Age-related mental decline can be traced to a tiny part of the brain's memory centre, according to research published today.

For the first time, scientists have been able to distinguish the impact of normal ageing on the hippocampus, a region of the brain linked to learning and memory, from the damage caused by early stage Alzheimer's disease to the same area.

To differentiate between the changes to brain structure and function caused by Alzheimer's disease and normal ageing, American scientists studied two animals that do not suffer from Alzheimer's - the monkey and the rat.

Brain scans of rhesus monkeys pinpointed a region of the hippocampus called the dentate gyrus, which experienced a significant decline in blood volume with increasing age.

Decreasing blood flow led to reduced activity in that part of the brain. In the rat, the scientists found that a gene in the dentate gyrus associated with learning became less active with age.

Other effects known to occur with the onset of Alzheimer's disease were not excessively present in neighbouring areas of the hippocampus in either the monkey or the rat.

The study in the journal Proceedings of the National Academy of Scientists suggests that the dentate gyrus is the region of the brain most sensitive to ageing.

The scientists, led by Scott Small, the assistant professor of neurology at Columbia University's college of physicians and surgeons, wrote: "By establishing differential patterns of hippocampal dysfunction, these studies confirm that ageing and Alzheimer's disease are indeed distinct entities."