Dr Crawford, of the department of Psychology and the Centre for Ageing Research, Lancaster University, said: "The diagnosis of Alzheimer's disease is currently heavily dependent on the results of a series of lengthy neuropsychological tests.

"However, patients with a dementia often find that these tests are difficult to complete due to a lack of clear understanding and lapse in their attention or motivation.

"Over the last 10 years, researchers in laboratories around the world have been working on an alternative approach based on the brain's control of the movements of the eye as a tool for investigating cognitive abilities, such as attention, cognitive inhibition and memory."

During the study, 18 patients with Alzheimer's disease, 25 patients with Parkinson's disease, 17 healthy young people and 18 healthy older people were asked to follow the movements of light on a computer monitor.

In some instances they were asked to look away from the light.

Detailed eye–tracking measurements showed stark contrasts in results.

Patients with Alzheimer's made errors on the task when they were asked to look away from the light.

They were unable correct those errors, despite being able to respond normally when they were asked to look towards the light.

These uncorrected errors were 10 times more frequent in the Alzheimers' patients than the control groups.

Researchers also measured memory function among those Alzheimer's patients who found the test difficult and were able to show a clear correlation with lower memory function.

Dr Crawford added: "The light tracking test could play a vital role in the diagnosis of Alzheimer's."