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  • Ajay Macherla

Prosopagnosia: A Face-Specific Neurological Disorder?




Abstract

Prosopagnosia is a disease characterized primarily by an inability to recognize faces. The condition occurs due to neural damage in the right fusiform gyrus of the brain, a region responsible for facial recognition and coordination. Symptoms of the disease include an inability to recognize other people’s faces, or even one’s own face, as well as social withdrawal and isolation. Face recognition tests are primarily utilized in order to diagnose patients with prosopagnosia. However, as of the time of this paper, there is no current treatment for this disease.


The operationalization of prosopagnosia has been ambiguous in the medical community, leading to debates concerning the true definition of the disease. The aim of this paper is to identify an operationalization of this disease.


In particular, some believe that prosopagnosia is a face-specific disease characterized by deficits in only facial recognition. On the other hand, others believe that prosopagnosia is non-face specific, citing voice recognition, word processing, and topographic orientation as damage in prosopagnosia patients. These arguments will be labeled the face-specificity and the nonface-specificity arguments, respectively. Identifying a solution to this problem will aid clinicians in diagnosing and treating prosopagnosia patients.


The paper concludes that a compromise between both sides is necessary for a solution to be derived. What is common amongst all patients with prosopagnosia is that they have difficulties in facial recognition. However, some have additional difficulties in other recognition capabilities, due to neural overlap in the brain. Thus, although the nonface-specificity argument is correct in some respect, the face-specific argument is also correct in that all prosopagnosia patients have facial recognition deficits.


In the end, the following definition was proposed for the condition: prosopagnosia is a neurological disorder in which patients share deficits in recognition capabilities in tandem with face recognition difficulties.


 


Introduction

Interactions between individuals and societies all begin with one subtle and subconscious action: face recognition. Whether it may be speaking to a friend, family member, or completely new person, facial recognition is more often than not the first occurrence in a particular interaction. Face detection extends beyond simply remembering that a curved green sphere is a tennis ball or a white rectangle is a piece of paper; it requires detection of several components of the human face, including the mouth, eyes, nose, and ears.


However, consider the possibility of not being able to recognize the face of other individuals. It would be difficult to remember people, let alone build meaningful social relationships. Let us go one step further: what if one could not recognize his or her own face? Although seemingly impossible, it is a reality for approximately one in fifty people, who are diagnosed with the rare condition of prosopagnosia (Bate, n.d.).


In conducting preliminary research on the topic, an overarching controversy could be taken away: the face-specificity versus the nonface-specificity argument. These two perspectives arise from differences in defining prosopagnosia. Some believe that those with prosopagnosia are only limited to difficulties in facial recognition, whereas people who believe the nonface-specificity argument believe that prosopagnosia can even extend to objects as well. Both arguments have evidence to support their respective viewpoints.


In this situation, it is important to gain an impartial, third party perspective on the issue. This literature review aims to offer a conclusion on the more compelling argument in defining prosopagnosia. With this information, more research can be conducted to gain more empirical evidence for the more appropriate argument for prosopagnosia. As a result, clinical research in developing treatments for the condition will be more streamlined as there is a common and definitive definition of prosopagnosia to refer to when conducting research.


The literature review will first overview prosopagnosia and summarize both arguments, providing evidence for each respective view. After that, a conclusion will be provided and defended.

 

Disease Overview

Although controversial, the definition of prosopagnosia is the inability to discern new and old faces. This condition can range from an inability to discriminate between faces, recognize one’s own face, or distinguish between faces and their environment. Individuals without this condition are able to remember over 5000 faces, as per an entry from a medical textbook. However, this number is significantly reduced for individuals who have been diagnosed with prosopagnosia (Cabrero & Jesus, n.d.).


This disease overview will be divided into five sections: etiology, pathology, diagnosis, symptoms, and treatments.


Etiology:

Although unclear, prosopagnosia is a neurological disorder that is considered to be due to damage in the right fusiform gyrus, a brain region characterized by its ability to control facial perception and coordination. Accordingly, injury to this area can lead to an inability to discern faces, which is also known as acquired prosopagnosia. The condition can also arise from other neurodegenerative diseases, although the exact etiology behind it is relatively unknown. Genetically, prosopagnosia can arise from genetic deletion mutations that can run through family history, known as developmental prosopagnosia. The National Institute of Neurological Disorders and Stroke also notes that children with autism and Asperger’s syndrome may lead to prosopagnosia, at least to some extent (Prosopagnosia Information, 2019).


Pathology:

The pathological aspects of prosopagnosia are understudied but nevertheless important to document and discover. A research article suggests that cerebral lesions localized to the right fusiform gyrus can lead to the lack of facial perception in affected individuals (Meadows, 1974, p. 489-501). Specifically, the article notes a right occipitotemporal lesion, meaning the lesion is localized to the occipital and temporal lobes of the brain. However, another study found a left occipitotemporal lesion (Barton, 2008). It seems that the location of the lesion varies to some degree, but the studies did note that in one way or another, the fusiform gyrus, either right or left, was affected in some way. Intuitively, this notion makes logical sense; therefore, the current argument behind the pathology of prosopagnosia remains appealing.


Diagnosis:

Perception tests can be utilized to measure an individual’s ability to recognize faces. This can include simply recognizing visual stimuli to being shown faces and asking the ability to distinguish between them based on given information. The Cambridge Face Perception Test, the Glasgow Face Matching Test, the Benton Facial Recognition Test, and the Caledonian Face Test are all examples of diagnostic tests used by physicians to assess the onset of prosopagnosia in their patients. Brain imaging using MRI scans can also be used to determine a localized occipitotemporal lesion in the fusiform gyrus area, which can then be speculated to have some sort of relationship to prosopagnosia by physicians viewing the MRI scan. However, the former diagnostic test is more efficient for diagnosis (Logan et al., 2016).


Symptoms:

Due to the damage in the fusiform gyrus of the brain, symptoms of prosopagnosia, either developed genetically or acquired, include the following (Bate, n.d.; Prosopagnosia (face, n.d.):

  • Inability to discern faces

  • Fail to recognize familiar people

  • Approach foreign faces, mistakenly thinking it is a familiar face

  • Social withdrawal

  • Difficulty in processing complex concepts, such as the plot of a TV show or movie

  • Describing people using only facial information

  • Inability to identify known faces in a photograph

  • Inability to discern his or her own face


Treatments:

As of right now, there is no known cure for prosopagnosia. More research is and should be conducted to gain a better understanding of the condition and thus develop effective strategies and treatments to alleviate the symptoms of the disease. Rehabilitation programs and matching visual and environmental cues to faces can help patients with prosopagnosia identify faces easier. However, these clues are not effective all the time, meaning they are not a sustainable strategy to employ. This is why more specialized research into the treatments of prosopagnosia must be conducted (Prosopagnosia (face, n.d.).


 

Face-Specificity Argument

Proponents of the face-specificity argument have their views more explicitly reflected in the current operationalization of prosopagnosia: patients with this rare condition have impairments only in their ability to recognize faces. Although these faces do not necessarily have to be human faces, as some empirical evidence shows, supporters of this argument cite studies demonstrating that prosopagnosia patients do not have difficulties in recognizing faces.


A follow-up study on a patient with prosopagnosia, WJ, was conducted and published in 1993 on the PubMed database to explore the controversy of this condition as a face-specific disorder. WJ was a farmer who had a flock of sheep. Although he did not have difficulties in recognizing his sheep, as shown through face recognition tests, comparable tests on human faces lead to WJ having more trouble recognizing these faces. Furthermore, when tested on recognizing unspecified objects, WJ did not appear to have any struggles to recognize them. This study ultimately demonstrates that perhaps prosopagnosia is limited as face-specific (McNeil & Warrington, 1993). Of course, there is a lack of generalizability to all patients with prosopagnosia due to the case study nature of the research, but the study provides compelling evidence to further the current debate on the operationalization of prosopagnosia.


Another study conducted in August of 2010 focused on a patient with severe prosopagnosia in order to investigate and draw a conclusion on the current debate regarding the condition’s operationalization. The study first had 3 different kinds of stimuli presented to the participant: novel 3D geometric objects, morphed common objects, and highly familiar objects (cars). The study noted that there was no apparent increase in error rate for the participant, as the patient was able to easily recognize these objects. A fourth stimulus was added to the experiment, which was human faces. The researchers found that compared to a control group of patients without prosopagnosia, the prosopagnosia patient had difficulties recognizing the faces at higher levels of discreimination, such as the global shape of the face. Ultimately, the study concluded the following: “Overall, these observations indicate that the alternative view of prosopagnosia as a more general impairment for fine-grained discrimination in visually homogeneous object categories does not hold” (Busigny et al., 2010). In other words, the argument that prosopagnosia is not limited to objects is invalid, as the experimental results show that the patient did not have difficulties in remembering and recognizing the objects, but struggled in recognizing human face stimuli. The publication also notes the following (Busigny et al., 2010):


Rather, it seems that brain damage in adulthood may lead to selective recognition impairment for faces, perhaps the only category of visual stimuli for which holistic/configural perception is not only potentially at play, but is strictly necessary to individualize members of the category efficiently.


Nonetheless, the study does carry some limitations, again, with the generalizability of the results to all populations of prosopagnosia patients. The focus patient could simply have a severe case of prosopagnosia, and perhaps an inability to recognize objects completely disappears in later and more severe stages of the condition. With the case study research design of the experiment, it is difficult to generalize the results. Furthermore, perhaps a previous expertise with the objects presented to the patient, such as cars, may have superseded the effects of prosopagnosia in the patient. This may have skewed the results of the study. However, the results, taken by itself, are statistically and qualitatively significant.


Another aspect of the current debate is whether the mechanisms used in the brain for face perception is limited to only faces. Of course, proponents of the face-specificity argument argue that the mechanisms are limited to only facial perception, so if neural damage to this area were to occur, then the only perception that would be damaged would be facial perception –– not object perception. A study published in 2005 on the PubMed database leveraged a face specific “M170,” which is a type of magnetoencephalography (MEG) response element that is involved in facial perception 170 milliseconds after a face stimulus is presented. In two experiments, the researchers found that cars, a common and familiar object, did not lead to a high M170 response. However, in experiment 2, a high M170 response was found in successful facial perception (Xu et al., 2005). The researchers were able to conclude that face perception is distinct to object perception in terms of M170 response. Although the number of participants was unspecified in the experiment, the study did not include other stimuli, such as word processing and voice recognition, that may be impaired in prosopagnosia patients.


In conclusion, the face-specificity argument contends that patients with prosopagnosia are limited to damage in their ability to recognize faces. Literature has been published supporting this view, as patients have experienced no difficulties in recognizing faces but difficulties in recognizing faces. Furthermore, additional research in the biological underpinning of the condition found that facial perception is distinct from object perception, in terms of M170 response, and perhaps other forms of perception that may be impaired by prosopagnosia.

 

Nonface-Specificity Argument

In contrast to the previous argument, the nonface-specificity argument contends that prosopagnosia is not limited to deficits in facial recognition; it can even extend to an inability to recognize familiar objects. In clinical research for this condition, multiple instances of this idea have been cited and empirically demonstrated. In this section, the empirical evidence for this argument will be summarized.


A study conducted in 1999 by a trio of researchers aimed to provide “unequivocal evidence” on whether prosopagnosia was limited to facial recognition or whether it extended to object recognition. The study recruited two prosopagnosia patients, SM and CR, to test their ability to recognize faces and familiar objects. They tested the subjects with recognizing faces and recognizing snowflakes and Greebles. In short, the results indicated that the patients were impaired on both facial and non facial fronts. The evidence seems to indicate that clinicians should test for object recognition on top of the traditional face recognition tests in order to accurately diagnose and treat a patient with prosopagnosia (Gauthier et al., 1999).


However, the study carries several limitations that must be considered. As the research was a case study, the small sample size cannot be generalized to larger populations of prosopagnosia patients. Nonetheless, the study is fairly easy to replicate to confirm the results, provided the researchers have access to a sample of patients with prosopagnosia. Granted, as the condition is relatively rare, the feasibility of replicability may be compromised.


Another study conducted in 2008 found that 9 out of 10 patients in the sample had “significant difficulty” in recognizing or even identifying common fruits and vegetables. In fact, one of the participants was a chef (Busigny et al,, 2010). Likewise, a study on six children with developed prosopagnosia found that one had severe deficits in recognizing both faces and objects (Dalrymple et al., 2017).


A common question asked to those who support the nonface-specificity argument is whether experience or expertise with a particular object, as seen with the chef having difficulty recognizing common kitchen foods, has an impact on the effects of prosopagnosia. In 2016, a study attempted to evaluate this possibility by leveraging the Cambridge Car Memory Test to test a sample of patients with prosopagnosia. The researchers asked the participants to identify cars, and in the results, attempted to control for any car expertise the participants may have had. The assumption was that with this expertise, the patients could override the effects of prosopagnosia and remember the object, which would be concerning to proponents of the nonface-specificity argument, as it nullifies their beliefs. The study found no significant difference between expertise and ability to recognize objects, indicating that some prosopagnosia patients may have trouble recognizing objects regardless of prior expertise with these objects (Esins et al., 2016).


Similarly, inabilities to recognize words are noted amongst patients with prosopagnosia. Although facial perception is more active in the right hemisphere of the brain, where the right fusiform gyrus is located and localized to facial perception, some studies have shown bilateral overlapping between facial recognition and word processing. Assuming that damage to this area causes the symptoms of prosopagnosia, it is plausible that there will be consequences to processing and recognizing words in patients with this rare condition (Nestor et al., 2012). One study found slight impairments in word processing in three subjects of their sample. Notwithstanding the small sample size that limits generalizability to larger populations, the study concedes that the results may have been due to visual agnosia, not prosopagnosia. However, the results are intriguing and are a great stepping stone for future research in this possibility.


Another study found that in a sample of prosopagnosia participants with parts of their right hemisphere lesioned, their ability to read words as a function of the word’s length, as well as their ability to sort letters by word content, was significantly impaired. Not only does this study localize parts of the right hemisphere to word processing, but it may also indicate that prosopagnosia includes inabilities to recognize words (Hills et al., 2015).


Some literature has also shown deficits in voice recognition. A study published in 2016 by a team of researchers attempted to assess the ability to recognize voices of patients with prosopagnosia. They found that subjects with bilateral anterior temporal lesions to the right hemisphere of their brains experienced difficulties in recognizing voices (Liu et al., 2014). Another study found similar results, although the voice impairments were only observed in one subject (Liu et al., 2015).


Finally, topographic disorientation, or a disorder in which an individual has difficulties in recognizing their surroundings, has also been associated with acquired prosopagnosia. A review found that some sort of topographic disorientation in 29% of their 147 sample size of patients with the rare condition (Schmidt, 2014). A literature review attempts to provide a biological explanation for this phenomenon when it states: “One possible explanation is the close proximity of the parahippocampal place area (PPA), an area activated when viewing scenes, to the fusiform face area (FFA), which is activated by viewing faces” (Barton et al., 2016). Intuitively, damage to the FFA may cause some sort of aftermath effects on the PPA, thus resulting in topographic disorientation. Another study found similar deficits in topographic skills and cognitive mapping in patients with prosopagnosia (Corrow et al., 2016).


In essence, the nonface-specificity argument contends that patients with prosopagnosia are not limited in their inability to recognize only faces. Empirical evidence has also established associations between prosopagnosia and word processing, voice recognition, object recognition, and even topographic disorientation.


 

Conclusions

In this section, we aim to provide an answer to the long-standing question on whether prosopagnosia is a face-specific neurological disorder. This section will be informed by a synthesis of evidence from the face-specificity argument and nonface-specificity argument sections.


First, it is necessary to state that all evidence provided for each side was valid. There have been no deception or manipulation involved on either side, meaning we cannot simply rule out an argument due to an apparent blunder in research design or logical reasoning. An accurate operationalization of prosopagnosia must be based on an impartial synthesis of all available evidence on both sides.


The face-specificity argument cites case studies and experiments with samples of prosopagnosia patients to inform its views. Multiple prosopagnosia patients studied have only reported difficulties in facial recognition, not object recognition. However, the nonface-specificity argument counters this claim, providing case studies on patients with both object and facial recognition deficits, including a chef who could not recognize common fruits and vegetables despite previous expertise with the items. As all cited evidence is valid, it can be concluded that prosopagnosia patients vary in symptoms, and not all patients will experience deficits in object recognition. What is common amongst all prosopagnosia patients is difficulties in facial recognition. Most likely, this similarity across patients informed the current accepted operationalization of prosopagnosia as a face-specific disease.


Even more, the nonface-specificity argument cites voice, word processing, and topographic orientation difficulties as characteristic of prosopagnosia patients, providing empirical evidence from case studies and other research. Although these deficits are not present in all patients, they are present in tandem with facial recognition difficulties, which is why the sample is characterized as prosopagnosia patients.


From this information, we can conclude that prosopagnosia patients are varied in symptoms, but all patients have one symptom in common: facial recognition deficits. However, as the nonface-specificity argument suggests, most likely, patients with prosopagnosia might experience additional symptoms such as voice, object, word, and topographic orientation difficulties. This is due to the fact that there is some overlap in neural pathways involved in facial recognition and other recognition capabilities (Nestor et al., 2012).


Although the face-specificity argument is partially correct in that prosopagnosia patients share deficits in facial recognition, the nonface-specificity argument is also correct in that some patients may also experience difficulties in other recognition skills. For this reason, the current operationalization of prosopagnosia is not applicable to all patients, meaning there is some ambiguity when diagnosing particular patients.


Considering all the available evidence and literature on the topic, it can be ultimately concluded that prosopagnosia is a neurological disorder in which patients share deficits in recognition capabilities in tandem with face recognition difficulties.


It must be noted that more research into this topic and operationalization should be conducted to refine this definition for clinical use. There are many unknowns on whether word processing, voice, and topographic disorientation deficits that proponents of the nonface-specificity argument cite are distinct from the symptoms experienced in prosopagnosia. However, compiling and synthesizing the evidence into an accurate definition will ensure that diagnosis of prosopagnosia in the clinic are more accurate and streamlined.



 

References

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