Yen-Tung Lee

Body ownership concerns what it is like to feel a body part or a full body as mine, and has become a prominent area of study. We propose that there is a closely related type of bodily self-consciousness largely neglected by researchers—experiential ownership. It refers to the sense that I am the one who is having a conscious experience. Are body ownership and experiential ownership actually the same phenomenon or are they genuinely different? In our experiments, the participant watched a rubber hand or someone else’s body from the first-person perspective and was touched either synchronously or asynchronously. The main findings: (1) The sense of body ownership was hindered in the asynchronous conditions of both the body-part and the full-body experiments. However, a strong sense of experiential ownership was observed in those conditions. (2) We found the opposite when the participants’ responses were measured after tactile stimulations had ceased for 5 s. In the synchronous conditions of another set of body-part and full-body experiments, only experiential ownership was blocked but not body ownership. These results demonstrate for the first time the double dissociation between body ownership and experiential ownership. Experiential ownership is indeed a distinct type of bodily self-consciousness.

Self-location—the sense of where I am in space—provides an experiential anchor for one's interaction with the environment. In the studies of full-body illusions, many researchers have defined self-location solely in terms of body-location—the subjective feeling of where my body is. Although this view is useful, there is an issue regarding whether it can fully accommodate the role of 1PP-location—the sense of where my first-person perspective is located in space. In this study, we investigate self-location by comparing body-location and 1PP-location: using a head-mounted display (HMD) and a stereo camera, the subjects watched their own body standing in front of them and received tactile stimulations. We manipulated their senses of body-location and 1PP-location in three different conditions: the participants standing still (Basic condition), asking them to move forward (Walking condition), and swiftly moving the stereo camera away from their body (Visual condition). In the Walking condition, the participants watched their body moving away from their 1PP. In the Visual condition, the scene seen via the HMD was systematically receding. Our data show that, under different manipulations of movement, the spatial unity between 1PP-location and body-location can be temporarily interrupted. Interestingly, we also observed a “double-body effect.” We further suggest that it is better to consider body-location and 1PP-location as interrelated but distinct factors that jointly support the sense of self-location.

Recent studies of the rubber hand illusion (RHI) have shown that the sense of body ownership is constrained by several factors and yet is still very flexible. However, exactly how flexible is our sense of body ownership? In this study, we address this issue by investigating the following question: is it possible that one may have the illusory experience of owning four hands? Under visual manipulation, the participant adopted the experimenter’s first-person perspective (1PP) as if it was his/her own. Sitting face to face, the participant saw four hands—the experimenter’s two hands from the adopted 1PP together with the subject’s own two hands from the adopted third-person perspective (3PP). We found that: (1) the four-hand illusion did not occur in the passive four-hand condition. (2) In the active four-hand condition, the participants tapped their index fingers, imitated by the experimenter. When tactile stimulations were not provided, the key illusion was not induced, either. (3) Strikingly, once all four hands began to act with the same pattern and received synchronous tactile stimulations at the same time, many participants felt as if they had two more hands. These results show that the sense of body ownership is much more flexible than most researchers have suggested.

In a recent study (Chen et al., 2018), we conducted a series of experiments that induced the “four-hand illusion”: using a head-mounted display (HMD), the participant adopted the experimenter’s first-person perspective (1PP) as if it was his/her own 1PP. The participant saw four hands via the HMD: the experimenter’s two hands from the adopted 1PP and the subject’s own two hands from the adopted third-person perspective (3PP). In the active four-hand condition, the participant tapped his/her index fingers, imitated by the experimenter. Once all four hands acted synchronously and received synchronous tactile stimulations at the same time, many participants felt as if they owned two more hands. In this paper, we argue that there is a philosophical implication of this novel illusion. According to Merleau-Ponty (1945/1962) and Legrand (2010), one can experience one’s own body or body-part either as-object or as-subject but cannot experience it as both simultaneously, i.e., these two experiences are mutually exclusive. Call this view the Experiential Exclusion Thesis. We contend that a key component of the four-hand illusion—the subjective experience of the 1PP-hands that involved both “kinesthetic sense of movement” and “visual sense of movement” (the movement that the participant sees via the HMD)—provides an important counter-example against this thesis. We argue that it is possible for a healthy subject to experience the same body-part both as-subject and as-object simultaneously. Our goal is not to annihilate the distinction between body-as-object and body-as-subject, but to show that it is not as rigid as suggested by the phenomenologists.

We investigate two issues about the subjective experience of one's body: first, is the experience of owning a full-body fundamentally different from the experience of owning a body-part?Second, when I experience a bodily sensation, does it guarantee that I cannot be wrong about whether it is me who feels it? To address these issues, we conducted a series of experiments that combined the rubber hand illusion (RHI) and the “body swap illusion.” The subject wore a head mounted display (HMD) connected with a stereo camera set on the experimenter's head. Sitting face to face, they used their right hand holding a paintbrush to brush each other's left hand. Through the HMD, the subject adopted the experimenter's first-person perspective (1PP) as if it was his/her own 1PP: the subject watched either the experimenter's hand from the adopted 1PP, and/or the subject's own hand from the adopted third-person perspective (3PP) in the opposite direction (180°), or the subject's full body from the adopted 3PP (180°, with or without face). The synchronous full-body conditions generate a “self-touching illusion”: many participants felt that “I was brushing my own hand!” We found that (1) the sense of body-part ownership and the sense of full-body ownership are not fundamentally different from each other; and (2) our data present a strong case against the mainstream philosophical view called the immunity principle (IEM). We argue that it is possible for misrepresentation to occur in the subject's sense of “experiential ownership” (the sense that I am the one who is having this bodily experience). We discuss these findings and conclude that not only the sense of body ownership but also the sense of experiential ownership call for further interdisciplinary studies.