‘Face-blindness’ — prosopagnosia — is a condition I find fascinating, perhaps because I myself have a touch of it (it’s now recognized that this condition represents the end of a continuum rather than being an either/or proposition). The intriguing thing about this inability to recognize faces is that, in its extreme form, it can nevertheless exist side-by-side with quite normal recognition of other objects.
Prosopagnosia that is not the result of brain damage often runs in families, and a study of three family members with this condition has revealed that in some cases at least, the inability to remember faces has to do with failing to form a mental representation that abstracts the essence of the face, sans context. That is, despite being fully able to read facial expressions, attractiveness and gender from the face (indeed one of the family members is an artist who has no trouble portraying fully detailed faces), they couldn’t cope with changes in lighting conditions and viewing angles.
I’m reminded of the phenomenon of perfect pitch, which is characterized by an inability to generalize across acoustically similar tones, so an A in a different key is a completely different note. Interestingly, like prosopagnosia, perfect pitch is now thought to be more common than has been thought (recognition of it is of course limited by the fact that some musical expertise is generally needed to reveal it). This inability to abstract or generalize is also a phenomenon of eidetic memory, and I have spoken before of the perils of this.
(Note: A fascinating account of what it is like to be face-blind, from a person with the condition, can be found at: http://www.choisser.com/faceblind/)
 . Three cases of developmental prosopagnosia from one family: Detailed neuropsychological and psychophysical investigation of face processing. Cortex [Internet]. 2010 ;46(8):949 - 964. Available from: http://www.sciencedirect.com/science/article/B8JH1-4WXC28C-1/2/e0a845c1f7ba28ced034d2a938ce451d