Economics of Celebrity Endorsement

I've never understood why companies pay famous celebrities to be in commercials, when there are much cheaper actors available. Do we really care that Usher uses MasterCard? Apparently, we do:

A new study suggests the answer involves superstar-specific happy memories stored in our cerebral cortex. Using brain-scan technology, researchers found those positive emotions get transferred from the personality to the product, producing a more positive impression of the item in question and, presumably, a greater probability of purchasing it.