Jenny seems relieved to be talking to someone about her problem and I am absorbed by her story. “I witnessed a murder,” she tells me. Responding to a cry from across the street, she looked up to see a man striking a woman. She didn't see the knife; the blade was already embedded in the woman's body. The man calmly walked away. It's more than 15 years since she witnessed the attack, but Jenny's recollection is precise. At the time, her evidence was dismissed as useless. Intelligent, articulate and keen about detail, she might have been the perfect witness but for a twist of fate: Jenny is face blind.

People with face blindness (prosopagnosia) have difficulty discriminating one face from another and fail to recognise even close family members. They sometimes fail to recognise themselves. Other aspects of face perception may be unaffected. Jenny, for example, has no difficulty reading facial expressions of emotion and feels that she's rather good at intuiting other people's states of mind.

Prosopagnosia was once considered a rare neurological disorder caused by injury to the right side of the brain, but a German study estimates that the incidence of face blindness could be as high as 2.5 per cent of the population. When there are so many other clues to a person's identity - voice, hairstyle, clothes - it's possible to get by most of the time without having to rely on the face.

Jenny homes in on specific features to identify people: “their hair, the way they walk”. But she is easily thrown and, to her embarrassment, is liable to “cold shoulder” colleagues. Context is crucial. Remarkably, her prosopagnosia has led Jenny to create two separate identities for the same person. When her husband visited her at work she was surprised to find he knew one of the secretaries. It turned out the woman was a next-door neighbour.

Perhaps my own face perception skills are not so sharp. I made a faux pas at a social gathering recently when, mistaking her for someone else, I went over and warmly embraced my GP, a woman I'd met just the once for a consultation five years ago. She didn't seem to mind, although quite likely thinks I'm mad. But maybe that was the opposite of prosopagnosia: seeing familiarity in the unfamiliar. Such “overrecognition” can be a rare consequence of brain damage. I remember assessing a man who had been knocked unconscious in a motorcycle accident. When he came round, every face in the crowd that had gathered seemed familiar. Months later he was still greeting strangers in the street as old friends.

Prosopagnosia may shed light on fundamental questions of brain function. It seems to indicate that the brain is highly modular, with separate systems dedicated to specific tasks. The other view is that perception works in a more general-purpose fashion. Can I help Jenny? Not much; there is no effective treatment. But she can help me and others to understand the workings of the brain.

Paul Broks is Senior Clinical Lecturer in Psychology at the University of Plymouth

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