It's that experience we all have when we just know we're being watched. You get that tingling feeling and look up — sure enough, a pair of eyes is trained on you. How did you know?
"Many mammals can tell when another animal is looking at them, but the human 'gaze-detection system' is particularly good at doing this from a distance," writes MNN's Laura Moss in an article explaining the skill. "We’re able to easily discern where someone is looking."
Studies have shown that particular cells fire in our brain when someone is looking directly at us, and different cells fire when someone is looking just a few degrees to the left or right of us. In other words, we have an uncanny ability to judge where someone else is looking even when we don't think we're paying any attention.
As Ilan Shrira explains in Psychology Today, "One factor goes back to our gaze detection system, which makes us more sensitive to the position of others' eyes than we realize. Another factor can be deduced by asking yourself this: How are human eyes different in their appearance from the eyes of other animals?"
We have a dark pupil surrounded by plenty of white, a contrast that allows us plenty of information to judge where someone is looking.
"We use a simple rule: dark in the middle of the eye = eye contact; dark on the right = looking right; dark on the left = looking left," writes Shrira. Our eyes are made to reveal where we're looking, just as our brains are wired to be particularly accurate at judging where eyes are looking.
Want to know more about this phenomenon? Read "Why can we sense when people are looking at us?"