| Author: Amedeo Gagliardi e Guglielmo Tosatto |
Prosopagnosia, or face blindness, is a cognitive disorder where the ability to identify and recognize faces is defective, while other aspects of visual processing, such object recognition and decision making are unaltered.
The terms was coined for condition derived by brain damages, but now it refers also to the congenital and hereditary form of the disorder.
The first cases of face blindness were discovered in early 19th century, but the term “Prosopagnosia” was used the first time by Joachim Bodamer, a German neurologist. He described a case of a 24 years old man, with a bullet wound to the head, who can’t recognize friends, family and his own face but can use other clues like voices, tactile to identify those faces.
The term “Prosopagnosia” derive from Ancient Greek πρόσωπον (Prosopon, face) and αγνωσία (Agnosia, non-knowledge) and Face Blindness was used the first time by Bill Choisser in 1996.
Face recognition
Normal face recognition process
Face recognition is a multistage process. First, a face is recognized as such; this stage is often described as detection phase. In a second step, individual and non-individual facial information (emotional expression, eye gaze, gender, age, and health) must be analyzed. Individual face recognition includes a structural encoding and a comparison with stored mental images of faces.
There are three models that describe the facial recognition:
• The ‘classic’ functional face recognition model of Bruce and Young (1986) depicts this process. It does not connect the tasks involved with any cerebral structures though.
• A later model by Ellis and Lewis assumes conscious face recognition and the generation of a feeling of familiarity to be processed by separate modules. This dissociation can be demonstrated in some cases of Capgras delusion, a rare psychiatric condition where the patients are convinced that close relatives are replaced by identically looking imposters. Ellis and Lewis assume that in the Capgras delusion the conscious face recognition is preserved but no longer triggers the feeling of familiarity.
• Based on functional imaging research, Gobbini and Haxby (2007) proposed a face recognition model which ties functional units to certain brain regions. They distinguish between a core and an extended system.
The core system comprises the occipital face area (OFA) in the inferior occipital lobe, the fusiform face area (FFA) in the middle fusiform gyrus, and the face area in the dorsal superior temporal sulcus (STS). In several fMRI studies, these areas showed an enhanced activation for faces as compared with other visual objects. The STS is supposed to process dynamic facial information (expression, eye gaze, and facial speech) while the OFA and FFA focus on invariant facial features for identification.
The core system encodes the visual appearance of familiar faces while the extended system provides the person’s information and the emotional response.
Dysfunctions of face recognition
Any neural tissue damage in the face recognition network of the brain can cause Face blindness.
Prosopagnosia can be caused by lesions in various parts of the inferior occipital areas (occipital face area, OFA), fusiform gyrus (fusiform face area, FFA), and the anterior temporal cortex.
Positron emission topography (PET) and fMRI scans have shown that in individuals without prosopagnosia, these areas are activated specifically in response to face stimuli
A prosopagnosia caused by an accident or stroke in adulthood is mostly called ‘acquired’ prosopagnosia or simply ‘prosopagnosia’.
Prosopagnosia can be also inherited or acquired by an early brain tissue damage (Congenital).
Figure 3. Brain area damaged.
Acquired prosopagnosia
It can be defined as a dissociation of face and object recognition impairment to the disadvantage of face processing. The individual range of impairments depends on the extent and the location of the damage, causing a unique pattern of symptoms in each patient. An acquired pure prosopagnosia is rare, only very few cases have been published.
The widely differing patterns of impairment make it difficult to identify the face-processing brain regions.
In a comprehensive meta-study of 100 cases of acquired prosopagnosia, Bouvier and Engel
(2006) found a high lesion overlap near the OFA while the FFA and the STS were less often affected. Instead, they identified an area medial to the FFA which was damaged in a number of cases.
Acquired prosopagnosia can develop as the result of several neurologically damaging causes. Vascular causes of prosopagnosia include posterior cerebral artery infarcts (PCAIs) and hemorrhages in the infero-medial part of the temporo-occipital area. These can be either bilateral or unilateral, but if they are unilateral, they are almost always in the right hemisphere. Recent studies have confirmed that right hemisphere damage to the specific temporo-occipital areas mentioned above is sufficient to induce prosopagnosia. MRI scans of patients with prosopagnosia showed lesions isolated to the right hemisphere, while fMRI scans showed that the left hemisphere was functioning normally. Unilateral left temporo-occipital lesions result in object agnosia, but spare face recognition processes, although a few cases have been documented where left unilateral damage resulted in prosopagnosia.
Hier, Mondlock, and Caplan (1983) examined 41 patients within 7 days after a right-sided stroke and diagnosed prosopagnosia in 19 of them, mostly accompanied by more dramatic symptoms like visual field defect, hemineglect, anosognosia (denial of illness) or weakness of arm and leg. Therefore, a temporary prosopagnosia after a right-sided stroke may well be hidden by more apparent symptoms and not be recorded at all.
In some cases of acquired prosopagnosia, a residual autonomic (covert) response to familiar faces has been reported. Some authors therefore postulated a second pathway for the covert, unconscious recognition of faces.
Congenital and hereditary prosopagnosia
McConachie was the first to report a case of prosopagnosia without any history of brain tissue damage. Twenty years later, Ariel and Sadeh (1996) published a second case. By 2002, less than a dozen cases of congenital prosopagnosia were published worldwide, three of these had been found in one family.
A study investigated the relatives of people with a congenital prosopagnosia and found seven pedigrees with 38 cases of congenital prosopagnosia (Grueter, 2004; Grueter et al., 2007).
All pedigrees were in accordance with a simple autosomal dominant mode of Inheritance. This indicates that the hereditary type is quite frequent if not prevailing among the cases of congenital prosopagnosia. It should be noted that hereditary prosopagnosia is the first identified hereditary disorder to affect the central visual cognition system. The familial segregation of hereditary prosopagnosia in a simple autosomal dominant mode suggests that it may be caused by a single mutation in one or more genes (point mutation). This would imply that at least within families, but probably also across families, the underlying defect is the same. Until now, the responsible gene has not been identified yet.
In hereditary prosopagnosia, there is no subjective or objective perceptual deficit in the recognition of facial emotions. While in acquired prosopagnosia, a central achromatopsia and a quadrantanopsia (loss of vision in one quarter of the visual field) is quite frequent, it has not been observed in hereditary prosopagnosia.
The affected persons also reported that their judgments of attractiveness and gender are not different from other people. They had no problems to recall semantic information about persons and recognized other people easily from non-facial clues.
Another functional signature of face processing is an EEG component called the N170, which shows a larger response to faces than nonfaces. Some individuals with DP showed N170 with normal face selectivity whereas others did not. What lies behind these mixed results is unclear, but is likely related to the heterogeneity of DP. The existence of face-selective regions and N170 in DP may, however, mask subtle impairments. For example, case C exhibited normal face-selective regions, but the regions did not show repetition suppression (i.e. reduction in fMRI response to repeated stimuli). Similarly, an EEG study of 16 DP individuals found normal face-selectivity for the N170 at the group level but observed that the N170 component was not enhanced for inverted faces as it was in controls. Extending previous reports, this study suggests that individuals with DP process upright and inverted faces similarly. These examples illustrate that studies of neural functions in DP may benefit from not only examining the existence of signatures of face processing, but also whether they exhibit the properties characteristic of normal face recognition.
Structural correlates of DP have also been identified. A diffusion tensor imaging study found reduced connectivity in two major tracts that project through the fusiform region to more anterior areas, indicating abnormal integrity of white matter in ventral cortex. Other investigations using voxel-based morphometry found gray matter reduction in cortical regions implicated in face processing including fusiform gyrus, inferior temporal gyrus, and superior temporal sulcus.
In sum, neural studies have begun to characterize functional and structural correlates of DP. An important next step will be to map particular neural correlates onto specific cognitive deficits. This step is challenging because the relationship between neural and cognitive mechanisms in face processing remains unclear. Despite substantial effort, attempts to localize specific cognitive operations onto focal neural regions have met with limited success.
In an fMRI study with four congenital prosopagnosics, wasn’t possible to demonstrate conclusive differences in the anatomy or in the FFA activation to faces.
A recent structural imaging study by Behrmann, Avidan, Gao, and Black (2007) with six prosopagnosics found them to have a significantly smaller anterior fusiform gyrus, though.
Comparison of hereditary and acquired prosopagnosia types
The hereditary and acquired types have a completely different etiology.
The acquired type is the consequence of a localized tissue damage, while the hereditary type is more probably caused by a defective neural development. As most cases of acquired prosopagnosia are caused by occipito-temporal tissue losses, adjacent anatomical structures are quite often damaged as well, though they are not functionally linked to face recognition. This explains the frequent coincidence of acquired central achromatopsia and prosopagnosia.
While emotion recognition is always spared in hereditary prosopagnosia, it is frequently defective in acquired prosopagnosia. Hereditary prosopagnosics show a frequent impairment in the visual recognition of objects and scenes, indicating a general impairment in visual recognition most prominent in face recognition. While acquired prosopagnosia causes a loss of familiarity feeling, people with hereditary prosopagnosia cannot judge familiarity, all faces seem vaguely familiar or vaguely unfamiliar.
SYMPTOMS
Though the affected people have a lifetime to compensate for their deficit, a careful interview conducted by Güter group showed a set of typical symptoms.
The test results confirmed the diagnosis of hereditary prosopagnosia in each case without exception.
The leading symptom of hereditary prosopagnosia is a lack of confidence about the familiarity of faces. The affected people do not complain that all faces look unfamiliar, but that they cannot determine the familiarity on a valid basis. Therefore, they not only overlook familiar people, but also confuse strangers with familiar persons (T. Grueter & Grueter, 2007).
This symptom is found throughout and therefore should be regarded as a diagnostic hallmark.
Nearly, all of them find gaze contacts unnecessary for social communication – as did 8 out of 56 controls. Eye-tracking experiments showed a peculiar scan pattern for faces in hereditary prosopagnosia. The gaze was more dispersed and more directed to external facial features.
The development of an adaptive behavioral pattern (avoiding critical situations, a ready set of excuses, etc.) indicates a long-standing perceptual deficit. Nearly, all hereditary prosopagnosics admitted problems with the visual recognition of objects and scenes.
The affected persons also reported that their judgments of attractiveness and gender are not different from other people. They had no problems to recall semantic information about persons and recognized other people easily from non-facial clues.
DIAGNOSIS
To asses prosopagnosia diagnoses some neurological and cognition tests were developed.
Until now there are three main tests used:
• Famous faces tests: individuals are asked to recognize the faces of famous persons. The problem is that this test is difficult to standardize.
• The Benton Facial Recognition Test (BFRT): another test used by neuropsychologists to assess face recognition skills. Individuals are presented with a target face above six test faces and they have to identify which test face matches the target face. The images are modified to eliminate hair and clothes, as many people with prosopagnosia use hair and clothing clues to recognize faces.
• The Cambridge Face Memory Test (CFMT): a new test developed by Duchaine and Nakayama to better diagnose people with prosopagnosia. This test initially presents individuals with three images each of six different target faces. They are then presented with many three-image series, which contain one image of a target face and two distracters. Duchaine and Nakayama showed that the CFMT is more accurate and efficient than previous tests in diagnosing patients with prosopagnosia.
TERAPHY
There is currently no cure for Prosopagnosia and there are few successful therapies. However, it is possible to manage Prosopagnosia through the use of various alternative cues, as previously mentioned.
With further research into these cues used by Prosopagnosics, it is possible to gain a greater understanding of how normal people recognize faces with respect to what kind of information processes are particularly important. Additional studies into the types of Prosopagnosia will generate better understanding of the locations and critical roles of brain areas involved in normal face perception, recognition and individuation. Hopefully such information will lead to treatments and cures of Prosopagnosia in the future.
BIBLIOGRAPHY
Neural and genetic foundations of face recognition and prosopagnosia. 2008
Advances in developmental prosopagnosia research. 2013
WEBOGRAPHY
Faceblind.org
