“That is what makes the idea so promising,” said Richard McNally, a psychologist at Harvard whose lab recently completed a study of 338 people using a simple program accessible on their smartphones. “But there are big questions about how it could work, and how robust the effect really is.”

The smartphone study is only one of the most recent tests of an approach called cognitive bias modification, or C.B.M., that seeks to break some of the brain’s bad habits. The premise, pioneered by Colin MacLeod of the University of Western Australia, is straightforward. Consider people with social anxiety, a kind of extreme shyness that can leave people breathless with dread. Studies have found that many who struggle with such anxiety fixate subconsciously on hostile faces in a crowd of people with mostly relaxed expressions, as if they see only the bad apples in a bushel of mostly good ones.

Modifying that bias — that is, reducing it — can interrupt the cascade of thoughts and feelings that normally follow, short-circuiting anxiety, lab studies suggest. In one commonly used program, for instance, people see two faces on the screen, one with a neutral expression and one looking hostile. The faces are stacked one atop the other, and a split-second later they disappear, and a single letter flashes on the screen, in either the top half or the bottom.

Users push a button to identify the letter, but this is meaningless; the object is to snap the eyes away from the part of the screen that showed the hostile face, conditioning the brain to ignore those bad apples. That’s all there is to it. Repeated practice, the researchers say, may train the eyes to automatically look away, or the frontal areas of the brain to exercise more top-down control.

“It’s a little boring, because it’s repetitive, but you’re only doing it for a few minutes a few times a day,” said Stefanie Block, 26, a University of Michigan graduate student who took part in the Harvard study while living in Boston. “I just did it when commuting to work on the subway; it’s crowded, there isn’t much you can do, it was the perfect time.”