Prosopagnosia (Greek; prosop, face + agnosia, lack of knowledge) is a brain disorder that results in people being unable to recognize faces by looking at them. Prosopagnosiacs aren't blind in the traditional sense of the word, their ability to see and recognize objects other than faces is often normal. Instead, the very specific part of the brain -- the anterior region of the right fusiform gyrus for you neuroscience geeks out there -- which is responsible for recognizing and categorizing faces is broken down. This disorder doesn't affect other kinds of recognition either; a prosopagnosiac can still tell who people are by their voice, hair, clothing, walking gait, etc. Prosopagnosia can be congenital, caused by the person's genetics and present from birth, or acquired, caused by an injury or stroke to the right fusiform gyrus.

Interestingly, the disability seems to only effect conscious access to identification information about the face. For instance, prosopagnosiacs show a strong electrodermal response to well known faces, a weaker response to faces they've seen before in the laboratory, and no response at all to new faces. This finding has been confirmed by EEG/ERP work, and also in a study that measured reaction time to faces. In other words, on some level of mental processing every face is recognized as such, that recognition just doesn't trigger any other memory processing or categorization as a known face.

Since people with prosopagnosia cannot recognize faces, they also sometimes cannot recognize the emotional cues communicated by facial expression. One figure I read (not cited, so don't ask) said that emotional communication during speech was 60% facial, 30% in prosody, and 10% by what words were actually spoken. Regardless of the actual figures, a great deal of emotional context is unavailable to these prosopagnosiacs, making it hard to understand how people feel and how to appropriately react to them. A congenital prosopagnosiac has never seen the emotional content in others' faces and thus doesn't know to transmit emotional information through his or her own expression, making emotional communication with others difficult.

Related problems are Capgras' and Cotard's syndromes, in which the facial recognition works perfectly, but the emotional content is cut off. Whereas prosopagnosiacs can't recognize others but can still have feelings for them, the Capgras' sufferer has no problem recognizing people, he just can't attach any emotional significance to that recognition.