faciality is eye structure. Compared to other primates, humans have more salient eyes:
the human eye lacks certain pigments found in primate eyes, so the outer fibrous covering, or “sclera,” of our eyeball is white. In contrast, most primates have uniformly brown or dark-hued sclera, making it more difficult to determine the direction they’re looking from their eyes alone. … Humans are also the only primates for whom the outline of the eye and the position of the iris are clearly visible. In addition, our eyes are more horizontally elongated and disproportionately large for our body size compared to most apes.
Experimental evidence indicates that a chimpanzee’s gaze direction responds primarily to a person’s head movements, while human gaze tracks another’s gaze direction. But if the objective is to indicate direction, the advantage of using eye movement rather than head movement isn’t obvious.
Eye contact, however, has more subtle value in communication. Seeing someone’s head move doesn’t mean that she knows that you were looking at her in a situation in which you would track her change in gaze direction. Eye contact generates common knowledge of gaze direction (you both know that you’re looking at each other, you both know that you both know you’re looking at each other, etc.) and common sense of whether a change in gaze direction would be tracked (nervous distancing or needed shift in attention?). Just looking at each other’s head doesn’t work this way, because head orientation doesn’t imply eye orientation in humans and other primates.
Making sense of presence is probably more valuable to humans than to other primates. Across species, a larger neocortex, both in absolute size and relative to total brain volume, is correlated with greater social complexity (pdf link). Relatively salient human eyes, like the relatively large human neocortex (particularly prefrontal cortex), support sense of presence. Direct gaze is a powerful way to produce sense of presence. I relay to you fourth-hand a plausible reported fact: “human infants look at the face and eyes of their caregiver twice as long on average compared with other apes.”
Given the importance of gaze to humans, a video viewer’s ability to discern the whites of the eyes of persons on a video might be a useful measure of video quality with real human relevance. My video of the JDRF Spin to Win has little interest other than the eyes, faces, and expression of the participants. Viewing the video on YouTube, the faces are distorted and whites of the eyes are barely discernable. But viewing the video on Blip.tv, you can see whites of the participants’ eyes much more clearly. The point is not simply that Blip.tv offers better quality video than YouTube. High-density, huge-screen television offers much better video quality than either. People want to see the whites of others’ eyes. That is a human-relevant measure of video quality.