The only problem was that Harry couldn't actually recognize his wife or children by looking at their faces.

His wife, for example, had to identify herself to Harry by wearing conspicuous articles of clothing, like a big red hat.

By taking one quick look at something like... my face... you can tell my age, gender, race, where I'm looking and even my mood.

And if you've met me before, hello, you generally recognize me in a fraction of a second.

We have this amazing ability to recognize faces.

There's even a dedicated area in our brains for it, but we hardly ever stop to think just how amazing this really is.

For all the new faces we see, our brains figure out how different it is from our perception of an average face.

This is an average of all of the faces you've encountered before.

Our brains reduce the facial features, like eyes or lips, to a point and what we remember is just the distance and direction of that point from the centre.

This is called face-space, not like the FaceSpace the Oatmeal invented but a vector based mathematical model of face perception proposed by researchers in the 80s.

It allows us to remember a huge amount of faces, because what we store in our memory is this code, rather than having a photographic memory for faces as a whole.

Because of this tendency to construct an "average face" from all of ones that we see, we're more likely to remember distinctive faces,

like Gollum's huge eyes or Mr. Spock's pointy ears.

As for people we already know, like our friends, family and even celebrities, something interesting happens inside our brain when we see their faces.

Researchers recorded lots of single neurons, which are so incredibly tiny, in patients suffering from epileptic seizures who had electrodes implanted within their skull.

They found that single neurons fired only when subjects were shown pictures of Jennifer Aniston, or Halle Berry, compared other faces or objects they didn't recognize.

Your neurons aren't lightening up because you're jealous of Jen's hair or love her chin, it's simply because you recognize her.

This activation of your neurons happens for your friends and families too, it's quite appropriately called "the grandmother cell".

Some researchers question the existence of such a neuron, but perhaps the neurons firing is our way of retrieving our face space memory.

Sadly there are times when grandmother won't be there to help.

There are some people, just like Harry, who don't recognize any faces, ever.

People who suffer from this condition, face blindness or prosopagnosia, have a warped face space.

Most of the time it's due to brain damage in facial recognition areas.

Harry actually sustained head injuries from a car crash, other people are born with it.

It's like people suffering from this condition just can't join the dots,

some face blind people will only recognize those they see very often, some won't recognize anyone at all.

So things like Facebook are even better at identifying you than some people are.

And facial recognition technology actually uses the face-space principles to work.

This technology identifies you, or me,

by measuring things like the distance between your eyes, the width of your nose, the shape of your cheekbones and the length of your jaw line.

In other words, your faceprint.

While most of our brains do this intuitively,

I hope there's some way for facial recognition technology to help face blind people recognize their wife or children in years to come.

So the next time you just can't remember a person's name, be thankful that you can recognize their face.

And that you don't have to wear this for your significant other to recognize you.

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