The experiment is one of several at the University of Manchester that suggest that the meaning of words and concepts is processed by a brain region called the anterior temporal lobe, just in front of the ears.

The findings promise ultimately to improve therapy for a condition known as semantic dementia, a form of fronto-temporal dementia, in which patients lose the ability to recognise and name common objects, and to grasp their significance.

The quadruped duck was the result of a test in which a female patient with semantic dementia was asked to copy an image.

When the original picture of a duck was in front of her, she sketched an excellent copy, showing that she was perfectly capable of seeing correctly and taking in information, but when asked to wait and draw from memory, a very different effect was seen.

With a ten-second delay, she gave the duck a turkey-like head, a mouth and an eyebrow, and started to draw a third leg before remembering that the bird had only two. With a 60-second delay, the duck became a four-legged abstraction with a wild tail, a more pronounced eyebrow and a smile.

Matthew Lambon-Ralph, who conducted the study, said: “The initial copy was very good, but with a delay you have to rely on recalling the meaning of what you have seen. As understanding is distorted, confusion sets in. Birds start to be confused with other animals, and that’s when you get a four-legged duck.”

Scans of the patient’s brain, like those of others with semantic dementia, show a loss of tissue in the anterior temporal lobe on both sides of the brain.

This has surprised scientists, as it was thought that semantic dementia was caused by problems in a region called Wernicker’s area. This could lead to new treatments.

DISEASES IN THE BRAIN

Explore Health News

• Swine flu

Times Recommends

• Oh no, Mr Darling: you have failed
• BBC survives dogfight over Kennel Club expose
• Where the Rosetta belongs can’t be set in stone