Sunday, November 19, 2006

White eyes, gaze-detection, and sexiness   posted by agnostic @ 11/19/2006 01:55:00 PM

John Hawks comments on a new study suggesting that the reason human beings have white sclerae is to facilitate detection of what another person is looking at (press story here). Though the article is not out yet, the gist is that humans pay more attention to another person's eye movements, while other primates pay more attention to another's head movements in order to infer what the other is looking at. The pupil through which we look is only color-differentiated from the surrounding iris in people with light irises, but since the iris and pupil are concentric, if we could track the iris, we could track the pupil as well. And because our visual system is tuned to pick up on contrasts, especially between figure and background, a contrasting iris-sclera form would be ideal for tracking someone's eye direction. This in turn would have been useful in cooperative and learning situations where individuals need to focus on the same objects so that background information is shared, minimizing the need to spell out assumptions. This is similar to the Gricean Maxims in the linguistic field of pragmatics -- the more we share assumptions and follow cooperative norms, the less longwinded and lawyerly we have to be in communicating.

Some other thoughts, which may or may not be discussed by the authors:

1) Aside from detecting eye direction, a white sclera would also facilitate detecting emotional eye expressions, as the white sclera contrasts with the full range of human skin colors -- especially the darker ones, but even Irish skin isn't that white. When you narrow your eyes in suspicion or incredulity, for instance, darker shapes (the eyelids) overtake whiter shapes (the sclerae). Conversely, when you express surprise, the darker eyelids recede and open up more of the white sclerae. We look at muscular contractions in the brow area as well when detecting suspicion or surprise, of course, but color contrast between the eyelids and sclerae is also informative. The more varied and subtle an organism's emotional range becomes (i.e., more so in humans than other primates), the more crucial this information may become.

2) The study mentions that gaze-detection would be useful during mother-child learning, and John adds tool-making and tool use, but many cooperative behaviors would not be aided by gaze-detection -- namely, those where individuals are physically separated beyond the threshold at which judging another person's gaze based on eye movement becomes unreliable. I couldn't find any study quantifying this threshold, but I think at the range of about 40-50 feet, a moving head with stationary eyes would convey a stronger signal than a stationary head with moving eyes. If you recall any battle scene you've ever seen where two units were separated by such a distance, they usually communicate by jerking their heads or motioning with their hands and/or weapons, and surely combat is a cooperative and learning situation par excellence. Thus, it's really the close-range, intimate cooperative behaviors that are most facilitated by gaze-detection.

3) The news release mentions that "our eyes are more horizontally elongated and disproportionately large for our body size compared to most apes." That makes sense: if you're trying to detect a figure moving across a background, or the frame closing in or opening up by say 25%, these tasks would be easier if the background were larger on an absolute level.

4) It follows from the above three points that larger, whiter eyes would be of greater use to females than males. I tried Google and PubMed for info on sexual dimorphism of eye size and came up with this, though I can't access it. Hormones affect the eye, so there may be dimorphism. Judging from experience, it seems females do have larger eyes, though the magnitude isn't as pronounced as for, say, breast size or height. Dimorphism is slow to evolve, but we're talking about something that likely happened at least before the major human races diverged and sometime after we split from chimpanzees. The primate-human comparison apparently measures the area occupied by the eyes compared to that occupied by the face or body, but that may not be the best way to measure large eyes when the purpose of those large eyes is to make it easy to track the iris' movement. What you'd measure, then, would be the area of the visible iris divided by the area of the entire visible part of the eye. I think it's by this measure that you can tell girls have larger eyes, and that "babyfaces" like Johnny Depp and Pete Doherty do as well.

5) Once eyes become whiter and larger, they could be used to gauge a mate's health since discolorations due to infection will be more apparent against a white background, which would set off a round of sexual selection for more ornamental eyes. Thus does evolution strive to create Penelope Cruz, a dual-mooned beauty if ever there was one.