Training patients to tune out the signals that cause major depression alters the chemicals in the brain as much as drugs do, a new study has found.

It found that as patients in therapy learn to turn off the thinking that leads them to dwell on negative thoughts and attitudes, the chemical activity in certain parts of the brain decreases as well.

Brain scans may one day become a useful part of treatment for clinically depressed patients, said Dr. Helen Mayberg, a senior scientist at the Rotman Institute, helping doctors to decide in advance what would be the most effective treatment and to monitor its effectiveness.