Wall Street researchers have again downgraded the U.S. and global ad spending outlook - and may have uncovered a new relationship, possibly due to digital media, between ad industry growth and economic expansion.

"Interestingly, advertising growth seems to be tracking real growth instead of nominal GDP growth, as it did in the past plus some," Merrill Lynch ad industry analyst Lauren Rich Fine is quoted by MediaPost as writing in a report released Thursday. "[M]edia no longer enjoys the benefit of above average rate inflation, rather the opposite where increased competition & measurement is putting pressure on rates."

GroupM Futures Director Adam Smith has suggested that the change may in part be due to the increasing efficiencies of digital media as marketers begin shifting budgets to lower-priced online inventory.

Not surprisingly, then, Merrill said the internet is "the bright spot" within the overall advertising outlook, also citing "some indications that advertisers are putting some money towards new digital initiatives (i.e., mobile advertising, games, video on demand) rather than just online."

Merrill Lynch now expects overall U.S. ad spend to grow 4.7 percent in 2006, down from 5.1 percent in its previous forecast, and global ad spend is expected to grow only 4.3 percent rather than the previously projected 4.9 percent.