Special to the Director's Cut Blog By Ryan Nakashima, AP Business Writer LOS ANGELES - Americans who watch the most video online tend to watch less TV, according to The Nielsen Co., a finding that overturns a longstanding belief that people who watch more programming do so over all devices. The ratings agency said Wednesday that starting last fall, it noticed a segment of consumers who were starting to make a trade-off between online video and regular TV. The activity was more pronounced among people ages 18-34. The finding could be troubling to television networks that have been putting shows online in order to reach new audiences. The hope was they wouldn't diminish viewership on television, where they still make most of their advertising revenue. Nielsen polled about 2,600 people who said they watched videos online in the first three months of the year, and divided them into fifths based on how much they watch. The fifth that watched the most video online consumed ...