“We are working with Fox on a viewer study that involves watching fans watch the game, including their biometrics,” said Amanda Herald, vice president of media strategy and business development at the N.F.L. She noted that networks have been using the double-box format in 30-second increments this season.

When the six-second ads run, “we’ll look at whether they’re more engaged at that moment — for example, heart rate, skin response,” she said. “We’ll also then ask those fans after the game about their sentiment toward the broadcast. Did it feel more commercialized than typical or less because it had one less ad break?”

The effort shows the willingness of both broadcasters and sports leagues to experiment with their long-held formulas for ads as they grapple with cord-cutters and viewers in the internet era, who are increasingly impatient with frequent commercial breaks. Roger Goodell, the N.F.L.’s commissioner, said in March that the league was taking several steps to improve the fan viewing experience, including working with broadcasters to “avoid untimely breaks in the action.”

“We know how annoying it is when we come back from a commercial break, kick off, and then cut to a commercial again,” Mr. Goodell said in an open letter to fans. “I hate that too. Our goal is to eliminate it.”

Spending on commercials during N.F.L. regular-season games reached $4 billion last year, according to Kantar Media. Fox Networks Group has been able to charge brands roughly as much for six-second ads as it does for 15-second ads, people familiar with the negotiations told The New York Times earlier this year.