Prosopagnosia is a rare disorder that can result from strokes where the individual is unable to recognize faces but maintains the ability to recognize other non-face objects. Disorders like prosopagnosia suggest to neuroscientists that the machinery for processing faces in the brain is in part special and segregated from the machinery for processing other objects.
It turns out that there is also a wildly underreported and surprisingly prevalent form of hereditary prosopagnosia (HPA), as shown in a new paper in American Journal of Medical Genetics. HPA has a prevalence of almost 2.5% and shows nearly no evidence of incomplete penetrance. Even more surprising, the inability to recognize faces seems to be the only symptom in the disorder -- a truly puzzling finding considering how intertwined the genetics controlling visual cortical development must be. As opposed to many disorders of cognition, this is not a syndrome associated with autism. These individuals lack only the ability to recognize faces.
The patients show interesting behavioral strategies to hide the deficit, partially explaining how such a prevalent disorder could go so unnoticed:
We found secondary behavioral adaptations in all affected persons. A detailed analysis showed three distinct strategies: a compensation strategy, an explanation strategy, and an avoidance strategy. We define compensational behavior as the attempt to recognize people by means other than their face. Most affected persons name voice, gait, bearing, favorite clothing, hair color, and style, accessories such as earrings or glasses as their favorite means of recognition. Others include the laugh, teeth, shoes, or the typical facial expression. People with HPA recognize other people readily in their usual settings, because they expect to meet them there. Explanatory strategies are defined as excuses or explanations for a socially inadequate behavior caused by HPA. Most affected persons have a ready set of excuses for not recognizing someone in the street, like being deep in thought, needing new glasses, being tired or distracted, or suffering from a bad headache. We define an avoidance strategy as behavior intended to avoid situations where the HPA can cause affected persons to behave in a socially inadequate or unaccepted way. Affected persons, therefore, avoid going to large functions without company, or meeting others in crowded places. All affected persons stated that they suffered from their disability to some extent, though none of them saw a doctor about it. All affected persons seem to be well integrated into society.
These behavioral adaptations are present in all affected persons studied. The exact severity of the individual cases of HPA is difficult to assess, because the adaptive behavior balances the deficit to an unknown extent. Still, our in-depth assessment showed that the underlying disorder always was fully expressed; we did not see intermediate forms. The impairments in additional family members - exclusively seen in an autosomal dominant pattern - further assists in the diagnosis of PA and especially of the hereditary form rather than the acquired form. (Emphasis mine.)
You would also not think that the disorder would be autosomal dominant.
Here is a picture of some sample pedigrees:
With respect to the issue of this being the only symptom, the authors argue that this is nearly unprecedented:
Congenital PA is the only known monogenic dysfunction of a higher cognitive visual skill. Among more than 90 different cognitive functions (e.g., musical mind, absolute pitch) and dysfunctions (e.g., agraphia, dyscalculia, dyslexia) related to specific cognitive behavioral and neurological disorders we could only find a few monosymptomatic conditions in the OMIM database (http://www.ncbi.nlm.nih.gov/Omim/) with proven or suggested heredity: perfect musical pitch, syn. absolute pitch [OMIM 6159300]; specific language impairment (SLI) [OMIM 602081]; specific dyslexia, syn. word-blindness (DYX1, [OMIM 127700], DYX2 [OMIM 600202], DYX3 [OMIM 604254], DYX5 [OMIM 606896]; DYX6 [OMIM 606616], DYX8 [OMIM 608995], DYX9 [OMIM 300509]), familial developmental dysphasia [OMIM 60017]; tune deafness; syn. dysmelodia [OMIM 191200]; congenital anosmia; syn. odor blindness [OMIM 107200]; inability to smell musk [OMIM 254150]; hereditary whispering dysphonia [OMIM 193680]; indifference to pain, syn. congenital analgesia [OMIM 243000]. Gene mapping was successful, in one disorder [OMIM 602081], a single but large family in which half of the members had orofacial dyspraxia and severe speech and language impairment. A point mutation was found in the FOXP2 (forkhead box protein 2) gene co-segregating with the disorder [Lai et al., ]. There are several susceptibility genes described in the heterogeneous group of dyslexia with regular segregation. The resulting phenotypes do not represent this in any case.
This is totally fascinating. I will be very curious to see what the associated gene they find does.
i hit this before too. see here, and the contents of email from the lead researcher i received.
Intriguing. I accept that there is a wide variation in our ability to recognize faces/individuals out of context or after time (I went to a conference last month and recognized some people immediately after 14 years, and others not at all, but was shocked by those people who recognized me after the same break, some of whom were just acquaintances).
I wonder if there are other deficits associated with developmental prosopagnosia (there certainly are with the cases I've seen after stroke). This page suggests there are some auditory processing issues, and gives a nice presentation of the phenomenology.