Prosopagnosia

Here is a bit from a post on Prosopagnosia by Vaughan Bell at the Mind Hacks blog:

The condition is often called 'face blindness' but the discussion gives a great illustration of why the label is so inaccurate because Chuck Close is famous for his detailed and evocative portraits of people's faces.

At this point, it's worth saying that there are various forms of prosopagnosia, an acquired version which people get after brain damage, and an inherited form, which Oliver Sacks and Chuck Close have.

You can see Close's portraits online but you really need to see them in real life to experience their impact because they are typically huge (2-3 metres high) and incredibly detailed.

This shows that prosopagnosia is clearly not 'face blindness' - people with the condition can see faces fine - what they can't do is distinguish people by their facial features. Faces just seem all the same - in the same way that you or I might have trouble distinguishing sheep by their faces.

We know a significant part of the difficulty is making sense of the structure of faces rather than their details. Statistically, human faces are very similar, and we have developed a way of perceiving faces that includes their overall layout.

You can demonstrate this process in action by simply by turning faces upside-down and showing that our ability to pick out differences is suddenly markedly reduced.

The Thatcher effect is probably the most striking example of this where changes to the eye and mouth seem hideous when the face is the right way up but when inverted we struggle to notice them.

This is because upright faces engage our perception of face structure into which the details are integrated. With upside-down faces we're left having to do piecemeal feature-by-feature comparisons like a newspaper 'spot the difference' competition.

Go read the whole post and follow the link to the NPR's Radiolab blog site where there is a dicussion about prosopagnosia with Oliver Sacks and Chuck Close.

Update 6sep2010: You can listen to an interview with Oliver Sachs discussing his new book The Mind's Eye at this New Yorker site:

Here Sacks talks with Blake Eskin about the comic and painful situations that arise when one is unable to recognize faces, how he compensates for face blindness, and what public awareness of the condition could do for those who suffer from it.

At 10:48 into this interview Oliver Sacks mentions that 2-3% of the population (i.e. 6-8 million people) suffers from face blindness. That is amazing!

Here's a nice video interview with Oliver Sacks about his book:



Update 7jan2011: Here's a video from the Yonas lab at the University of Minnesota:



And here is an interesting example of a specific person with prosopagnosia. This gives an insight into what it must feel like to be "face blind"...



Update 2011jan18: Here is a bit from an interesting article by Kevin Mitchell in Scientific American which links prosopagnosia with tone deafness & amusia:

Most of us are familiar with people who are tune deaf – these are the people who not only cannot sing in tune but are also unaware of that fact. Individuals with severe forms of this condition, known as amusia, are unable to detect whether particular notes within a melody are out of tune or out of key. Many are also unable to recognise melodies without lyrics or to hold a tune in their heads, even if they have just heard it. These difficulties arise despite normal hearing and also a fairly normal ability to hear the difference between isolated tones. The defect lies in connecting this sensory input with some implicit knowledge of musical structure and contours. Amusia thus falls into a class of conditions known as agnosias, which are characterised by the lack of knowledge of some, often very specific, category of object.

Another, equally curious, example of this class of condition is prosopagnosia – the lack of knowledge of faces. People with severe prosopagnosia may be completely unable to recognise the faces of famous people, friends, loved ones, even their own faces. As with amusia, this reflects a high-level deficit – people with prosopagnosia have normal vision and the ability to distinguish specific facial features, gender, even facial emotions. Both conditions thus seem to reflect the inability to link incoming sensory information (a person’s face or a specific note) with stored, implicit knowledge about that category (the person’s identity or a specific melody or general rules of melodic stucture).

...

This conclusion is strongly supported by the results of functional and structural neuroimaging experiments. In people with congenital amusia, frontal areas are more weakly coupled to posterior auditory areas. These findings thus suggest that the brains of people with amusia can detect discordant notes just fine – the people are simply not aware of it. Their brain knows but their mind does not.

Very similar effects have been observed in neuroimaging experiments of people with prosopagnosia. Normally, the activity of a brain area in response to a specific stimulus (such as a particular face) will decrease with repeated presentations, but will increase again in response to a new example from the same category (a new face). If the brains of prosopagnosics are really unable to discriminate between different faces then the increase in response to a new face should be absent. In fact, the “face areas” of prosopagnosics are still quite sensitive to differences in facial identity. What is different is that these responses are not communicated to areas in the frontal and parietal lobes, where conscious awareness is triggered.

These results are consistent with studies which use galvanic skin responses to detect emotional responses. These studies have shown that even though prosopagnosics may not consciously be able to distinguish a loved one’s face from a stranger’s, they still experience a specific autonomic response when shown a loved one’s face. Again, structural neuroimaging supports the notion that the reason for this failure in communication lies in a structural disconnection – a reduction in the nerve bundles linking these areas.

Go read the whole article. It has a number of interesting tidbits about these conditions and their similarities.