Is the body schema sufficient for the sense of ownership? Glenn Carruthers Macquarie Centre for Cognitive Science (MACC

1. Is the body schema sufficient for the sense of ownership?Glenn CarruthersMacquarie Centre for Cognitive Science (MACCS) Macquarie University, Australia De Vignemont’s Model The sense of ownership arises from the localisation of sensation on a multimodal, spatial and mereological map of the body that is part of the body schema. Body sensation seems to occur at a specific point of the body because it’s represented in terms of this map. This gives rise to the sense of ownershipThe body image is a set of beliefs, attitudes and perceptions that are about one’s body. The body schema is a system of representations involved in motor control. “Body schema is for action and body image is for identification” (de Vignemont 2007 pg 439) Problems for de Vignemont’s ModelIt is not clear what is causing the illusion in the case of the active RHI. 2 differences between the active condition and the tactile stimulation condition: 1st, as de Vignemont notes, the body schema is needed to generate the action, 2nd there is a sense of agency over the action. Perhaps the sense of agency over the hand causes the illusion in this case. As such the active rubber hand illusion does not unambiguously support de Vignemont’s model. Prosthesis again: And not too long ago I was lying in bed with my wife. I had removed my limb. We were eating some food I had cooked and I decided to get up and do the dishes. I reached over to take her plate, got up, and forgot I did not have my limb on. I fell on the floor, landing on the distal end of the stump (…) So, I guess I have reached a point where I am capable of such foolish acts as that and forget my leg was not on (de Vignemont 2007 pg 441). A clue comes from normal experience. If I were to ask you if you had two legs you wouldn’t need to check yourself to find out. You could just respond based on a generic understanding of what your body is usually like. Online (short-term) versus offline (long term) representations of the body: distinguished by their content. Online (short term) representations represent the body as it is currently: Current movements; Current limb position; If and where one is itchy.Offline (long term) representations represent the body as it is usually: Actions afforded by the body; Usual spatial relations between parts (legs side by side, head above shoulders).Both online and offline representations can be part of the image or schema. Offline representations constitute the generic understanding of one’s body that is used to answer the question: ‘do you have two legs?’ It is possible that it is by including the prosthesis in an offline (long term) representation of the body that subjects experience an sense of ownership for it. Similarly mistaking image of a hand for the hand represented in the offline representation could explain RHI. That prosthetics are included in the body schema does not unambiguously support de Vignemont’s model. There are cases where subjects experience asomatognosia without a deficit in their body schema. Hard to test, patients are usually paralysed. Need an alternative method, such as the localisation of sensation in the denied limb. This hasn’t been tested directly but there is one study which is suggestive. Patients tested during preparatory surgery to treat severe epilepsy using Wada test (= deactivation of one hemisphere of the brain). 82% of subjects have asomatognosia for arm/hand contralateral to deactivation. Also somatoparaphrenia as they attribute hand to experimenter. Asked to hold hands in the air before deactivation, after deactivation paralysed hand placed on the table. Then asked to say where the hand is and point to it. 9 of the 19 patients suffering somatoparaphrenia tested in this way were able to point to the hand. In this study around half of those suffering from somatoparaphrenia can point to their denied hand. Suggests they still have a body schema representation for that hand. It seems unlikely that a representation that is part of the body schema underlies the sense of ownership. Key Texts: Carruthers, G. (2008). "Types of body representation and the sense of embodiment." Consciousness and Cognition 17(4): 1302-1316. 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. de Vignemont, F. (2007). "Habeas Corpus: the Sense of Ownership of One's Own Body." Mind and Language 22(4): 427-449. Meador, K., D. Loring, T. Feinberg, G. Lee and M. Nichols (2000). "Anosognosia and asomatognosia during intracarotidamobarbital inactivation." Neurology 55(6). Moro, V., M. Zampini and S. Aglioti, M (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. Tsakiris, M., G. Prabhu and P. Haggard (2006). "Having a body versus moving your body: How agency structures body-ownership." Consciousness and Cognition 15: 423-432. Evidence that body schema underlies the sense of ownership Asomatognosia= denial of ownership of a limbSomatoparaphrenia= attribution of limb to another de Vignemont considers 2 patients’ suffering somatoparaphrenia for left arm and hemispatial neglect for left side of space. When arm held out of neglected space (right side of patients’ bodies) they experience sensations in the limb, but continue to deny ownership. Why is one limb represented as part of the body and the other not? Lost a representation that represents body parts qua one’s body. Lost a representation of the boundaries of one’s body (the spatial hypothesis). De Vignemont suggests that these patients have lost their representations of the left arm that are part of the body schema. Can body schema representations define the apparent boundaries of one’s body? If this is done by localising sensation on the boundaries of one’s body then perhaps. Cases of numbsense show that body schema representations can localise sensation. Sensation is localised and acted on apparently without representations of the body part that are a part of the body image. RHI elicited with action. Instead of rubber hand subjects see an image of their hand projected 0.15m above their real hand. When they move a finger up and down and see this in the projected image they come to experience a sense of ownership over the image of their hand. In active condition the projected image of the hand is represented as the hand that the subject is moving, i.e. It is included in the body schema. Perhaps doing this also gives rise to a sense of ownership over the hand. Patients who undergo amputation and use prosthetic limbs come to experience a sense of ownership for the prosthesis: Many amputees feel that their artificial limb is somehow a part of them, a simple example of this is that I wouldn’t like just anyone putting their hand on my artificial knee, even though it is not actually a part of my body’s flesh, it is still mine even though it’s a piece of plastic and metal (de Vignemont 2007 pg 431). In order to move around effectively the prosthesis needs to be included in the body schema. Perhaps this provides a sense of ownership.