Dr Duchaine says that he recently found two families in the US each of whom had seven or eight members who were affected.

Recognising faces must have been a useful evolutionary skill, I said. He agreed but pointed out that it is becoming ever more demanding.Centuries ago when we lived in small communities, you might need to identify 200 faces in a lifetime. Today, in cities and thanks to television, we might see that number in a day. If you can’t recognise them, there is no cure. You would hardly risk brain surgery, after all. But sufferers can work out strategies for coping , Dr Duchaine says. “For instance, you can look for a key facial trait every time you meet someone and memorise it.”

Oh yes, I said, I do that. “Like what?” he asked. Well, I said, I don’t know if it is just me but Robert De Niro and Al Pacino always seem very similar so I have trained myself to look for the wart on De Niro’s right cheek. “I see,” said Dr Duchaine.

Anyhow, I couldn’t be that bad. No, he said. Some people can’t recognise members of their family when they run into them at airports. But I was bad enough, probably in the bottom 16 per cent of the population at face recognition.

And there was one test I was really terrible at. I had to identify celebrity faces shorn of their hair. I knew I had not done well, failing to recognise even the first one, which was of Dr Duchaine, and I had only just met him. I also drew a blank with both Beckhams, Prince William and Einstein. I thought Kate Moss was Natalie Portman and Arnie Schwarzenegger was Jim Carrey. Save for the brain-damaged, Dr Duchaine had never met someone so rubbish at this test. Interesting, I said, given my job. “Which is?” he asked. Celebrity interviewer, I said. And I had interviewed Arnie twice.

Dr Brad Duchaine can be contacted at b.duchaine@ucl.ac.uk ; more information is available at www.faceblind.org

VISUAL TRICKS OF THE MIND

Patients with Capgras syndrome typically believe that they are surrounded by phoneys, robots and impersonators. This eerie sense of alienation appears to result from damage to a pathway in the brain that gives us a sense of familiarity about people and things we know well. Sufferers say the sense of alienation is strongest among family and friends but can extend to pets and places. Some years ago, a Midlands man was so sure that his father had been replaced by a robot that he slashed his throat open to find the wires.

With Anton’s syndrome, sightless people staunchly believe that they are not blind. It seems to happen when two different areas of the brain have been damaged: the one needed for seeing and the one needed for knowing that you are seeing. It occurs suddenly, often after a stroke, and victims of Anton's syndrome may walk around for some time bumping into things until they become convinced that something is wrong.

Lhermitte’s peduncular hallucinosis causes people to experience cartoon-like hallucinations, such as strange-looking chickens and toy dragons jiving around. It was discovered in 1922 by the French neurologist Jean Lhermitte, who performed an autopsy on a 72-year-old woman who had reported a catalogue of strange sights in later life. He discovered the probable cause: a lesion in her midbrain. Other Lhermitte’s patients’ visions include lions jumping into the room through a window, silver and golden fish swimming below the bed, and oriental people walking in a single line down a hallway with their hands clasped in prayer.

JOHN NAISH