Prosopagnosia is a disorder colloquially referred to as face blindness. While cases of inability to recognize faces have been documented throughout history, the term prosopagnosia was first coined by the German neurologist Joachim Bodamer . The disorder is classified into one of two types, acquired and developmental, based mainly on etiology and age at which symptoms appear. Prosopagnosia is caused by trauma or malformation of the occipitotemporal area of the brain, although affected areas and degree of impairment often differ from patient to patient, resulting in varied treatment and prognoses . Most commonly, prosopagnosia is linked to damage in the right fusiform gyrus, an area of the brain heavily activated during both facial detection and identification . For many years it was believed that there was no remedy for prosopagnosia, but recently, both facial recognition training and medical treatment have seen some successes. Prosopagnosia is a rather obscure yet surprisingly common disorder that will likely receive more attention in the coming years as it becomes better known in both the medical and scientific worlds.
History, Classification and Diagnostics
In 1947, German neurologist Joachim Bodamer published several accounts of a rather interesting deficit he had encountered amongst certain patients of his. In particular, Bodamer described a 24-year old man who had a bullet wound to the head, who he referred to as “Patient S”. The bullet had damaged the parts of Patient S’s brain that were responsible for visual processing causing him to experience difficulty with vision. Interestingly, even as his vision returned, Patient S had trouble assigning objects to their names, a symptom characteristic of general visual agnosia .
Bodamer noted that S had trouble recognizing faces; in particular, he could identify a face as a face, but struggled to differentiate between the faces of his close friends and those of strangers. In his published case study, Bodamer wrote that all faces appeared “sober” and “tasteless” to his patient. Bodamer called this disease Die Prosop-agnosie, combining the Greek root for face, prósopōn, and lack of knowledge, agnōsía. Bodamer is widely credited with publishing the first clinical account of prosopagnosia and identifying it as a distinct disorder, separate from other types of visual impairments and agnosias.
Even now, prosopagnosia is often loosely defined, as patient cases often present varying degrees of severity . Some sufferers report that they are unable to recognize previously encountered faces. Others may be unable to even identify a face as different from other objects; although, in these cases, prosopagnosia is often coupled with other visual agnosias, such as a difficulty in recognizing places, objects, and emotions . In spite of these impairments, all prosopagnosics tend to retain normal intelligence and intact visual acuity .
Generally, prosopagnosia is classified into acquired and developmental types. Patients with acquired prosopagnosia exhibit normal facial recognition and then subsequently lose this ability after head trauma, stroke, or a degenerative disease. The onset of developmental prosopagnosia is typically much earlier, as patients suffer from face blindness prior to adolescence. The specific causes of developmental prosopagnosia are much more varied, as the term is broadly applied to prosopagnosia as a result of genetic, prenatal, or childhood brain damage, as well as to idiopathic cases .
Acquired prosopagnosia, such as in the case of Patient S, is much more commonly diagnosed because individuals who have experienced on-level facial recognition ability are more likely to notice and articulate such an issue. Since the disorder typically manifests itself through observable anatomical changes to the brain, fMRI imaging, CT scanning, and PET scanning are often used in case studies and diagnosis.
Several visually-based exams of facial recognition also exist. The Warrington Recognition Memory for Faces (RMF) test and the Benton Facial Recognition Test (BFRT) were commonly used by clinicians and cognitive neuropsychologists; however, studies have shown that they tend to be unreliable at identifying prosopagnosics because stimuli presented during the test contained an excess of non-internal facial information—hair, clothes, eyebrows—which could be used in identification . As a result, they have been slowly phased out of clinical usage. Instead, the Cambridge Face Memory test has emerged as a relatively strongly assessor of facial recognition ability and an important diagnostic tool for prosopagnosics .
Researchers have also developed a self-report tool, the 20-item prosopagnosia index (PI20), which targets developmental prosopagnosics who have yet to be diagnosed. The PI20 presents 20 statements that the subject is asked to rate on a five-point scale, with 5 indicating ‘strongly agree’. These statements include “When people change their hairstyle of wear hats, I have problems recognizing them.” The PI20 has been efficient at diagnosis, and its success indicates, to some extent, that even those with developmental prosopagnosia, whom have never experienced normal facial recognition, are generally aware of their deficit and how it affects them .
Prevalence and Distribution
Although diagnosis is much more likely to be obtained for those with acquired prosopagnosia, it is estimated that, in the general population, there is a much higher prevalence of those with the developmental type. In recent years, some research has been done with the purpose of determining a genetic basis of prosopagnosia. Congenital, inherited prosopagnosia was found to have a surprisingly high prevalence, especially among the German-Caucasian population, where it affects up to 2.5% of the population. Surprisingly, the majority of those with this form of prosopagnosia did not display the brain lesions that typically characterize prosopagnosics. The pattern of inheritance for this type of prosopagnosia appeared to be autosomal dominant, although the specific genes which are affected are not yet clear .
Overall, there is a scarcity of studies measuring prevalence of this disease. While media attention has made it better known, the clinical world is currently more focused on etiology rather than epidemiology with regards to prosopagnosia. One study conducted in Varanasi, India found that among 689 students of Banaras Hindu University, only one female student was afflicted with prosopagnosia. Some of her relatives reported symptoms similar to the ones that she had reported, suggesting that the hereditary form of prosopagnosia is not confined to those of Caucasian ancestry . Nevertheless, more comprehensive studies are needed before conclusions about the prevalence of prosopagnosia in the general population can be made.
The Role of the Fusiform Gyrus and Its Associated Areas
Prosopagnosia is generally characterized by injury or malformation of the brain (save for the rare exceptions mentioned above). Clinicians and researchers have long associated the symptoms of this disease with damage to areas of the brain related to facial processing and recognition. By 1982,it was known that lesions of the central visual system were common in prosopagnosics .
Bilateral damage tends to be widespread among patients, but unilateral lesions of the right temporo-occipital region are also sufficient to engender impairments, a finding consistent with evidence that facial processing is heavily dependent on the right hemisphere [7, 8]. It is now believed that although regions of the left temporal and occipital lobes display activity during facial recognition, they are not critical such that in most patients unilateral left-hemispheric lesions would not result in prosopagnosia. Nevertheless, the degree of hemispheric dominance in facial processing tends to vary among individuals: in some left-handed individuals where hemispheric specialization is reversed, localized lesions in the left hemisphere have been known to cause prosopagnosia .
More recently, the advent of advanced neuroimaging techniques such as fMRI has allowed researchers to identify and isolate the areas of the brain that specifically respond to facial stimuli. Certain loci in the visual extrastriate cortex, regions of the occipital lobe bordering the primary visual cortex, are now hypothesized to constitute a core facial network (CFN). The CFN includes, primarily, the lateral middle fusiform gyrus, known as the fusiform face area (FFA), as well as the occipital face area (OFA) and the face-selective posterior superior temporal sulcus (pSTS) [9, 10].
The fusiform gyrus is the best known of this triad, likely because it displays the strongest response to faces and the largest difference in activity between perceptions of faces versus objects . It was widely believed that the FFA was critical for facial processing and it was thus dubbed a “face module.” However, cases documenting patients of prosopagnosia who retain structurally intact FFA have recently arisen . As such, it is likely that this area coordinates with other areas of the brain, such as the OFA, in the process of facial recognition.
Many hypotheses have been put forward with regards to the relationship between the FFA and other areas of the brain. It was originally suggested that the OFA was more involved in processing structural elements of faces, such as the individual features, information from which would then be transmitted in a feedforward system to the FFA for whole-face processing .
This hypothesis was countered by evidence demonstrating that processing of whole faces elicits a faster response than encountering isolated features, suggesting that facial processing happens first in the FFA then in the OFA. In particular, opponents cited the behavior of the N170 wave, a component of the event-related potential, a specific electrophysiological response to facial stimuli . The N170 response is faster when viewing the face as a whole, essentially signifying that in facial processing, humans tend to process other human faces first as a whole, then subsequently by individual features . The pSTS, while somewhat absent from extensive clinical review, is thought to play a role in transient aspects of facial recognition such as expression .
Accepted models of facial recognition generally include not only the core face network, but also an extended face network (EFN). The CFN includes the posterior areas highlighted above and is considered an “entry-point” for face processing. From there, information travels to the anterior temporal cortex, which contains areas of the EFN. Further identity processing occurs here, and loss of functional connectivity between these anterior regions and the CFN is also thought to play a role in prosopagnosia. Loss of connection to the amygdala, the area of the brain responsible for emotional response, has also been observed and may contribute to the failure of prosopagnosics to recognize emotional expressions and link a familiar face with a particular emotion (e.g. feeling affection when seeing your mother) .
While there is still debate surrounding which neural networks are responsible for the varying symptoms that prosopagnosics experience, the hope is that increasingly accurate models of facial processing will pave the way for better and more accurate treatments for both acquired and developmental types of prosopagnosia.
Treatments and Ongoing Research
As far as we currently know, there is no fully effective cure for prosopagnosia. As recent as 2005, a renowned Australian cognitive scientist Max Coltheart articulated the belief that face processing could belong to a domain of cognition that, after severe impairment from brain injury, could not be restored . While effective treatments have begun to emerge in the last half-century, there remains much room for improvement.
Treatment usually takes the form of cognitive training, in which facial processing deficits are addressed either through compensatory or remedial training. Compensatory treatment focuses on allowing patients to live a normal life despite their impairments rather than attempt to re-engage facial recognition skills which have been already lost. Remedial treatment attacks the problem directly and actually attempts to restore facial recognition to some extent. Unfortunately, this approach has been much less effective .
Clinicians employing the compensatory treatment approach often engage their patients in feature training, in which they are taught to work around their deficits by promoting recognition through specific facial features . Patients are often asked to verbalize individual facial characteristics, such as “long thin face” or “blue eyes”. This approach has been particularly effective in young developmental prosopagnosics, although it has seen some success in patients with acquired prosopagnosia as well . Remedial treatment is often an extension of feature training in which patients are eventually expected to reach a level wherein they can process multiple individual features almost simultaneously, thus leading to an almost normal holistic face response. Remedial approaches have seen more success in those with the developmental subset as well, but overall success rates are low .
Ultimately, prosopagnosia is an incredibly difficult disorder to treat. Whereas some may respond extremely well to training, others may show no signs of improvement. Most recently, researchers have seen some promise in the administration of intranasal oxytocin which has been shown to correlate with higher scores on the Cambridge Face Memory Test, at least in those with developmental prosopagnosia .
Moreover, variations in the oxytocin receptor gene, OXTR, were associated with performance on the Warrington Face Memory Test . The exact physiological mechanism is unknown , but oxytocin is heavily involved in social cognition, bonding and trust, all of which requires sensitivity in visual processing. The same region also contains the main gyri responsible for facial recognition, which are damaged or dysfunctional in prosopagnosia .
While simple at a glance, prosopagnosia is a complex disorder which serves as a model from which neurologists have gleaned many insights on the facial processing mechanisms of our human brain. Prosopagnosia has not yet had its success story, but the ongoing and collaborative work of psychologists, biologists and the general community of those raising awareness for this possibly very prevalent disease have brought about some truly innovative solutions. As we learn more about the genetic and anatomical basis of facial processing in the brain, the path to finding a cure will sharpen into focus, the pieces of the prosopagnosic puzzle finally fitting together.
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