Prosopagnosia or "face blindness"
Primates, especially humans and chimps, have varied faces presumably reflecting the importance of visual senses over olfactory in recognizing individuals. There is rampant speculation that primate brain size was driven to expand in order to deal with conspecific recognition (recognizing one's fellow group members -- larger groups require larger brains.e.g. Dunbar, 2001)
Primates, as Darwin knew, share mammalian facial expressions and have evolved even more fine-grained musculature enabling subtle variations not possible in other species. (Huber, 1931)
Face-blind humans are forced to adopt other senses and techniques to maintain a normal social interaction -- and sometimes this is very difficult.
Recent research, specifically on face recognition and related work on cerebral specialization suggests a specific region of the right primate hemisphere embodies a crucial component of face recognition circuits. The right hemisphere also is significant in perception of emotions expressed by the face.
Individuals with damage to the right hemisphere sometimes find themselves unable to recognize familar faces -- including close relatives' and even their own. There appear to be all sorts of variations on this theme.
A more insidious variation is developmental prosopagnosia, perhaps inherited, that predisposes entire families to be poor at face-recognition. These individuals may not even be aware they have a deficit.
(See the FaceBlind Lab and other web sources.)
Fernández-Carriba et al (2002) Asymmetry in facial expression of emotions by chimpanzees. Neuropsychologia, 1395, 1-11.
Kanwisher, N. (2006) What's in a Face? Science,
A similar region appears in macaques--not drawn to scale on this image. A paper by Tsao et al in this issue utilizes a new methodology -- both fMRI and cell recording to define these areas.
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