Posted by alexandra_k on December 21, 2004, at 1:09:07
Physiological Findings and Delusions of Misidentification
The Capgras and Frégoli delusions are different kinds of delusions of mis-identification. In the Capgras delusion the subject seems to be mis-identifying someone they were previously close to such as a husband, wife, or child by maintaining that they have been replaced by an impostor, robot, or clone. In the Frégoli delusion the subject would seem to be mis-identifying strangers for people who are familiar to them when they maintain that people they know are disguising themselves as strangers and are following them around. It would seem plausible to consider that delusions of mis-identification may arise from a difficulty with processing perceptual information that would normally enable subjects to recognize people they know.
Interestingly, it has been found that subjects who have developed the Capgras delusion in response to cerebral trauma do process information regarding faces who are familiar to them differently from non-delusional controls, though perhaps in an unexpected way. While non-delusional subjects produce a heightened skin galvanization response to familiar faces as opposed to the faces of strangers, subjects with the Capgras delusion have been found to lack such a response, or to display a response that is significantly diminished compared to the responses of non delusional controls (Bruyer, 1991; Young and de Haan, 1992 and Young, 1994 in Stone and Young, 1997 p. 337).Subjects with the neurological condition of prosopagnosia have been found to display the usual heightened skin galvanization response to familiar faces. They are able to report that the faces seem familiar to them as normal subjects do, but they are unable to identify the face, or recall biographical information pertaining to the face that they have been shown. Ellis and Young (1990) consider these findings to provide some support for their thesis that there are two dissociable pathways involved in face recognition; a perceptual pathway, and an affective one. They maintain that these two pathways should be a part of a cognitive model of face recognition and they also consider these pathways to be realized in the brain on the dorsal and ventral routes. Breen et al., (2000) have critiqued the notion that the two cognitive pathways are realized on the dorsal and ventral routes, but they also maintain that the dual mode cognitive model could be realized on a single neural pathway. This does not seem to disrupt the cognitive model of face recognition; it just calls into question issues around how the model is realized on the neural wetware of the brain.
Ellis and Young (1990) link the perceptual pathway to the subject’s ability to recall and verbalize information pertaining to the face of the person they have been shown, such as biographical information, and the person’s name. This is the pathway that is damaged in subjects with prosopagnosia. Ellis and Young, (1990) do not explicitly consider the function of the affective pathway, except to postulate that its malfunction is responsible for the production of the anomalous experience that features in the Capgras delusion.
It still needs to be explained how such a lack of normal affective response to familiar faces is relevant to an explanation of the Capgras delusion. Although normal subjects are not typically aware of producing a heightened skin galvanization response to familiar faces and not to strangers it is thought that the lack of normal response would trigger consciously experienced ‘alarm bells’ that would serve to signal to the subject that something is wrong. It would seem plausible to consider that prior to head injury the delusional subject would have produced the greatest affective response to people who were close to them such as a wife, husband, or child. Post head trauma the difference between the response that should have occurred and the response that does occur would thus be the most anomalous for their loved ones. These findings would also seem relevant to an explanation of the Frégoli delusion if, for example, it was found that subjects with this delusion produced a heightened skin galvanization response to strangers. While this has not been empirically tested it would seem to be a plausible hypothesis in light of the findings of the physiological responses of subjects with the Capgras delusion. If it was found that subjects with the Frégoli delusion do produce a heightened response to at least some strangers then this would seem to go some of the way towards explaining why they say that strangers are people who are familiar to them.
poster:alexandra_k
thread:432333
URL: http://www.dr-bob.org/babble/write/20041210/msgs/432333.html