(Warning: Parts of this video can be distressing to watch.)

Posted on YouTube in 2009, the “Still Face Experiment” video makes its point bluntly.

“Babies this young are extremely responsive to the emotions and the reactivity and the social interaction that they get from the world around them,” Edward Tronick explains in the video.

To illustrate this, Tronick, a UMass Boston psychology professor and child development expert, has a mother interact with her baby using a playful, happy voice.

The experiment? After a moment, the mom turns away from the baby, turns back, and holds her face still: no smiles, no conversation, just stone-faced eye contact. The baby tries but fails to reestablish the happy connection. The longer the baby struggles to engage the mom, the more distressed the baby becomes.

It’s excruciating to watch. But the video shows the power of warm, engaging, and responsive relationships between babies and adults. It also shows how harmful abuse and neglect can be for infants.

Last year in the Washington Post, Brigid Schulte wrote, “To even begin to understand just how profoundly child neglect — to say nothing of abuse — can shape every aspect of a child’s life, I dare anyone to try and watch the two-minute experiment that researchers call ‘The Still Face.’” Schulte adds: “I couldn’t.”

“What’s really striking about the still face experiment is that the infants don’t stop trying to get the parents’ attention back,” Tronick told the Post. “They’ll go through repeated cycles where they try to elicit attention, fail, turn away, sad and disengaged, then they turn back and try again.”

“When it goes on long enough, you see infants lose postural control and actually collapse in the car seat,” Tronick continued. “Or they’ll start self-soothing behaviors, sucking the back of their hand or their thumbs. Then they really disengage from the parent and don’t look back.”

Tronick told the Post that when he “began doing these experiments in the 1980s, we just didn’t have any idea how powerful the connection with other people was for infants, and how, when you disconnected, how powerfully negative the effect was on the infant.”

Before the video ends, the mom reengages with the baby, and the two return to their upbeat interaction.

Writing in the Post, Schulte compares the baby’s reaction to those of abused and neglected children, noting, “Tronick and others have found that even abused and neglected children, once surrounded by loving support, too, begin to thrive and the brain can rewire in a positive, healthy way.”

In addition: “The report found that one of the biggest risk factors for child abuse and neglect is if the parent him or herself was abused or neglected. So Tronick and others are working to train professionals and educate and treat parents in an effort to break the cycle. And, one hopes, put an end to the wrenching effects of The Still Face.”