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Artist's depiction of Prospagnosia-http://t1.gstatic.com/images?q=tbn:ANd9GcRPitc6_gVO9RbE5HQaHI94xa2QixdS2-f4YwYKAOJULj3sOgpECQ
Prosopagnisa is type of agnosia or inability to recognize faces. Those with this disorder are often considered “face blind” because they cannot recognize others on the basis of visual perception. Instead, other cues are utilized such as voice intonation, hair style and clothing.This disorder effects about 2% of the population and can be congenital but it is often acquired by severe head trauma. Its effects are precipitated by lesions of the fusiform gyrus ( medial temporal occipital gyrus) and can produce localized visual system deficits. In this wikispace, I will describe the symptoms of the disorder and also give background about the anatomical structures involved, effects on the visual system and common diagnostic tests.
Prospagnosia is a disease that effects facial memory. Individuals with this condition are able to make non-facial visual discriminations and can often categorize visual stimuli as a face. However, if they are instructed to idenitify a person without the use of voices, clothing or specific attributes, they cannot identify a familiar person (Mayer and Rossion) or even themselves. A study done by Bukach and Caldara found that patients with the disease typically have difficulty recognizing the upper part of the face such as the eye region and instead
the mouth and other external aspects to identify faces.
Figure 1: Areas of the face stimulus used by a brain-damaged prosopagnosic patient (PS) with deficit restricted to faces ( Rossion). PS used mainly the mouth area to recognize faces, as identified by a response classification method. In contrast, normal participants extract diagnostic information to recognize faces from the eyes on average, with a left visual field bias (Caldara).
Overview of the Visual System
Figure2: Visual Pathways-http://t2.gstatic.com/images?q=tbn:ANd9GcRcRzYXxdmpKKxrZIKXxBcVh8xN7fn1S40nd3O6-fk4mzwutx9I
The visual system is a complex organization of fibers that project from the retina to the visual cortex. Information about the external visual field is processed by rod and cone cells that connect to M (motion and spatial perception) type and P (form and color) type ganglion cells. They ultimately join to form the left and right optic nerves. When these fibers reach the optic chiasm they bifurcate; temporal retinal information remains ipsolateral, while nasal retinal information decussates and travels contralaterally.
Fibers then proceed to subcortical regions such as the lateral geniculate nucleus LGN of the thalamus and the superior colliculus. However, most fibers travel through the LGN and terminate in the magnocellular (location of P type cell termination) and parvocellular (location of M type cell termination) layers. The LGN then projects to the primary visual cortex (V1).The primary visual cortex (Brodmann Area 17) processes information by way of two different streams: the ventral stream and dorsal stream. The dorsal stream’s principle role is visually guided prehension without comprehension and the ventral stream is responsible for visual thought, planning and memory offline (Milner 2012).
Visual Processing and Prospagnosia:
Figure 3: The Ventral and Dorsal Streams-http://en.wikipedia.org/wiki/File:Medial_surface_of_cerebral_cortex_-_fusiform_gyrus.png
Acquired prospagnosia involves lesions of the ventromedial regions of the occipitotemporal cortex which encompasses the following regions: the lingual fusiform, parahippocampal gyri, and more anterior aspects of the temporal lobes (Hoffman). These region are significant in prospagnosia because they participate in facial memory. Meadows’ research on The Anatomical Basis of Prospagnosia speaks about the effect of lesions on the visual system and highlights connections between lesions of the occipitotemporal cortex as having significant effects on the deep white matter; particularly the tapetal fibers from the corpus collosum, the inferior longitudinal fasiculus (ILF) and the optic radiations (Meadows). Effects of these lesions are manifested as contralateral quadrantic visual field defects. However, Meadows proposes that lesions of the occipitotemporal cortex are not the only way the disease presents, but that those within the ILF may cause similar symptoms.
Congenital Prosopagnosia (CP)
Individuals with congenital prospagnosia are born with an inability to recognize faces. This form is less common than acquired prosopagnosia (AP) and is not the result of brain damage. Mechanisms and symptoms of CP are not clear but an article by Behrmann and Avidan highlight different types of visual impairments that may be related to CP. They include the following: distingushing a face from non- face stimuli, determining whether two faces are the same or different and recognizing the individual identity of a face. The article also discusses the two different forms of prosopagnosia: apperceptive and associative. Apperceptive describes the lack of an intact perception while the an associative deficit is one of recognition and assessment of meaning. Insight into CP is significant as it allows for a greater understanding of the psychological and neural mechanisms of facial processing.
Figure 4: Example of Benton Facial Recognition Test-http://t3.gstatic.com/images?q=tbn:ANd9GcSzO_vssxGZLYKx_Jf590sNtnvJkIv3-ZqBjEqSh2cJWdfejKUO
The most common diagnostic tool is a matching test called the Benton face recognition test. Subjects are presented with several photographs one of which is called the “target photograph” (depicted in the photo above as A) which serves as the point of reference. Prosopagnostics are evaluated on the following matching tasks: matching a frontal view of the target with an identical photo, matching a frontal view of the target individual with three photos of the target taken from different angles and matching a frontal view taken of the target individual with three photos of the target taken under different lighting conditions (Duchaine). However, there has been some debate about the accuracy of this test because some prosopagnostics have been able to achieve normal scores.
There is currently no cure for prosopagnosia but individuals with this disorder are encouraged to practice facial recognition tricks. Prosopagnostics may have difficulty in social situations as they are not able to remember those they come in contact within unless these people have distinguished external features. Researchers are continuing to study this disease and are especially considering ways of understanding the mechanisms of congenital propagnosia. The video below gives great perspective on the severity of the disease.
Broadman Area 17- the primary visual cortex (striate cortex) is apart of the occipital cortex that receives information from the optic radiations.
Benton Face Test- diagnostic exam for prospagnosia that involves matching faces.
tapetal fibers- fibers connected to the tapetum which is the layer of tissue immediately behing the retina.
corpus collosum- nervous tissue that connects the two cerebral hemispheres allowing communication between right and left side of the brain.
inferior longitudinal fasiculus (ILF)- fibers that allow for connection between the occipital and temporal lobes.
acquired prosopagnosia (AP)- inability to recognize and remember faces due to brain injury.
congenital prospagnosia (CP) - one born with an inability to recognize and remember faces.
1) T/F the posterior fusiform gyrus is involved in prosopagnosia
2) T/F congenital prosopagnosia is less common than acquired prosopagnosia
3) T/F lesions of the occipitotemporal cortex can produce ipsolateral visual system defects
4) T/F Broadmann area 17 is not a major player in the visual system
5) T/F visual system information crosses at the optic chiasm and goes directly to the primary visual cortex
6) Describe the difference between congenital prospagnosia and acquired prosopagnosia.
7) What are the symptoms of prosopagnosia? Why is this disorder so difficult to deal with?
8) Describe the effect of prosopagnosia on visual processing.
Solutions to T/F. Solutions to 6-8 can be found within the wiki.
Prosopagnosia General overview.pdf
gives information on possible mechanisms of prosopagnosia
great website to find links to other scholarly articles!
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