This youtube clip shows an experiment with a woman with prosopagnosia:

http://www.youtube.com/watch?v=vwCrxomPbtY

Renzi proposes two different types of Prosopagnosia: apperceptive prosopagnosia and associative prosopagnosia (McNeil et al. 1993). People suffering from apperceptive prosopagnosia have trouble perceiving the differences between faces and they cannot distinguish between factors such as ages or gender (Dan, 2007). Associative prosopagnosia is less severe, and people suffering this syndrome can recognize faces but have trouble identifying someone. However, this classification is still under debate.

As Mcneil et al. discuss in their paper (1993), there are different ways in which prosopagnosia disembosoms: there was for example a farmer that could not recognize human faces but was able to recognize his own cows. On the other hand, there is also an example of a man that could recognize faces, but not his own cows.

In order to understand this syndrome, knowledge about the way we recognize and perceive faces is necessary. Bruce and Young’s model for face recognition, made in 1986, makes a distinction between two different neurological systems: one for expressions and one for recognition (Calder & Young, 2005). However, Calder & Young (2005) argue that many people with prosopagnosia have trouble identifying faces, but have an impaired expression-recognition as well. They argue that: “much of the evidence for impaired facial identity with intact facial expression recognition comes from studies in which the cause of the identity impairments has not been established” (Calder & Young, 2005).

   This model from Calder & Young (2005) analyzes which parts of the brain contribute to face perception:

So, how can we learn more about face recognition and prosoapagnosia? According to Calder & Young (2005), functional imaging studies are inconsistent and has not been researched extensively. Williams et al. (2007) conducted a research with an fMRI scan on woman with prosopagnosia, which did not result from an acquired brain damage and their research suggests that the fusiform face area cannot “maintain stable representations of new faces”.

Concluding, it is hard to identify what prosopagnosia exactly is and what parts of the brain are involved. However, the resolution of fMRI scans is improving and further fMRI research might give a clearer view on how we recognize faces and why some people are unable to do this.  This interesting syndrom shows how limited the knowledge is on the cognition of faces and how much there is yet to learn.