Michael hasn’t a clue which student is facing him. But he has a way of finding out. “Now,” he says, “I’ve forgotten what your surname is...” He judges, correctly, that it is less insulting to own up to forgetting a student’s surname than to reveal that he just can’t remember their face.

Michael (not his real name) suffers from prosopagnosia, commonly known as face-blindness. He can recognise hair, clothes, shape, gait - but not faces.

You might think that teaching is an odd profession to find him in. But, says Brad Duchaine, a cognitive neuroscientist at University College London (UCL) and one of the world’s leading experts on the condition, you’d be surprised how many face-blind people go into teaching. In fact, you’d be surprised by how many people are face-blind to some degree - up to 2.5 per cent, according to recent research.

Maybe you are a bit face-blind yourself. You might pride yourself on your ability to recognise people but you may just be relying on other visual clues. Do you, for instance, drive your nearest and dearest mad by asking, when the baddy appears on screen for the second or third time, “Who’s that?” That’s one of the signs.

Many face-blind people go through life quite happily without realising they have a problem - or rather, that they have more of a problem than others.

Until they are tested, most sufferers do not realise they have trouble recognising faces.

“But teachers are more likely to notice than others,” Dr Duchaine points out, “because they’re working with people all the time. If you’re sitting looking at a computer, it doesn’t matter. Many people notice it when they first go to college or start a deep relationship with somebody - they notice the other person is much better at it.” The disorder was originally thought to be caused only by brain injury. It was first identified in 1947 by a German neurologist when he examined a 24-year-old man who had lost his ability to recognise his friends and family, and even his own reflection, after a bullet wound to the head.

Most cases are now thought to be “developmental” - either present from birth or occurring very early in life. Studies suggest that a baby being unable to see with its left eye during the first few months of life is a major risk factor. This raises the possibility that there is a developmental “window” for face recognition.

Working out the causes of face-blindness is complicated by the fact that each sufferer has a slightly different problem. Some have difficulty only with faces, others with recognising objects or animals as well. Some cannot recognise faces, but have no problem telling whether a face is attractive or what emotion it conveys.

Dr Duchaine has been working on prosopagnosia for eight years and says he and his colleagues have still to find answers to important questions. For example, understanding face-blindness could solve key questions about the way the brain works - in particular, whether we have specialised modules for each task, such as face recognition.

“What is the mechanism that is not working on information processing?” he asks. “What is the neural basis for this condition? Does it run in families?” His latest research suggests it does.

He also wants to work out strategies so that people severely affected with facial recognition problems can at least be trained to recognise the five people they most need to know. But most face-blind people have already developed instinctive strategies to get round the problem, such as a keen awareness of people’s hair, shape and movement.

How on earth do teachers cope in the classroom? “Seating charts,” says Dr Duchaine succinctly. “But then teachers can get confused when pupils are not where they expect them to be.”

So, teachers who depend on a seating chart are advised not to leave this issue of The TES Magazine lying around in class. Puckish pupils might just decide to make their lives even harder than usual

* In 2006, a web survey of 1,600 people conducted jointly by a team from Harvard University and UCL suggested that up to 2 per cent of people have some degree of face-blindness, while a survey of 700 secondary school pupils by the Institute for Human Genetics in Munster, Germany, found that 2.5 per cent had trouble recognising faces. Readers can find out more and contact Dr Duchaine and his team via the website www.faceblind.orgFor online tests, visit: www.icn.ucl.ac.ukfacetestsmyabilities.php

I didn’t recognise myself when I looked in a mirror

Dawn Lloyd faces severe challenges teaching English to university students in Oman.

While the government there has banned the wearing of veils in schools and universities since incidences of impersonation during exams were discovered, female students wear the hijab and the males wear either caps or turbans.

Dawn, who is extremely face-blind, thus lacks most of the clues to her students’ identity. “The only thing you can see for the girls is their face, and the only thing for the boys is face and neck,” she says.

She deals with it by making notes on all the students and keeping them with her attendance sheets. Her notes may not be complimentary: “Often the only distinguishing features aren’t exactly what you want to draw attention to,”

she remarks, “such as ‘acne’, ‘scar under eye’, ‘chubby’, ‘big nose’ and so on. I’m always careful to keep these notes hidden in other papers.

“I then make a big deal of using their names as often as possible, so I learn them. In a class of 25, I can usually identify all the students by the end of the class.

“Of course, I’ve forgotten or don’t recognise a lot of them by the next class, but they’re all so impressed that I remember any at all that they’re very willing to forgive me for not remembering everyone.”

Then she comes clean. “Two to three weeks into the semester, by which time the students know me, I explain prosopagnosia to them.

“I tell them how, at high school, I looked through a window into another classroom in which I thought I should know everyone, but there was one person I just couldn’t figure out. I puzzled and puzzled over this for a long time until I finally noticed the clothes and realised I was looking in a mirror at myself.

“I grin broadly, and shrug, and tell them my memory is fine - I just don’t recognise faces. I tell them that if I ever see them outside campus, and if I ignore them, it’s not because I don’t like them - I just don’t recognise them. I assure them that they should come and say hello and tell me their name anyway.”

I didn’t know it was Mick Jagger, despite the lips - but I picked out Tony Blair

Jo Livingston remembers vividly when she first realised she had a problem.

Then in her mid-twenties, she was being interviewed for a post as a librarian. The interviewer was a dark-haired man in a dark suit. They chatted for a while, then he left the room. He returned and they resumed their conversation.

Or that’s what she thought. In fact, the first interviewer had been replaced by another dark-haired man in a dark suit. He thought they were starting a fresh conversation; she assumed they were carrying on with one.

The interview finally hit the buffers of mutual incomprehension. She did not get the job.

Fast forward a few years and Jo is a librarian in an inner-London school and is also responsible for occasional teaching of research skills. “That was particularly tough for me because I had to interact with the whole school population, not just an individual class or subject,” she says. “And clues are much more limited with pupils - they wear uniform, have a comparatively small range of hairstyles and no dangly earrings or special glasses.

“Regulars to the library were OK, but casuals doing projects - I hadn’t a clue who they were. I made a lot of mistakes. But the pupils didn’t say anything. They think all adults are pretty weird.”

After taking a PGCE, Jo landed what she says is the ideal job for a face-blind teacher: lecturing on IT in a further education college. Not only are classes smaller (typically 10 to 12 students), making seating plans easier to learn, but: “It’s all backs of heads. It’s easier for me to recognise somebody’s back view.”

Two years ago, an article in The Times about prosopagnosia alerted Jo to the prevalence of her condition. “It’s such a relief,” she says. “You begin to think you’re mad.” She visited Dr Duchaine at UCL and took various tests to assess the extent of her face-blindness.

“On the Harvard face recognition test (where the faces of famous people are shown with hair and other clues removed) I scored 30 per cent - a normal person would score 85 per cent,” she says. “I didn’t get JFK, Marilyn Monroe or Mick Jagger, despite the lips. But I did get Tony Blair and the Queen. Some people don’t.”