Weeks before depression patients show any visible benefits from taking medication, their brain waves change -- and this could be picked up by the scan.

The method could save patients from wasting time taking drugs that are expensive and have potential side-effects, said the researchers at the University of California, Los Angeles. "This is the first study to detect specific changes in brain wave activity that precede the clinical changes in a way that can usefully predict response," said team leader Dr Ian Cook, a psychiatrist.

"Up to 40 per cent of depressed patients do not respond to the first medication they try.

"Since it takes several weeks for an effective treatment to produce clear improvement, doctors often wait six to 12 weeks to decide that a particular medication just isn't right for that patient and move on to another treatment."

Studies have also shown that patients may respond to placebos -- dummy pills -- in much the same way they respond to anti-depressants. And research suggests that cognitive therapy -- talking with a psychiatrist instead of taking pills -- may work as well, or better, than drugs.

Dr Cook said his team's methods, using electroencephalograms or EEGs, might shed light on these issues. Different people might have different forms of depression that responded to different treatments, he said.

"We were looking at the prefrontal cortex -- the part right behind the forehead," Cook said.

"It's an area involved in judgment, motivation -- many of the things that make us human."

The EEGs, done simply by pasting an electrode to the patient's scalp with gel and measuring brain activity, showed changes soon after patients took medications.

"We were seeing EEG changes at 48 hours to one week and people didn't show clear clinical differences until about four weeks into the study," Dr Cook said.

"We are continuing to follow them and see what the changes are like at three months, six months, a year."

His team studied 51 volunteers, in groups that got either fluoxetine, made by Eli Lilly and Co under brand name Prozac, venlafaxine, made by Wyeth under brand name Effexor, or a placebo.

An estimated 20 million Americans suffer from depression, which can lead to suicide.

- The research team used electroencephalograms (EEGs) to predict if certain anti-depressant drugs would have demonstrable effects on patients

- The team studied brainwave changes in the prefrontal cortex, the area of the brain involved in judgment, motivation and many of the things "that make us human"