NEW YORK (AP) -- Chennel King, a nurse from Norwalk, Conn., went Christmas shopping the other day with a new holiday companion: a budget.Despite a tough economic situation - her husband was laid off almost a year ago - King didn't want to disappoint her five children. So she still went to a mall in suburban New Jersey, but with a limit of $200 per child.Plenty of Americans are having to hold back this year as the lure of flashy ads, tempting bargains and family expectations clashes with the realities of the economy. Experts in consumer behavior say that situation can strain the brain.Scientists say we are to some extent wired for shopping. It seems to tap into circuits that originally spurred our ancestors to go out looking for food, says Brian Knutson, an associate professor of psychology and neuroscience at Stanford University.\We are built to forage