The experiment is thought to be the first true demonstration of electronic mood control. Those behind it emphasise more trials are needed but hope it could offer a drug-free therapy for millions suffering long-lasting clinical depression.

In the study, carried out in Toronto, Canada, people who had suffered years of untreatable clinical depression had electrodes planted deep in their brains to stimulate one of the areas involved in mood control.

Before the treatment the patients had led the miserable existence typical of the deeply depressed, lacking motivation, refusing to get out of bed for days and often feeling suicidal.

They had not responded to conventional therapies such as drugs, electroconvulsive therapy or psychotherapy. Afterwards, some of the test cases started going to the gym and established new businesses.

One was a woman struggling to cope with her children. She told researchers: “I want to hold my children and actually feel them.” After undergoing the £10,000 operation she is back in control of her life and has become active in her local parent-teacher association.

In the procedure surgeons adopted a treatment previously used for severe cases of Parkinson’s disease by drilling into the skull and inserting electrodes into the brain.

With Parkinson’s disease the aim is to neutralise the brain impulses that cause patients to suffer constant tremors. For patients with clinical depression the surgeons had a different target: the subgenual cingulate region or Cg25. This is located in the frontal lobes and plays a critical role in modulating sadness.

Six people, all severely depressed, volunteered for the electrode implant treatment. Each underwent local anaesthetic before doctors drilled two small holes in their skulls.

Using magnetic resonance imaging to guide them, they inserted two thin electrode-tipped wires into the Cg25 area. The other ends of the wire were threaded under the scalp down to the lower neck area.

Next the patients underwent a general anaesthetic to have a pulse generator implant — the pacemaker — sewn under the skin in their chest. The wires were hooked up to this to provide constant brain stimulation.

The results, to be unveiled this week in the American neuroscience journal Neuron, have been described by those involved as “like a miracle”.

One severely depressed woman in her forties who previously suffered such mental lethargy she would not answer the telephone has started a business dealing in antiques. A former competitive cyclist has got married and returned to the gym to get himself in shape. Another woman, a former veterinary technician, has revived a previous ambition to open a kennel and cattery.

All six volunteers reported acute effects once the current was switched on. These included a sudden brightening of the room, a “disappearance of the void” and a feeling of “connectedness”.

Dr Helen Mayberg, a neurologist now based at the Emory University School of Medicine in Atlanta, Georgia, who led the research, said: “These people weren’t just having a bad day. They were beyond suicidal; they were too apathetic and disengaged to be bothered. They described their state as dead and deader.”

She added: “I see depression as a brain disease not as a chemical inbalance like most psychiatrists. The brain is not a bowl of soup. You cannot just add a chemical and stir. It is a very intricate wiring system. Some circuits were not working for these people. Once we turned on the stimulator the changes were astounding.”

Two out of the six, both men, lapsed back into depression within six months. But the scientists believe that fine-tuning of the implant treatment could eventually cure most cases of severe depression.

Clinical depression affects 20% of Britons at some time in their lives and up to 2.3m have the condition at any one time.

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