Despite decades of research, 99% of clinical trials on new drugs for Alzheimer's disease have failed, meaning there is still no cure. But scientists believe it is not the drugs which do not work, it is the trials.

To test the theory, work is about to begin on the world's biggest and most in-depth study to find the earliest signs of Alzheimer's in people.

Scientists believe that if the drugs were used earlier, before the clinical symptoms of memory loss, they could slow the onset of the disease or even stop it entirely.

Andrew Bomford takes a closer look at the research.

(Photo: Brain scans. Credit: BBC)