Patients have been implanted with a tiny "brain pacemaker" in an attempt to slow the progress of Alzheimer's disease.
Three patients were fitted with the device, which fires electric pulses into surrounding cells, as part of the ground-breaking trial.
Doctors report that it reduced the speed at which their brain function declined. In two patients, the effects were significant.
Patient groups are warning that the study shows the technique has no major side effects, but many more patients will need to be treated to prove that it works.
Dr Douglas Scharre, who led the research at Ohio State University Wexner Medical Centre, said: "The study was for 18 months, not just a brief window, so it had a prolonged effect on these patients."
Deep brain stimulation (DBS) has already proved highly effective in more than 130,000 patients with Parkinson's disease.
But for the first time, doctors implanted thin electrodes in the frontal lobe of three patients with Alzheimer's. A device similar to a heart pacemaker was then used to stimulate the brain tissue.
Dr Scharre said: "The frontal lobes are responsible for our abilities to solve problems, organise and plan, and utilise good judgements.
"By stimulating this region of the brain, the Alzheimer's subjects' cognitive and daily functional abilities as a whole declined more slowly than patients not being treated with deep brain stimulation."
There are several treatments that aim to increase memory in patients with Alzheimer's, but problem solving and decision-making skills are also affected by the disease.
When LaVonne Moore had the implant in 2014, she was unable to prepare a meal. But now, the 85-year-old can now cook, choose her own clothes and plan a day out.
Her husband Tom said: "Three and a half years later she's doing pretty well."
But Dr Doug Brown of the Alzheimer's Society said: "This study showed that DBS is likely to be safe for people with Alzheimer's disease but as the study only involved three people who showed varying degrees of improvement, it may not work for everyone in practice.
More from Tech
"Given that we haven't had any new treatments for dementia in over a decade it is encouraging to see techniques from other diseases being tested for dementia, but it will need further, more in-depth research before we can draw any firm conclusions."
The study is published in the Journal of Alzheimer's Disease.