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Posted by alexandra_k on December 23, 2004, at 16:18:22

Perceptual Versus Experiential Anomaly.

Davies et al. (2002 pp. 150-151) go on to consider that the rather than the general sort of anomalous experience that they outlined earlier that was similar to Maher’s,

Suppose, on the other hand, that the patient’s unusual experience represents the situation as follows: “This is someone who looks just like my close relative but is not really her/him.” If the delusional hypothesis is already part of the representational content of the patient’s perception, then the route to a delusional belief involves nothing more than accepting the perception as veridical.

Such a rich content attribution would seem to bridge the gap between anomalous experience and delusional belief to the extent that all that is required is for the delusional subject to accept their anomalous experience to be veridical in much the same way that subjects with normal experiences do. Davies et al. consider the relevant anomalous experience to be a perceptual experience, however, and they go on to consider that the delusional subject accepts this perceptual experience as veridical despite rational grounds to doubt. By considering there to be rational grounds to doubt such a move from perception to belief would seem to constitute a second factor in the sense that normal subjects are able to inhibit this response whereas delusional subjects are unable to do this.

By considering the relevant anomalous experience to be a perceptual experience Davies et al. are led straight into the problem of the unwanted prediction. If the relevant kind of anomalous experience is any sort of erroneous perception then it would indeed follow from Davies et al.’s account that the delusional subject should accept an illusory visual perception as veridical. It would seem to me, however, that the relevant kind of anomalous experience for delusions of misidentification is not so much a perceptual experience – as in prosopagnosia – so much as an experience that arises from a disconnection between perception and appropriate affective response.

Hohwy & Rosenberg (forthcoming) maintain that instead of intensity and duration being the crucial difference between delusional and non-delusional subjects, as Maher maintained, delusions are a function of the recurrence of the anomalous experience in the face of no alternative ways to reality test. Typically we can reality test the information provided by one sensory modality with information provided by another. We can check things we hear with what we can see, and what we can see with what we can touch. They consider that the nature of the anomalous experience of delusional subjects is one that is not able to be tested by any alternative sense modality. In the case of the Capgras delusion, for example, the lack of affective response may be considered to provide information that is not duplicated by any other available mechanism. One might want to maintain that the remaining pathway would be enough for reality testing; however, this does not seem to be the case. I would like to suggest that the reason for this is that the affective response system takes priority because of the evolutionary advantage of monitoring for strangers and possible threat potential. In this case the affective response system would take priority or precedence.

What this line on delusions requires, however, is that delusions are not inferences that are called on to explain why it is that the subject has such an anomalous experience, as Maher maintained. Rather the delusional subject may be trying to do one of two things: they may be falsely reporting on the way that things are in the world by accepting certain kinds of anomalous experiences to be veridical (whether or not they have rational grounds to doubt) or alternatively, they may be attempting to express their anomalous experience. If delusional subjects are indeed expressing the nature of their anomalous experience then it may be that the evidence that is typically offered against the delusion misses the point somewhat.

It is as Walkup notes:

>The distinction between a description of the experience (sometimes called a phenomenological description) and the description of the factual state of affairs is scientifically and clinically important. Scientifically, a subject who consistently failed to describe the perception of certain illusions would be suspected of some visual or neurological abnormality. Clinically, the therapist who challenges a patient’s description of his or her experience may sound absurd, just as would a vision researcher who insisted to an experimental subject that the two lines in the Muller-Lyer illusion actually look the same length (Walkup, 1995 p. 326).

If, on the other hand, delusional subjects are falsely reporting on the way that things are in the world by accepting certain kinds of anomalous experience to be veridical then this would not constitute a second factor if it is the case that non-delusional subjects use this very same process to arrive at their beliefs. If Hohwy and Rosenberg’s point is accepted that the subject is unable to reality test with a mechanism that duplicates the information provided by the familiarity mechanism then it would seem that delusional subjects do not have rational grounds to doubt that the person in front of them is unfamiliar to them. As such there would seem to be a single factor, that of a certain kind of anomalous experience that is both necessary and sufficient for a subject to arrive at a delusional belief.




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