In my last post I promised to tell you about the anatomy of prosopagnosia, or the inability to recognize familiar faces. I will begin with a short general introduction on the difficulties of investigating the anatomy of neuropsychological syndromes.
Studies of individual patients have been paramount in defining neuropsychological syndromes such as prosopagnosia. They have also been necessary when describing very rare symptoms of brain damage, such as the inability to perceive motion. The downside of studying individuals with brain lesions is that the lesions are accidents of nature. They do not come in one size fits all. Rather, heterogeneity is the rule. Patients with prosopagnosia are never entirely alike in terms of clinical symptoms. Although they all have prosopagnosia they may differ in many ways. Some also have difficulties recognizing objects, some have low-level visual problems and so on and so on. In addition, the patients have lesions in different areas of the brain and with chronic lesion there may be some reorganization of function. Different etiologies (e.g., stroke vs. congenital problems) may also entail different clinical profiles. Finally, the brain is a neuronal network but not a collection of individually functioning modules. This means that a lesion in one area can affect the functioning of another area. Thus, correlating anatomy and clinical symptoms in clinical populations can be quite a challenge.
Right or left?
Early on it was often claimed that lesions in both hemispheres were necessary to cause prosopagnosia. In 2002 it was still regarded as controversial to claim that a lesion confined to the right hemisphere was sufficient. But now we know better. It has been well documented that lesions confined to the right hemisphere are sufficient to cause prosopagnosia.
Lesions in the left hemisphere can result in problems recognizing faces but this is different from the prosopagnosia proper seen after right hemisphere lesions. Patients with left-hemisphere lesions appear to process faces normally. That is, as a whole, in a configurational manner. Problems with recognizing faces after left hemisphere lesions are also more likely to be the result of a more general visual recognition defect. These patients also lack what can be considered the hallmark of prosopagnosia. That is, they do not lose the feeling of familiarity associated with looking at familiar faces. This familiarity feeling is an important first step in recognizing faces and this feeling is dependent on right hemisphere structures. Thus, patients with prosopagnosia following right hemisphere damage do not experience a feeling of familiarity when looking at familiar faces. They might as well be looking at total strangers. They also rarely have problems recognizing objects. Patients with face recognition problems following right hemisphere lesion also have problems recognizing faces holistically and their eye movements when looking at faces are erratic and quite different from what is seen in healthy individuals. These patients have a more specific problem recognizing faces than patients with left sided lesions.
The anatomical basis of prosopagnosia
There is no single area of the brain that is responsible for our ability to recognize faces. Rather there is a dynamic network of cortical areas underneath the brain which subserves this function. In the posterior part of the brain there is the occipital face area (OFA). More to the front, in the temporal cortex, is the fusiform gyrus where the fusiform face area (FFA) is located. The anterior temporal areas are also of importance in face recognition. These areas are both on the right and left side of the brain and are shown in the animated illustration in this post. The most specific form of prosopagnosia is caused by damage to the facial recognition areas on the right side.
Here you can listen to a fascinating interview with Heather Sellers on NPR. Heather has developmental prosopagnosia: http://www.npr.org/2010/11/13/131267727/living-with-face-blindness-who-are-you-again and has also written a book on her life with this condition: http://www.amazon.com/Dont-Look-Like-Anyone-Know/dp/B00476WMRC
Atkinson, A. P., & Adolphs, R. (2011). The neuropsychology of face perception: Beyond simple dissociations and functional selectivity. Philosophical Transactions of the Royal Society, 366, 1726-1738.
Gainotti, G., & Marra, C. (2011). Differential contribution of right and left temporo-occipital and anterior temporal lesions to face recognition disorders. Frontiers in Human Neuroscience, 5, 1-11.
Rossion, B., Hanseeuw, B., & Dricot, L. (2012). Defining face perception areas in the human brain: A large-scaled factorial fMRI face localizer analysis. Brain and Cognition, 79, 138-157.
Sorger, B., Goebel, R., Schiltz, C., Rossion, B. (2007). Understanding the functional neuroanatomy of acquired prosopagnosia. NeuroImage, 35, 836-852.
Uttner, I., Bliem, H., & Danek, A. (2002). Prosopagnosia after unilateral right cerebral infarction. Journal of Neurology, 249, 933-935.