BERLIN — At a laboratory in Germany, volunteers slide into a donut-shaped MRI machine and perform simple tasks, such as deciding whether to add or subtract two numbers, or choosing which of two buttons to press. They have no inkling that scientists in the next room are trying to read their minds — using a brain scan to figure out their intention before it is turned into action.

In the past, scientists had been able to detect decisions about making physical movements before those movements appeared. But researchers at Berlin’s Bernstein Center for Computational Neuroscience claim they have now, for the first time, identified people’s decisions about how they would later do a mental activity — in this case, adding versus subtracting.

While still in its initial stages, the techniques may eventually have wide-ranging implications for everything from criminal interrogations to airline security checks. And that alarms some ethicists who fear the technology could one day be abused by authorities, marketers, or employers.

Tanja Steinbach, a 21-year-old student in Leipzig who participated in the experiment, found it a bit spooky but wasn’t overly concerned about the civil liberties implications.

“It’s really weird,” she said. “But since I know they’re only able to do this if they have certain machines, I’m not worried that everybody else on the street can read my mind.”

Researchers have long used MRI machines to identify different types of brain activity, and scientists in America have recently developed brain scans designed for lie detection. But outside experts say the work led by Dr. John-Dylan Haynes at the Bernstein Center is groundbreaking.

“The fact that we can determine what intention a person is holding in their mind pushes the level of our understanding of subjective thought to a whole new level,” said Dr. Paul Wolpe, a professor of psychiatry at the University of Pennsylvania.