What is a Difference Between Disownership Feeling and Loss of Ownership Feeling Over One's Own Body?  Yuta Nishiyama , Chihiro Yamashita; Wataru Hasegawa (Department of Information and Management Systems Science, Nagaoka University of Technology, Niigata, Japan)   C4

For a couple of decades, it has been proposed that synchronous multi-sensory stimuli induce the conscious experience of ownership over an external object as one's body. Manipulating a sense of ownership not only of body parts but also of full body brought about a progressive change to pursue a nature of the self. In the meantime, it has been controversial how one's real body is dealt with while one experiences owning another object. It is still unclear even whether the fake and the real are coexistent or exclusive in a body image. This question is based on being at least a thing owned. Therefore, loss of ownership becomes an issue. In contrast, we would like to call loss of ownership requiring no alternative disownership feeling. Our previous study showed that healthy participants could feel as if their hand is not their own. In that experiment, participants were presented with live images of one side of their upper body via a head mounted display. That their elbow on the far side was out of view behind their upper body induced a disownership feeling of the ipsilateral hand even though they moved it. The evidence, however, grounded only on comparisons of subjective responses between experimental conditions, out-of-view and on-view. As one of possible measurements, several researches of bodily awareness have demonstrated a modulation of pain perception associated with sense of body ownership. Thus we investigated whether disownership feeling induced in our experimental setup causes a change of subjective experience associated with pain. We measured contact heat-pain thresholds by using a thermal stimulator that raises the probe temperature at 1 degree celsius a second from skin temperature. Fifteen undergraduates took part in this study. Temperature of the probe started to increase with experimenter's verbal notification after participants had experienced either experimental condition. Participants were asked to press a button on their contralateral hand at the moment of feeling pain in order to stop temperature increment. Measurements were performed four times each condition to calculate a mean temperature. In addition to a response to thermal stimuli, participants answered five questionnaire items by using a visual analogue scale. Although results of questionnaire statistically showed that participants felt disownership over their hand in the out-of-view condition rather than in the on-view condition, there is no significant difference in pain threshold. However, we found a positive correlation between differences of pain threshold values and those of responses to the question, "I felt as if the arm shrunk." In other words, the more they feel their arm shrink, the more analgesic the out-of-view condition is. According to researches of a relationship between body size perception and pain perception, this result is consistent with characteristics of patients with clinical conditions such as chronic pain and phantom pain rather than healthy participants. This suggests that the feeling in our experiments would differ from loss of ownership. In this presentation, we will also introduce an additional study about decrease of skin temperature during the experience.