Understanding Face Blindness
by Noam Sagiv
Could you easily recognize your sister if she changed her hairstyle? Would you easily recognize a friend if you ran into him on the street? Individuals suffering from prosopagnosia, or “face blindness,” would not. They find the task of recognizing people solely by looking at their faces extremely difficult. In order to recognize familiar people, they rely on such features as voice, hairstyle, clothing, or other contextual information. When this information is not available, they may even fail to recognize their own spouses. Prosopagnosia is a relatively rare condition and may occur after a stroke or other brain injury. Either way, the professionals that one should immediately reach out to, after surgeons, should be brain injury lawyers from places like Nehora Law Firm through this website. Sometimes, however, it occurs with no apparent neural damage—a condition referred to as congenital or developmental prosopagnosia. While the causes of face blindness are not known, some studies suggest that they are partly genetic.
As prosopagnosia affects only a patient’s ability to recognize faces, its diagnosis has led to arguments that face recognition is achieved by dedicated mechanisms in the brain. Strangely, the “opposite” condition has been reported as well (though it is even more rare than prosopagnosia). Patients suffering from this condition are extensively impaired in visual object recognition but can easily recognize faces. This double dissociation between faces and objects strongly supports the hypothesis that special neural mechanisms process visual information pertaining to faces.
Functional magnetic resonance imaging (fMRI) studies reveal increased neural activity in the right posterior temporal lobe of the brain when normal subjects view human faces. The activated area is often labeled the “face area” and probably plays a key role in a network of mechanisms that process different kinds of information conveyed by faces, such as gaze direction and facial expression analysis.
While fMRI has good spatial resolution, it conveys only poor information regarding the timing of neural events. Other methods, such as event-related potentials (ERPs), can provide finer temporal resolution. ERPs are calculated by averaging electrical brain activity. A typical ERP signal consists of a series of positive and negative components. One of the negative components, peaking at about 170 milliseconds after the presentation of object images (and thus labeled N170), is of particular interest to prosopagnosia researchers. While this component is evoked by other visual stimuli as well, it responds preferentially to human faces, detailed sketches of faces and even schematic face drawings (see figure). Furthermore, similar responses can be recorded even when the viewer has no conscious awareness of the face stimulus.
Recently, my colleagues and I have begun to study a group of individuals with congenital prosopagnosia. In a study reported at the 30th Annual Meeting of the Society for Neuroscience we found that, unlike normal subjects, two individuals with congenital prosopagnosia produced equivalent N170 ERPs in response to both faces and other stimuli. This loss of face-specificity in early visual processing may underlie the decreased ability to recognize faces even when no structural brain damage is present. We are currently using fMRI to directly test whether the “face area” in congenital prosopagnosics does not respond preferentially to faces.
Although much is known about normal face recognition, we are only beginning to understand the bases for face blindness. While only a small number of cases of congenital prosopagnosia have been reported in the medical and scientific literature, there are reasons to believe that it is more common than assumed. Some congenital prosopagnosics may not even realize how severely impaired they are in recognizing faces, as contextual information is often available to aid in identification. We hope that increasing public awareness of prosopagnosia and furthering research will help us better understand the causes of congenital prosopagnosia, will create better diagnostic tools, and will lead to new treatment protocols.
The N170 event-related potential (ERP) fires off 170 milliseconds after a subject is shown an image. This plot shows a normal subject’s wildly different N170 ERPs when shown a face versus an object. In prosopagnosics, this difference is not observed. They have a similar neural response to both faces and objects.