Mirrored from Wiring the Brain (http://wiringthebrain.blogspot.com/)
Faces are special. Humans are innately interested in faces and so good at detecting them that we see them in clouds, shrouds, pieces of toast, tree-stumps, and even simple yellow circles with a couple of dots in them. Even newborn infants (really, really newborn) are more interested in looking at faces than non-faces. Not too surprisingly, this preference and ability extends to other species too. Monkeys reared from birth with absolutely no visual exposure to either monkey or human faces for two years still showed a strong preferential interest in faces (both monkey and human) when they were shown them. Given the importance within social groups of recognising particular individuals and of reading emotional and social cues from people’s faces, it is perhaps not too surprising that face recognition is a built-in part of our cognitive toolkit.
This is not to say that experience plays no part in the skill of face recognition – we clearly improve with practice and exposure in the ability to distinguish large numbers of faces. This can clearly be seen in the effect of cultural exposure to people of different races on the ability to distinguish such faces. (Most people are significantly worse at distinguishing between faces in races other than their own). We are thus pre-wired to be interested in faces and to process them differently from other visual stimuli, but we still need experience to be good at it.
Both the specialisation and the effects of experience are evident in neuroimaging studies. A particular area in the fusiform gyrus (the “fusiform face area”) shows highly selective responses to visual facial stimuli, compared to other types of visual stimuli. The size and responsiveness of this area increases over time, from children to adults, correlated with improved abilities in discrimination of faces. (Despite the name, this is more accurately thought of as a series of small clusters, linked together by dedicated circuits).
It is well known that lesions to this area of the brain can result in a condition commonly called “face blindess”, but with the clinical term prosopagnosia (which translates literally, and more accurately as “lack of knowledge of faces”). This condition was popularised by Oliver Sacks in his classic book “The Man Who Mistook His Wife for a Hat”, in which he described one sufferer who really was face-blind – he was unable even to recognise faces as faces – they simply made no sense as visual stimuli. This is the most severe manifestation along a spectrum – more typical is that sufferers can distinguish faces from other visual stimuli but are unable to recognise whom they belong to.
Perhaps less well known is the fact that prosopagnosia can also be developmental – not associated with any kind of lesion or injury. Some people are just born that way. In fact, as many as 2.5% of the population may be prosopagnosic. The underlying deficit may be an inability to perceive faces holistically – to put the pieces together into a coherent picture – the process that allows instant and effortless recognition for most people. Importantly, this defect is quite specific to face processing – other types of visual information can readily be combined in a holistic fashion. Interestingly, most prosopagnosics can still extract some information from facial features, such as gender and age, though they are usually unable to tell whose face it is, or even if it is a familiar face. (At least, they are not consciously aware of whether the face is familiar or not – they may, interestingly, still show a subconscious emotional response to familiar faces). This deficit has obvious and severe social consequences, and many sufferers adopt alternative strategies to recognise people, based on individual distinctive features, voice, gait or other characteristics.
Remarkably, developmental prosopagnosia can be inherited. Studies of its familiality, by Grueter and colleagues, have shown that it tends to be inherited in a very simple manner, consistent with Mendelian dominant inheritance – i.e., it can be caused by a single mutation, which clearly segregates between affected and unaffected members within families. What the gene (or genes, as it could be different genes in different families) encodes is not known, but there is evidence from neuroimaging that it may control the development of connectivity within the ventral visual stream – the part of the visual system devoted to processing visual form, where the fusiform face areas is found.
In a study by Cibu Thomas and colleagues, patients with developmental prosopagnosia showed dramatic reductions in the number of nerve fibres projecting along this ventral visual stream. In addition, the degree of reduction correlated strongly with the degree of impairment in face recognition. Thus, while most people are pre-wired to process faces with ease, through specialized visual circuits, this appears to not be the case in these patients.
Recent work has shown that more graded differences in the ability to recognise faces extend across the entire population – some people are just better at it than others. Twin studies demonstrate that variation in this ability across the normal range is also strongly influenced by genetic variation, both in behavioural measures and in the responsiveness of the fusiform face area in neuroimaging studies.
The identification of the genes involved, either in inherited prosopagnosia or in the normal variation in face-processing abilities, should lead to the elucidation of how changes in the genetic programme of brain development can result in altered connectivity in very specific circuits, dedicated to an innate and evolutionarily conserved psychological function.
GRUETER, M., GRUETER, T., BELL, V., HORST, J., LASKOWSKI, W., SPERLING, K., HALLIGAN, P., ELLI, H., & KENNERKNECHT, I. (2007). Hereditary Prosopagnosia: the First Case Series Cortex, 43 (6), 734-749 DOI: 10.1016/S0010-9452(08)70502-1
Thomas C, Avidan G, Humphreys K, Jung KJ, Gao F, & Behrmann M (2009). Reduced structural connectivity in ventral visual cortex in congenital prosopagnosia. Nature neuroscience, 12 (1), 29-31 PMID: 19029889
Wilmer, J., Germine, L., Chabris, C., Chatterjee, G., Williams, M., Loken, E., Nakayama, K., & Duchaine, B. (2010). Human face recognition ability is specific and highly heritable Proceedings of the National Academy of Sciences, 107 (11), 5238-5241 DOI: 10.1073/pnas.0913053107