Continuing my theme of attempting easily digestible write ups of my papers here are some thoughts on the delusion somatoparaphrenia…
If I know anything at all surely I know that my hand is my hand and not someone else’s. Actually on that, I remember once as an undergrad, when getting frustrated in an analytic epistemology (trying to define knowledge) class, declaring that if there is knowledge at all it only exists in a form distributed across scientific communities (guess what class I did the previous semester). The tutor, a logician, made something like the above claim – pointing out that my view meant that I couldn’t know I had two hands. I was more than happy to bite that bullet at the time, these days I just think that knowledge isn’t justified, true, belief. Anyway that’s not the point… surely I know my hands are my hands. How could anyone be wrong about this? My hand just feels like it’s a part of me and I can do stuff with it – in a noticeably different way to how I can do things with yours.
I don’t think reflections like this tell us much in regards to developing a theory of knowledge, but my tutor was hitting on something interesting- we just know that our hands our ours and it would seem to be a catastrophic state of affairs were we to be wrong about this.
Indeed it is, but sometimes people don’t know one of their hands is their own and do think it is someone else’s. Such people have hands, but they say that one of them (usually the left) isn’t their own- they say it is someone else’s, or sometimes that it is a dead thing (Sacks, 1986) or give it its own personality (Feinberg & Keenan, 2005). Such people are usually considered to be suffering from a delusion, specifically a delusion called ‘somatoparaphrenia’- that said, for those who enjoy such things, there is plenty of work to be done to try a dictate a consistent set of definitions of ‘somatoparaphrenia’ and the sometimes interchangeable, sometimes not, ‘asomatognosia’ and ‘alien hand’. Let’s put nomenclature aside and ask about what’s really weird here: how can such patients be wrong about who their own hand (arm, foot) belongs to?
The option which I think is most likely is that they don’t experience the hand (arm, foot- you get the point) in the usual way. Specifically the hand doesn’t feel like it is a part of them anymore. This feeling that a body part is a part of oneself I usually call ‘the sense of embodiment’ (others, for reasons which needn’t detain us here, prefer ‘ownership’). I’ll hypothesise then that the delusion of somatoparaphrenia arises, in part, from a deficit in eliciting the sense of embodiment for the hand.
As with the other disorders of self-consciousness we’ve been talking about on here this problem arises in the context of severe brain damage- usually stroke- and is typically accompanied by a variety of other symptoms. One very important accompanying symptom is that paralysis. The hand which the patient attributes to another is paralysed; although hypnotic analogues may hold whilst subjects move the misattributed hand (Rahmanovic, Barnier, Cox, Langdon, & Coltheart, 2012) and it’s unclear whether or not patients sometimes experiencing moving supernumerary limbs (i.e. illusory extra limbs- something like the more well-known ‘phantom limb’) (Vallar & Ronchi, 2009). The delusion is also typically accompanied by an inability to attend to the left hand side of space and the body (various forms of hemi-spatial neglect). This is interesting as it may suggest that such patients don’t suffer a disorder of self-consciousness, but instead are unable to attend to their left hand. If so then the reports from patients that the hand is not their own could be a confabulation provoked by the examiners questions. There are a few facts which speak against such an account. First, there have been cases of somatoparaphrenia without spatial neglect and vice versa. Second, some patients apparently spontaneously act against the limb which they believe is not their own, trying to get rid of it even when not being asked about it (Sacks, 1986). Third, moving the hand into space (i.e. the right hand side of the patient’s body) doesn’t change the patient’s reports (Moro, Zampini, & Aglioti, 2004). Fourth, patients suffering spatial neglect will neglect the entire left side of space, but often only ‘disown’ one body part, e.g. the hand or leg.
Attempting to explain somatoparaphrenia in terms of a deficit in self-consciousness, specifically the sense of embodiment, doesn’t face these problems. Moreover if it is right that this delusion arises from a deficit in the sense of embodiment then contributing to an explanation of somatoparaphrenia is a good test ground for theories of self-consciousness.
Frederique de Vignemont realised this in developing her theory of the sense of embodiment (she says ‘ownership’- but we are talking about the same thing). In what is my favourite piece of philosophy (not because it’s right but because it’s an example of the depth and explanatory power of great philosophy) she argues that the sense of embodiment arises from the localisation of bodily sensations on a map of the body which is a part of the body schema (de Vignemont, 2007). Just what this means requires a little unpacking. Roughly her hypothesis is that we get a sense of embodiment when we represent bodily sensations at a particular location on the body. To specify this location requires a representation of the body. The specific representation which does this job is thought to be a part of the body schema, which means that it is the same body representation which is used to control motor actions performed with, and directed toward, the body (de Vignemont, 2007, 2010; Gallagher, 2005). With regards to somatoparaphrenia de Vignemont hypothesises that such patients have a deficit in their sense of embodiment due to a deficit in this body representation. Without this body schema representation sensations cannot be localised and so no sense of embodiment can be elicited.
From this we can derive a testable prediction. If the loss of the sense of embodiment is due to a deficit which prevents bodily sensations being localised then patients suffering somatoparaphrenia should show other signs of the deficit. For example, if they don’t represent where they are being touched then they shouldn’t be able to point to where they are touched.
There is something especially joyous about a philosophical hypothesis generating such a specific prediction – it’s a nice practicing what you preach moment. That’s not to say it’s easy to test… I know of more than one PhD submission being substantially delayed because of the belief that it would be possible to find patients suffering from one or other delusion… So, whilst it’s a shame, it’s hardly a surprising shame that this prediction hasn’t been directly tested. But all is not lost; there is suggestive evidence which will do for a blog post.
Meador and colleagues (2000) found that using what’s called the Wada test they could temporarily induce somatoparaphrenia. The Wada test involves the use of chemicals to deactivate one hemisphere of the brain – sounds horrible, but the point is to assess language dominance (i.e. find which hemisphere contributes the most to language production and comprehension) before brain surgery. During such a procedure Meador and colleagues asked the patients about their hand which had become paralysed. When asked directly as many as 82% of patients claimed that there now paralysed arm/hand was not their own, instead attributing it to the experimenter present. Despite this around half of those showing this temporary somatoparaphrenia were able to accurately point to their hand, the remainder pointing to where the hand had been held in the air prior to deactivation. It seems then that somatoparaphrenia is not necessarily related to an inability to point to the denied hand. This suggests that de Vignemont’s prediction may not be borne out, but more than this the fact such patients can point to where their hand is suggests that the body schema representations of that hand remain intact. It seems unlikely then that deficits in representations which are part of the body schema causes deficits in the sense of embodiment and somatoparaphrenia.
That said I do think there is something right about the idea that there is a problem with a way in which these patients represent their bodies, so, next time out let’s have another look at what types of body representation might be related to the sense of embodiment.
With love
For my paper on this see: Carruthers, G. (2009). “Is the body schema sufficient for the sense of embodiment? An alternative to de Vignemont’s model.”Philosophical Psychology 22(2): 123-142
Refs:
De Vignemont, F. (2007). Habeas Corpus: the Sense of Ownership of One’s Own Body. Mind and Language, 22(4), 427–449.
De Vignemont, F. (2010). Body schema and body image- Pros and cons. Neuropsychologia, 48, 669–680.
Feinberg, T. E., & Keenan, J. P. (2005). Where in the brain is the Self? Consciousness and Cognition, 14, 661–678.
Gallagher, S. (2005). how the body shapes the mind. Oxford University Press.
Meador, K., Loring, D., Feinberg, T., Lee, G., & Nichols, M. (2000). Anosognosia and asomatognosia during intracarotid amobarbital inactivation. Neurology,55(6).
Moro, V., Zampini, M., & Aglioti, S. (2004). Changes in spatial position of hands modify tactile extinction but not disownership of contralesional hand in two right brain-damaged patients. Neurocase, 10(6), 436–443.
Rahmanovic, A., Barnier, A. J., Cox, R. E., Langdon, R. A., & Coltheart, M. (2012). “That’s not my arm”: A hypnotic analogue of somatoparaphrenia. Cognitive Neuropsychiatry, 17(1), 36–63. doi:10.1080/13546805.2011.564925
Sacks, O. (1986). The Man Who Mistook His Wife for a Hat. Picador.
Vallar, G., & Ronchi, R. (2009). Somatoparaphrenia: a body delusion. A review of the neuropsychological literature. Experimental Brain Research, 192(3), 533–551.