LOS ANGELES (AP) — The use of Internet-ready devices like smartphones has seriously cut into American's TV-watching time, new Nielsen data shows, potentially undercutting the notion that mobile devices merely serve as "second screens" while people are plopped in front of the set.

Data provided to The Associated Press shows that the number of 18-to-34-year-olds who used a smartphone, tablet or TV-connected device like a streaming box rose 26 percent in May compared to a year earlier, to an average of 8.5 million people per minute.

Those in the same age group who watched TV, listened to radio or used a computer fell 8 percent over the same time, to 16.6 million people.

Nielsen's inaugural "Comparable Metrics" report for the first time introduces data that directly compares the time people spend on various devices.