While Ellen Milz and her family were watching the Olympics last summer, their TV was watching them.

Ms. Milz, 48, who lives with her husband and three children in Chicago, had agreed to be a panelist for a company called TVision Insights, which monitored her viewing habits — and whether her eyes flicked down to her phone during the commercials, whether she was smiling or frowning — through a device on top of her TV.

“The marketing company said, ‘We’re going to ask you to put this device in your home, connect it to your TV and they’re going to watch you for the Olympics to see how you like it, what sports, your expression, who’s around,’” she said. “And I said, ‘Whatever, I have nothing to hide.’”

Ms. Milz acknowledged that she had initially found the idea odd, but that those qualms had quickly faded.

“It’s out of sight, out of mind,” she said, comparing it to the Nest security cameras in her home. She said she had initially received $60 for participating and an additional $230 after four to six months.