Sure, individuals need to practice self control but there was an interesting feature article in the May-June 2006 issue of American Scientist, "The Allure of Fast-Paced Pictures", which was really more about interesting pictures and the brains natural tendency to focus on them. Blame your opioid receptors. A conclusion from this study would be that the people behind you are simply hard-wired to look at - at least for some period of time - your interesting, colorful, perhaps moving pictures instead of paying attention to the professor. It's hard to look away from the thing darting across the field, a TV or your neighbor's facebook page.