There is a story about an absent-minded professor who comes home from his college one afternoon only to remember when he gets there that he and his family had moved the previous week. The trouble is that he cannot recall the address of their new place, so he asks a couple of children playing outside if they happen to know where the family that lived there had moved to.

“It’s 22 Park Street,” one replies. “You go left at the junction, then turn right up the hill and it’s on the right-hand side. You really can’t miss it, Daddy.”

But maybe the professor was not just absent-minded. Maybe he also suffered from prosopagnosia, or face-blindness. The term was invented in 1947 by Joachim Bodamer, a German neurologist, who combined the Greek for face with the medical term for recognition impairment, agnosia, and a handful of scientists have been investigating the syndrome ever since. I sympathise with Professor Nuts. I have had prosopagnosia diagnosed.

It’s embarrassing, more than anything else. I thank complete strangers for kindnesses they have not performed. I misintroduce people I know quite well. In the case of two women writers, both of whom happen to be blonde and to have prominent noses, I frequently begin conversations on the assumption that they are the other one (such conversations always end badly).

More often I blank acquaintances and colleagues when I see them out of context; for example, in the street. To you, I appear rude, stand-offish, solipsistic or, as a colleague says, “a typical man”. Yet once I have satisfied myself as to who you are, I am really quite good at remembering your life stories and children’s names. Put it this way: it makes parties difficult.

The other day I caught the subject being discussed on the regional news by Dr Brad Duchaine, an American neuroscientist working at University College London. He was looking for prosopagnosics to test. A week later I was sitting in his lab in Bloomsbury, undergoing 90 minutes of visual tests on a laptop. In the first few quizzes I was shown 50 pictures, first of houses, then cars and, finally, slightly bizarrely, toy horses. Ten of these in each test would be shown twice and I had to press a key when I recognised a picture I had seen previously.

It really wasn’t that difficult, but then the test was repeated with photographs of bald, jowly men of the Mitchell brothers/Bob Hoskins type (incidentally, I sat next to Hoskins at the theatre the other night and did not recognise him). I tried to note particular characteristics — dimples, enlarged noses — but they all looked much the same to me. I was lost.

“Now, you see, control groups of non- prosopagnosics find all these tests really easy,” Dr Duchaine told me afterwards. “People clean up on them. My wife got only one wrong. I mean, we all make mistakes from time to time, but prosopagnosics do so much more often. They also over-recognise people; greet people they have never met.”

I have, I say, for years complained that all the young male actors in Hollywood tend to look alike. “They do,” he replied, consulting my results, “to you. Of course, there are some faces almost everyone can recognise because they are very distinctive, what we call the Gorbachev effect.”

Dr Duchaine has been researching the syndrome for ten years. At the beginning he thought the problem quite rare, but has found it to be more common. In one of her memoirs, the primatologist Jane Goodall owned up to being face-blind. I told him that the architect Richard Rogers may be another sufferer. He once told me he always stuck by his wife at parties so that she could remind him who everyone was. Since Lord Rogers can tell buildings apart and Goodall a chimp from a bonobo, the point seems to be that we prosopagnosics are not generally visually illiterate (I may be useless at faces, but I enjoy distinguishing typefaces).

There is evidence, Dr Duchaine says, that face recognition involves particularly the brain’s right hemisphere, popularly thought to be the intuitive, holistic side. Prosopagnosia is a result of an impairment to a mechanism that specifically identifies faces, which is why I could tell the cars, houses and horses apart. His hunch is that there is no one cause for it, that, indeed, prosopagnosia may actually be being used as an umbrella term for several distinct conditions. Acquired prosopagnosia, for instance, is caused by head injury or brain illness.

Others have had problems all their lives. The developmental prosopagnosics among them may have had eyesight difficulties as infants when the brain is learning to distinguish faces; even babies whose cataracts have been corrected at three months, when tested 15 years later turn out to have grown into teenagers poor at face recognition.

And then there are the genetic prosopagnosics, whose face-blindness runs in the family.