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What is Prosopagnosia?
Prosopagnosia is a condition characterized by the inability to recognize faces, due to impairments in the visual processing and visual association areas of the brain. These impairments can be present at birth as congenital or developmental prosopagnosia. In the case of congenital prosopagnosia the brain’s visual processing centers associated with visual recognition of faces does not develop properly. Prosopagnosia can also be present later in life in the form of acquired prosopagnosia which can be caused by traumatic brain injuries, neurological diseases and other various factors.  Most researchers agree that prosopagnosia is connected to dysfunctions in the fusiform gyrus of the brain, however the way in which the fusiform gyrus causes theses difficulties is still under investigation.  It is estimated that Developmental or congenital prosopagnosia is prevalent in 2 to 2.9% of the world’s population.  There are still a lot of unanswered questions about prosopagnosia and so more research is needed to deepen our understanding of this complicated disorder.
Anatomical Structures and Neural Pathways Associated with Prosopagnosia
In order for the brain to recognize faces, the following structures and neurological processes must be working correctly:
The first process required for facial recognition begins in the eyes. When a face comes into the visual field, the visual input will enter into the eye through the cornea where the image will be bent. The image will then be focused onto the fovea of the eye’s retina using the lens which can be adjusted by ciliary muscles. When the light hits the retina the light will bounce off of the retina and activate cone cells. Axons from the cone cells will hyperpolarize or depolarize the bipolar cells depending on whether bipolar cells are on or off center bipolar cells. Bipolar cells will then send neural input to ganglion cells which will exit the optic disc's of each eye as the optic nerves. Neural input that originated on the temporal retina will remain ipsilateral and enter into the optic tract on the same side of the body as it originated. Nasal retina neural input will decussate at the optic chiasm and enter the contralateral optic tract. The left and right optic tracts will then synapse with the Lateral Geniculate nucleus of the thalamus. 
Figure 1: Visual Processing
Figure 2: The Lateral Geniculate Nucleus
The information that is most important in facial recognition is in layers P3, P4, P5 and P6 of the LGN as those layers receive information on shape, form and color which are necessary for viewing facial features. Neurons from the LGN will then head towards the primary visual cortex as the optic radiation. In the primary visual cortex, neurons will synapse in layer 4C beta. From there, information about shape and form will synapse with inter-blobs in layers 2 and 3 of the primary visual cortex while neural input regarding color will synapse with blobs in layers 2 and 3 of the primary visual cortex. Neurons carrying information about color and form will then enter the ventral stream or the “what” pathway as they carry information about characteristics of an object. 
Figure 3: The Ventral and Dorsal Streams of the Brain
In the ventral stream, neurons from inter- blobs will synapse with neurons in the pale stripe of visual association area 18. Neurons originating in the blobs will synapse with the thin stripe of the visual association area 18. From there the neurons will synapse with the inferior occipitotemporal cortex which includes vital structures for processing faces. The inferior ocipitotemporal cortex includes the hippocampus and the fusiform gyrus. These structures are responsible for processing visual input and associating the input with previously stored information in order to recognize faces. 
Closer Look at the Fusiform Gyrus and the Hippocampus:
Figure 4: Regions of the Fusiform Gyrus
Figure 5: Anatomical view of the Fusiform Gyrus
Figure 6: Anatomical view of the Hippocampus
The fusiform gyrus is a section of the inferior occipitotemporal cortex located on the ventral surface of the brain. It is found in between the hippocampal gyrus and the inferior temporal gyrus and is a part of the ventral stream or the “what” pathway of the brain. Within the fusiform gyrus there is a specific anatomical region known as the FFA or the fusiform face area which is most active when your visual system is analyzing faces. 
The Hippocampus is a section of the inferior ocipitotemporal cortex that is located just above or superior to the fusiform gyrus. The hippocampus plays a large role in memory. Researchers have been trying to investigate whether there are any correlations between dysfunctions in the hippocampus and prosopagnosia. In some research, the hippocampus was found to play a helpful role in recognizing faces, however it may only be a supporting structure to aid other main facial recognition structures such as the fusiform gyrus. In the case of some injuries or pathologies to the hippocampus, facial recognition has been delayed due to these impairments . There is still more research to be done in order to gain more information on other possible correlations between prosopagnosia and the hippocampus.
Two Main Classifications of Prosopagnosia
Congenital Prosopagnosia is a neurological developmental disorder in which the necessary anatomical structures for facial recognition do not developing properly. Researchers do not know the exact reason as to why congenital prosopagnosia occurs, however they have come to a few possible reasons to help explain why congenital prosopagnosia might occur. 
4 Main hypotheses on cause of congenital prosopagnosia:
Researchers believe that prosopagnosia has a strong genetic influence. Patients with developmental prosopagnosia tend to have first- degree relatives that also have prosopagnosia showing there may be an autosomal component involved. In addition, twin studies have been used to compare monozygotic twins and dizygotic twins to compare prosopagnosia genetic prevalence in both children. Twins who were monozygotic with prosopagnosia had a 100% prevalence of prosopagnosia in both twins. In contrast, dizygote twins had a 50% prevalence of both twins having prosopagnosia. This study showed that there may definitely be a genetic component involved in developmental prosopagnosia. 
When children are born, they have an innate mechanism built into them that naturally draws them to faces. In one study, monkeys were denied the ability to see any faces for 6 to 24 months. After the selected amount of months ended, monkeys exposed to faces were still interested in looking at faces despite the initial deprivation of facial stimuli. This theory states that children were born lacking this innate mechanism that causes children to fixate on faces at a young age, thus causing prosopagnosia to occur. 
Early in development there is a chance that negative experiences can hinder a person’s ability to recognize faces. These negative experiences range from injuries that impact any anatomical region involved in facial recognition and visual processing to traumatic experiences such as being isolated from others or institutionalized at a young age. In one study, children who were less than 2 months of age that had to have bilateral cataract removal surgery had a reduced ability to recognize faces as their ability to process spatial differences between different facial features was impacted. Experiential factors tend to affect the holistic integration of visual input as their brain's are not able to associate the information in an efficient manner.
The neurological factors of prosopagnosia are still being researched due to their vast complexity, however a few hypotheses have been created in regards to neurological factors that impact facial recognition. The brains of patients with prosopagnosia generally have a temporal lobe with less volumetric mass than those who do not have prosopagnosia. This could include an underdeveloped fusiform gyrus which would negatively impact a person's ability to recognize faces.
In addition, patients with prosopagnosia tend to have less neural fibers and connections within the ventral stream. 
There are 2 main hypotheses as to why acquired prosopagnosia occurs.
Acquired prosopagnosia can be caused by damage to specific regions of the brain that deal with facial recognition and visual processing. Damage to the inferior occiptiotemporal cortex, or specifically, damage to the fusiform gyrus and the connections surrounding the fusiform gyrus can cause acquired prosopagnosia. The FFA or the facial fusiform area is the main region of the fusiform gyrus in which researchers have found to be most critical in recognizing and processing faces. In addition, diseases such as meningococcal meningitis can also cause problems in facial recognition. 
Connections to Autism and Aspergers:
Though a lot of researchers have found correlations between prosopagnosia and complications of the fusiform gyrus, other implications such as a possible connection to autism and Aspergers are important to analyze when looking at this disorder. People who have autism and Aspergers syndrome tend to avoid social aspects of life. Due to their disinterest in being social, the importance of remembering the faces of the people around them can generally be put low on the level of importance not only in their life, but also in their neural structures. People who do not wish to recognize faces or do not see it’s importance can lower activity in the fusiform gyrus and thus cause some prosopagnosia symptoms to occur. 
Research on acquired Prosopagnosia is still under way, as the above hypotheses are not valid for every case of acquired prosopagnosia.
Signs of Prosopagnosia
Patients with Prosopagnosia will not only have difficulty recognizing faces, but they will have trouble with social situations. When people with prosopagnosia interact with others it can be difficult for them to engage as they have to constantly figure out, without using facial information, who it is they are talking to. This can be difficult for them and so they tend to be less social for fear of embarrassing themselves in front of their peers when addressing them. In addition to withdrawing from people and social situations, they may feel guilty about not being able to recognize people even though they have met many times beforehand. This can cause a lot of anxiety and frustration for people with prosopagnosia.
There is currently no cure for prosopagnosia and so generally treatment involves giving them some tangible strategies in order to help them deal with their symptoms. Patients can learn how to rely on auditory information and gait patterns of those they are trying to remember. Additionally, memorizing clothing and hair styles of those closest to the person with prosopagnosia can help them to remember who they are interacting with. In addition, semantic information, or the context in which people have met or are used to seeing each other, can be helpful in recognizing others. 
Final Remarks on Prosopagnosia
In conclusion, congenital and acquired prosopagnosia are impairments in a persons’ ability to recognize faces. Congenital prosopagnosia generally occurs from conception to the early stages of development while acquired prosopagnosia generally occurs later in life. Though a lot of research has been conducted in order to figure out the exact causes of prosopagnosia, there are still a lot of unknowns that need to be investigated. In order to help patients who are struggling with prosopagnosia, teaching them how to analyze auditory information, gait patterns and other helpful tricks can help them to recognize those they wish to remember.  Prosopagnosia is a disease that negatively affects the lives of those who have it and so it is important to continue researching this disorder.
The above video is a real life example of someone with Prosopagnosia.
The above video is another real life example of someone with Prosopagnosia.
The above video offers some strategies or ways of coping with Prosopagnosia by Oliver Sacks.
Glossary of Important Terms:
Denoting a disease, predisposition or abnormality that is not inherited.
A form of autism spectrum disorder that is less severe than other forms, characterized by difficulty with social interaction and communication and by repetitive behavior or restricted interests.
Areas of the outer layer of the brain concerned with the integration of sensory and other datawith other aspects of brain function, and the elaboration of them into the complex processes underlying higher mentalfunctions such as language, imagination and creativity.
A complex developmental disorder distinguished by difficulties with social interaction, verbal and nonverbalcommunication, and behavioral problems, including repetitive behaviors and narrow focus of interest.
Existing at birth, referring to certain mental or physical traits, anomalies, malformations, diseases, and like findings,which may be either hereditary or due to an influence occurring during gestation up to the moment of birth.
A relative reduction in magnitude of polarization; in nerve cells, depolarization may result from an increase in thepermeability of the cell membrane to sodium ions.
Relating to twins derived from two separate zygotes, that is, bearing the same genetic relationship as full sibs but sharing a common intrauterine environment.
An increase in polarization of membranes of nerves or muscle cells; the reverse change from that associated withexcitatory action.
Twins resulting from one zygote that at an early stage of development separated into independently growing cellaggregations giving rise to two individuals of the same sex and identical genetic constitution.
the structural and functional manifestations of disease.
(also known as the"what pathway") travels to the temporal lobe and is involved with object identification and recognition, in order to plan behavior.
Follow up Questions
True of false:
1.) The two types of prosopagnosia are acquired and recessive.
2.) Prosopagnosia is a disorder characterized by the inability to recognize faces.
3.) Prosopagnosia is commonly present in the autistic population.
4.) Which one of the following statements were not listed as a possible explanation for the prevalence of developmental prosopagnosia?
b. Experiential Factors
c. Innate Mechanisms
d. Neurological factors
e. All of the above are correct statements
5.) Which anatomical structure plays a large role in facial recognition?
b. Fusiform gyrus
d. Frontal cortex
e. None of the above
6.) What treatments are available for people with Prosopagnosia?
b. Neuron replacement surgery
c. Equipping patients with strategies on how to deal with their symptoms
d. Electric stimulation of the hippocampus
Short Answer Questions:
7.) Why is facial recognition processed in the ventral stream?
8.) Describe 3 possible reasons as to why congenital prosopagnosia may occur.
9.) If you have a family member or friend that has prosopagnosia, what can you do to help them adapt to their condition?
7.) Facial recognition is processed in the ventral stream for a few reasons. First, the fusiform gyrus is located in the ventral stream which is the main center for facial recognition. Second, facial recognition requires information regarding shape, form and color rather than movement and so the ventral stream is the most effective system to use when analyzing facial features.
8.) Possible ways in which congenital prosopagnosia is present is due to genetic factors, experiential factors, innate mechanism dysfunction and neurological issues.
9.) If you have a family member or friend who is struggling with prosopagnosia you can help teach them tricks on how to recognize people without facial recognition. You could teach them how to recognize voices, gait patterns or many other strategies to help them cope with their issues.
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Journal Of Neuropsychology
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2. Bate, S. (n.d.). Information About Prosopagnosia. Retrieved November 26, 2016, from
3. Clark, Sean. Neurophysiology Lecture Notes (2016).
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5. DeGutis, J., Cohan, S., Mercado, R. J., Wilmer, J., & Nakayama, K. (2012). Holistic processing of the mouth but not the eyes in developmental prosopagnosia.
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Journal Of Neuroscience
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8. Kress, T., & Daum, I. (2003). Developmental prosopagnosia: A review.
9. Olsen, R. K., Yunjo, L., Kube, J., Rosenbaum, R. S., Grady, C. L., Moscovitch, M., & Ryan, J. D. (2015). The Role of Relational Binding in Item Memory: Evidence from Face Recognition in a Case of Developmental Amnesia.
Journal Of Neuroscience
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10. Rossion, B. (2013). The composite face illusion: A whole window into our understanding of holistic face perception.
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