Tim Hughes once caught a plane from New York to LA. Sitting next to him was a woman, a striking brunette with big hair.

Mr Hughes helped her with her bags and then sat back to enjoy the attention of the unusually solicitous airline staff. Upon landing he wished her an enjoyable stay.

"I live here," was her steely-eyed reply.

"It was only two weeks later, after I'd seen a magazine article about her, that I realised that the person I'd been sitting next to was Elizabeth Taylor," recalls Mr Hughes.

Tim Hughes first realised he had prosopagnosia in grade six at school. ( Supplied )

The former IT professional has a condition called prosopagnosia, sometimes known as "face blindness".

But people with prosopagnosia aren't actually blind; they know when they see a face, but their brain can't process who it belongs to.

An inability to tell the difference between two similar faces does not necessarily mean that a person has prosopagnosia, and difficulty perceiving differences is not the same as difficulty with remembering faces, though the result can be the same.

"One of the things that we think people [without prosopagnosia] do when they look at faces is to see the face more as a whole, rather than the individual parts that make up the face," says Romina Palermo, associate professor of psychology at the University of Western Australia.

"We know that people who see faces more as wholes tend to be better at recognising faces. We also know that people who have prosopagnosia don't tend to look at a face as a whole as much, they tend to see the parts more often."

What causes it?

It's much harder to recognise a face upside down, which may give you some idea of what it's like to have prosopagnosia. ( Pixabay.com, CC-0 )

Brain injury, most often a stroke can cause prosopagnosia, but there is also a congenital, or developmental, version in which certain brain mechanisms fail to develop properly, possibly due to genetics.

The neurological explanation for prosopagnosia is complex: to recognise a face, the brain relies on a neural network of at least three core regions in the occipital and temporal lobes of the right and left brain hemispheres which supply different aspects of face processing.

"There are a number of processes, and our understanding is sketchy," says Brad Duchaine, professor in psychological and brain sciences at New Hampshire's Dartmouth College.

"In our posterior lobes we have a fairly simple, image-based representation of the face that you're seeing — which is exactly what you're seeing — you're encoding the values of the features of that face.

"As you move to more anterior areas in the brain, those areas are giving what's called a 'view independent' manner, where you're extracting a representation of just who a person is. It's not tied to the particular image of the face that you're seeing at that moment."

The fear of failing to recognise a loved one

As for Mr Hughes, he first suspected something wasn't right back in Year Six.

"There were two similar looking blonde-haired boys. I never could tell them apart, even though the other kids could," he says.

Since then, Mr Hughes' inability to recognise faces has put him in some awkward situations, especially once he started dating.

People with prosopagnosia usually develop coping strategies to give them clues to the identity of the person they're interacting with. ( Pixabay.com, CC-0 )

"I knew my girlfriend would be getting on the same bus, and would be wearing school uniform… but I was petrified I wouldn't be able to tell who she was. Luckily, she recognised me," he says.

Things didn't work out so well another time: "One time I went to my girlfriend's house to pick her up… but after a short while, the woman I was talking to said, 'I think you want my sister.'"

Many people with prosopagnosia don't have as much trouble recognising family members, but unfamiliar situations can make things difficult.

"One time at the beach I thought I'd misplaced my son Stuart," says Mr Hughes. "There was a boy standing next to me who I thought was one of Stuart's friends, so I asked him if he knew where Stuart was. He replied, 'Yes, I'm Stuart.'"

How to develop coping strategies

People with prosopagnosia usually develop coping strategies to give them clues to the identity of the person they're interacting with.

They use extra layers of information such as gait, voice, eye colour, clothing, or hairstyle. For people with prosopagnosia, a new hairdo can be quite confusing.

Can you spot the Elvis? ( Getty Images: Eric Chu/AFP )

Mr Hughes' coping mechanisms include examining people "top to toe" and having a friend point out who's who, or who he might expect to see at an event.

He also lets people know he has the condition and that he might not recognise them next time he sees them.

He's not a fan of the term "face blindness" because it's not accurate. He prefers to say that he has "facial recognition issues" — once he puts facial features together, he can identify who a person is.

Insights into how the brain works

The Australian Prosopagnosia Register was begun about 10 years ago, and people can sign up for information, receive updates on research and do tests to help figure out what level of prosopagnosia they have.

There have also been studies on whether people can be trained to improve their facial recognition abilities.

They can, but it's intensive, involving tasks like looking at photos and trying to identify subtle differences in people's eyes or the distance between the eyes.

Is upside down Trump as recognisable as right side up Trump? ( Getty Images: Noam Galai )

Research into prosopagnosia also opens a window on to how the human brain processes faces. Facial recognition is a specialised task utilising different areas in the brain, and information about how this happens could assist with other research.

"It's useful to know how everybody processes faces," says Professor Romina Palermo.

"When some people are unable to do it, there can be insights into how the brain works.

"Facial recognition is a specialised task utilising a distributed network of areas in the brain, and information about how this happens can assist with other areas of cognition."