For instance, a music video for the song "All I Want" was added to the reel because some people might consider it "sweet" while others would see it as "sappy," the researchers explained. One of the clips presented a debate on whether college football should be banned; another featured a discussion about a speech by former President Barack Obama.
The reel also included video from a gay wedding, a presentation by an astronaut on the International Space Station showing what happens when you wring out a washcloth in space, a documentary about a baby sloth sanctuary and highlights from a soccer match, among other things.
While the students watched, the scanner recorded the responses of 80 separate regions of their brains. Then the researchers compared the responses of each student with the responses of every other student.
The 42 students could be paired up in 861 distinct ways. Some of those pairs were friends, and some weren't.
Sure enough, the responses of friend pairs were more alike than the responses of nonfriend pairs. And the more similar their responses, the shorter the distance between them in the social network.
In statistical terms, for each one-unit increase in neural similarity, the odds that two people were friends increased by 47 percent.
Even when the researchers controlled for the similarities of people in each of the 861 pairs -- including features like age, gender and nationality -- the correlation between cognitive response and position in the social network remained.
That correlation was most clearly seen in areas of the brain involved in motivation, learning, attention, language processing and determining the mental states of others, to name a few examples.
"A more specific understanding of precisely which cognitive and emotional processes underlie these effects will likely require complementary follow-up studies," the researchers wrote.
Parkinson and her colleagues also found that the brain responses alone could do a pretty good job of predicting whether two people were friends, mere acquaintances or total strangers.