On Wired Science, DeAnne Musolf writes about the neurological basis for staring at facial deformities or irregularities:
When someone unfamiliar approaches you in the aisle of a grocery store, a glance at his face and its expression helps your brain to sort that person into one of two broad categories: safe or potentially unsafe. The amygdala (the brain area associated with judgment) depends upon the emotion conveyed by the person’s facial features to make that crucial call. Is he happy? Angry? Irritated?
To decide, your eyes sweep over the person’s face, retrieving only parts, mainly just his nose and eyes. Your brain will then try to assemble those pieces into a configuration that you know something about.
When the pieces you supply match nothing in the gallery of known facial expressions, when you encounter a person whose nose, mouth or eyes are distorted in a way you have never encountered before, you instinctively lock on. Your gaze remains riveted, and your brain stays tuned for further information.
Why We Stare, Even When We Don’t Want To