They walk among us, apparently well and ordinary, but their world is an adventure full of people, places, and things they know but cannot recognise by sight. They are people with visual agnosia, the product of a damaged brain following a stroke or head injury.

A Cambridge neuropsychologist, Roz McCarthy, is to make short films illuminating the perspective of the visual agnosia sufferer, many of whom cannot recognise the faces of friends and family.

"What they see is the same as what you and I see," said Dr McCarthy yesterday. "What they don't have is the experience you or I have. Part of the idea of the films is to demonstrate that they have a valid and interesting experience in their own right."

A way to convey the strangeness of visual agnosia is to use images of chimeras, or imaginary animals - the babex, a cross between a baboon and an ibex; or the bunnyphant, half rabbit, half elephant.

Dr McCarthy described how one patient, Philip, was unable to tell which were real when shown chimeras and pictures of sheep or cows. All he could tell was they were animals.

"You could show him a dog and he might say that it was quite small, so therefore it was a domestic animal, but he wouldn't know that it was a dog. He's learned about zebras, because one student spent a long time explaining that they were like zebra crossings. At first he thought someone must have spray-painted it."

Philip was unsure about the babex, however. "He'll look at it and say, 'It's fairly big, it's got these tusks at the top, so it might be dangerous. I think it might be one of those things that you shoot in Scotland.'"

Philip, aged 42, received a head injury in a car crash 24 years ago. Yet he has no trouble recognising buildings, and drives around Cambridge.

Other agnosia sufferers can recognise people but not objects. A DIY enthusiast was downcast because his tools looked all the same to him. "He'd say: 'Not another bloody household object!'"

Philip is unable to recognise his partner of 20 years, or their daughter, although he can distinguish their voices and how they move. "He goes home and knows the woman is probably the same one he has always lived with, but is not totally sure until she speaks."

Dr McCarthy, of Cambridge University's experimental psychology department, has been given a £30,000 grant by the Wellcome Trust to work with two film-makers, and hopes the outcome will help her research into visual agnosia, and also reveal how the rest of us recognise familiar things, creatures and people.