(The original of this article first appeared in the Summer 2012 issue of I,Science, which can be read on www.isciencemag.co.uk)
Have you ever introduced yourself to someone only to find out you’d met them before? How about when someone approaches you and starts talking to you as if you’ve known each other for ages, but you can’t for the love of science figure out who they are? If this happens to you on a daily basis, then perhaps, like me, you suffer from a condition called prosopagnosia. It is caused by a defect in the visual cortex, which makes it impossible for me to recognise faces.
The visual cortex is a part of your brain responsible for the analysis of the stimuli it receives from your eyes. It recognises the shape, colour, texture, movement and placement of an object and labels it accordingly. When something goes wrong in this categorisation process, it is called visual agnosia. Visual agnosia are caused by damage in the visual cortex. How this condition expresses itself is dependent on which part of the cortex is damaged. In the case of prosopagnosia, the damage is in the fusiform face area (FFA), a section of the visual cortex that deals strictly* with the recognition of faces.
Prosopagnosia is also called ‘face blindness’, but this is perhaps a misleading term. Someone with prosopagnosia can see faces just fine. A better way to describe it is that although most of us agree that all the features of the front of a head together constitute what we categorise as a ‘face’, those with prosopagnosia don’t have this category available to them and just see the features as separate entities.
There are other visual agnosias including akinetopsia, which disables the perception of motion. Patients with akinetopsia visualise movement as a series of static pictures.
Then there is environmental agnosia, which makes it impossible for the patient to navigate his or her way to a location, even if they are very familiar with it. There is also a condition called ‘total agnosia’, which is most common with patients who recover from blindness. Because these patients have never processed visual information, their visual cortex is unable to provide them with any of the categories that the sighted rely on. It is possible to recover from total agnosia by training the visual cortex, much like a baby needs to learn to categorise the visual world.
Prosopagnosia is very much a spectrum affliction. Famous neurologist Oliver Sacks, for example, has very severe prosopagnosia and doesn’t even recognise his own face in the mirror. Others are able to recognise some faces, usually close relatives or loved ones.
Some people, like myself, are born with the condition and it is thought to have a strong genetic cause. However, prosopagnosia is mostly reported in people who have suffered brain damage after a stroke or accident. This may be because the loss of an ability is more noticeable than the original absence of it. One benefit of the condition is that I always have a way out of the awkward situation when I fail to recognise someone. And now, so do you.
For more information on prosopagnosia visit http://www.faceblind.org or read Oliver Sack’s book, The Mind’s Eye.
*On October 1st 2012 PNAS published a paper by Isabel Gauthier et al that showed that the FFA of car and plane experts is also used to recognise pictures of cars and planes. This suggests that perhaps the FFA isn’t so specialised for faces after all.