The same cannot be said for Leonard, 67, who had surgery in 1995. “There was no change at all,” he said. “I still don’t leave the house.”

Both men asked that their last names not be used to protect their privacy.

The great promise of neuroscience at the end of the last century was that it would revolutionize the treatment of psychiatric problems. But the first real application of advanced brain science is not novel at all. It is a precise, sophisticated version of an old and controversial approach: psychosurgery, in which doctors operate directly on the brain.

In the last decade or so, more than 500 people have undergone brain surgery for problems like depression, anxiety, Tourette’s syndrome, even obesity, most as a part of medical studies. The results have been encouraging, and this year, for the first time since frontal lobotomy fell into disrepute in the 1950s, the Food and Drug Administration approved one of the surgical techniques for some cases of O.C.D.

While no more than a few thousand people are impaired enough to meet the strict criteria for the surgery right now, millions more suffering from an array of severe conditions, from depression to obesity, could seek such operations as the techniques become less experimental.

But with that hope comes risk. For all the progress that has been made, some psychiatrists and medical ethicists say, doctors still do not know much about the circuits they are tampering with, and the results are unpredictable: some people improve, others feel little or nothing, and an unlucky few actually get worse. In this country, at least one patient was left unable to feed or care for herself after botched surgery.