The term, agnosia, stems from the latin word for “no knowledge”. Agnosia is a failure to understand, or recognise a specific area be it text (Alexia), music (Amusia) or in this case, faces (Prosopagnosia).
To understand facial agnosia (which from this point on shall be referred to as prosopagnosia), it is important to get a grasp of visual agnosia as a whole. Visual agnosia is a deficit in perception as a result of damage to the brain, for instance, a person suffering from visual agnosia may not be able to distinguish between different animals as illustrated by the study by Macrae and Trolle (1956). It is caused by damage to parts of the visual association cortex which contribute to the visual stream. Research has found that a large region of the ventral stream of the visual association cortex, named the lateral occipital cortex, responds to a variety of shapes and objects. Damage to this area in particular can result in visual agnosia.
Prosopagnosia (from the Greek prosopon, meaning “face”) is a form of visual agnosia that is exclusive to faces, those suffering from prosopagnosia are unable to recognise the faces of their friends, family or even themselves.
Buxbaum, Glosser and Coslett (1999) studied a patient titled WB, WB was fully capable of recognising objects but was unable to distinguish between faces. This study provides evidence for the possibility that prosopagnosia is a result of damage to a specific region exclusive to the identification of faces. The work of Grill-Spector, Knouf and Kanwisher (2004) found evidence suggesting just this. They carried out research into the fusiform-face area which is found in fusiform gyrus on the base of the temporal lobe. Grill-Spector et al used functional MRI scans to look at the activation of regions of the brain when participants carried out tasks which involved identifying faces. They found much greater activation in the fusiform-face area when participants carried out this task suggesting that there is a region of the brain responsible for the identification of faces. Although the cause of prosopagnosia has not conclusively been identified, it is highly likely that prosopagnosia is caused by damage to the fusiform-face area.
For more information of visual agnosia, it is advisable to look at The Man Who Mistook His Wife For a Hat by Oliver Sacks, although it is written by a neurologist and highly neurological and psychology in nature, it is a very easy to understand insight into various neurological phenomena.
References
Buxbaum, L.J., Glosser, G. & Coslett, H.B. (1999). Impaired face and word recognition without object agnosia. Neuropsychologia, 37, 41-50.
Carlson, N.R. (Eds.). (2009). Physiology of behavior. Boston: Pearson education.
Grill-Spector, K., Knouf, N. & Kanwisher, N. (2004). The fusiform face area subserves face perception, not generic within-category identification. Nature neuroscience, 7, 555-562.
