IF PAVLOV HAD BEEN A PAPARAZZO, he'd have had no trouble recognizing the conditioned response that occurs every time 10-year-old Macaulay Culkin faces a lens. Ever since he slapped on the Brut in Home Alone, he's expected to recreate the gesture for anyone who shows up with a camera. Is this good for a kid his age? How will he look at 40 if he keeps beating up on himself? Has anyone spoken with his dentist about the maxillofacial implications of whacking yourself upside the head on a daily basis?