Saturday, 5 April 2014

Explaining Delusions (6)

This a response to Phil Corlett's contribution to the blog, posted on behalf of Max Coltheart.


Max Coltheart
Dear Phil

Let’s focus for the moment on the best-studied monothematic delusion, Capgras delusion, and let me ask you two questions so that we can decide whether your account differs from ours.

First question: there are 3 studies of autonomic responding to familiar faces in patients with Capgras delusion, and all showed that these deluded patients don’t show greater response when faces are familiar than when they are not, and general show weak faces. Would you agree that this abnormality is not a coincidence, but instead plays a causal role in the delusion? And if your answer is Yes, what do you see this causal role as being? (Our answer to this question: the absence of autonomic response to the face of a spouse is unexpected i.e. unpredicted, and that triggers a search for an explanation of the prediction error, which takes the form of a candidate belief).

Second question: if you do agree that this absence of autonomic responding to familiar faces is causally implicated in Capgras delusion, would you agree that it can’t be sufficient for the delusion to occur, since the same absence is seen in patients with ventromedial frontal damage, and yet these patients do not exhibit Capgras delusion? If your answer is Yes, does that not imply that there must be a second impairment present for the delusion to occur?

It is true as you pointed out that no one has shown the predicted autonomic impairments that Ramachandran and Blakelee speculated to be involved in Fregoli and Cotard. But no one has looked for these, and so there is no evidence that they do not exist; there is just no evidence either way at present.

With respect to your point about the third factor, generation of candidate explanations: for us “factor” means “impairment”, and the generation of candidate explanation is not a process that requires any impairment for it to occur. It is one of the processes of belief formation and evaluation that, in our theory, is intact in patients with monothematic delusion. That’s why we would not refer to this as a third factor. There may well be patients in which the generation of candidate explanations is impaired. But we consider this process to be intact in patients with monothematic delusion.

Best,
Max

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