Following brain damage (trauma, stroke ...), some people are left with an inability to recognize familiar faces and encode new faces in memory, while their visual system appears largely preserved, and they can recognize people through their voice for instance. This condition was termed prosopagnosia by Bodamer (1947) and first described by Wigan in 1844, and then Quaglino & Borelli in 1867 (paper translated by Ellis & Florence, 1990).

Cases of acquired prosopagnosia with preserved visual functions are extermely rare, and most cases have also large deficits are object recognition. In fact, the question of whether prosopagnosia can be 'pure' (without object recognition impairments) is still a matter of debate for some authors, even though there are a few cases described in the literature who seem to have normal object recognition.

Studying cases of prosopagnosia can be particularly interesting, for two reasons:

1. They can help us understanding better the location and the critical role(s) of the brain areas involved in normal face recognition (see Rossion et al., 2003; Schiltz et al., 2006).

2. The kind of visual cues that they still can or cannot extract and remember on faces may help us understanding better how normal people recognize faces (what kind of information processes are particularly important). (see Caldara et al., 2005).

Because the patients are extremely rare and all have different associated impairments, we advocate the detailed investigations of single-cases of prosopagnosia rather than group studies of prosopagnosics.

For instance, we have been studying a fascinating case of prosopagnosia, PS, who sustained brain damage in 1992 at the age of 41. She has recovered all neuropsychological functions but has a massive prosopagnosia (for a paper in French describing PS's story and summarizing our results, see here). Unlike other cases of prosopagnosia that we have been studying, such as NS (Delvenne et al., 2004), PS recognizes objects normally, including living things. She can even discriminate visually similar objects at the individual level (e.g. discriminating two pictures of cars or birds) like normal people, at a normal speed (see Schiltz et al., 2006). Unlike many prosopagnosic patients, she is not achromatopsic either, nor does she suffer from topographical agnosia. She has a left paraccentral scotoma and a lower normal range color discrimination, but these cannot account for her prosopagnosia.

By presenting PS with little information on face pictures sampled randomly over hundreds of trials ('Bubbles', Gosselin & Schyns, 2001), we have recenlty shown that she suffered from a deficit at representing information at the level of the eyes of the faces, and mostly relied on on the mouth and the external contour (Caldara et al., 2005). We believe that this loss of ability to extract diagnostic information at the level of the upper part of the face, in particular the eye region, may be a dominant feature of prosopagnosia, reflecting an destruction of what constitutes our most developped skill for processing faces. Our most recent work using eye movement recordings support this view, showing that PS fixates mainly on the mouth during identification of personnally familiar faces (Orban de Xivry et al., 2008).

At the neural level, our fMRI studies of PS have shown that her main lesion in the right hemisphere destroyed part of the inferior occipital cortex, where preferential processing for faces is usually observed in neuroimaging studies of normal subjects ('occipital face area', 'OFA'), and which appears to be the region most often damaged in cases of prosopagnosia (see Bouvier & Engel, 2006). However, she has a preserved right fusiform gyrus and preferential 'FFA' activation for faces (Rossion et al., 2003), suggesting that in the normal brain the 'FFA' is not dependent on inputs from the posteriorly located 'OFA' in a feedforward model for face processing. Nevertheless, recent investigations show that PS's 'FFA' does not discriminate between individual faces, in line with the behavior of the patient (Schiltz et al., 2006).

Our current research on the neuro-functional basis of prosopagnosia gives clues about the reasons why PS's deficit is specific for that category: while her main lesion in the right hemisphere destroyed part of the inferior occipital cortex, the lesion spared the ventral and dorsal part of the lateral occipital complex (LOC), as well as parahippocampal areas, where recognition of non-face objects may take place (Sorger et al., 2007; Dricot et al., 2008). That the ventral LOC in the right hemisphere is structurally and functionnally intact is amazing, given the localization of the right hemisphere lesion.

For a review on prosopagnosia with an emphasis on PS's case, see Mayer & Rossion, 2007, or Rossion (2008): La reconnaissance des visages (Cerveau & Psycho, N°25).