Curved sixth fingers: Flexible representation of the shape of supernumerary body parts.

A recent perceptual illusion induces the feeling of having a sixth finger on one's hand. It is unclear whether the representation of supernumerary fingers is flexible for shape. To test whether we can embody a sixth finger with a different shape from our own fingers, we induced a sixth finger which curved laterally though 180°. Participants reported feeling both curved and straight sixth fingers, depending on the stimulation pattern. Visual comparative judgements of the felt curvature of the supernumerary finger, showed means of 182° in the curved condition, and 35° in the straight condition. Our results show we can feel a supernumerary finger with different shape from our actual fingers, indicating that shape is represented flexibly in the perception of our hands. This study also adds evidence to the independence of the supernumerary finger from the actual fingers, showing we can represent the sixth finger with its own shape.

The vestibular system modulates the contributions of head and torso to egocentric spatial judgements.

Egocentric representations allow us to describe the external world as experienced from an individual's bodily location. We recently developed a novel method of quantifying the weight given to different body parts in egocentric judgments (the Misalignment Paradigm). We found that both head and torso contribute to simple alter-egocentric spatial judgments. We hypothesised that artificial stimulation of the vestibular system would provide a head-related signal, which might affect the weighting given to the head in egocentric spatial judgments. Bipolar Galvanic Vestibular Stimulation (GVS) was applied during the Misalignment Paradigm. A Sham stimulation condition was also included to control for non-specific effects. Our data show that the weight given to the head was increased during left anodal and right cathodal GVS, compared to the opposite GVS polarity (right anodal and left cathodal GVS) and Sham stimulation. That is, the polarity of GVS, which preferentially activates vestibular areas in the right cerebral hemisphere, influenced the relative weightings of head and torso in egocentric spatial judgments.

Dissociating contributions of head and torso to spatial reference frames: The misalignment paradigm.

When we represent someone's view of a scene as egocentrically structured, where do we represent the origin of the reference frame? By analysing responses in a spatial perspective-taking task as a function of spatial location with respect to both head and torso, we isolated the respective contribution of each part to spatial judgments. Both the head and the torso contributed to judgements, though with greater contributions from the torso. A second experiment manipulating visual contrast of the torso showed that this does not reflect low-level differences in visual salience between body parts. Our results demonstrate that spatial perspective-taking relies on a weighted combination of reference frames centred on different parts of the body.

Where exactly am I? Self-location judgements distribute between head and torso.

I am clearly located where my body is located. But is there one particular place inside my body where I am? Recent results have provided apparently contradictory findings about this question. Here, we addressed this issue using a more direct approach than has been used in previous studies. Using a simple pointing task, we asked participants to point directly at themselves, either by manual manipulation of the pointer whilst blindfolded or by visually discerning when the pointer was in the correct position. Self-location judgements in haptic and visual modalities were highly similar, and were clearly modulated by the starting location of the pointer. Participants most frequently chose to point to one of two likely regions, the upper face or the upper torso, according to which they reached first. These results suggest that while the experienced self is not spread out homogeneously across the entire body, nor is it localised in any single point. Rather, two distinct regions, the upper face and upper torso, appear to be judged as where "I" am.

Shared contributions of the head and torso to spatial reference frames across spatial judgments.

Egocentric frames of reference take the body as the point of origin of a spatial coordinate system. Bodies, however, are not points, but extended objects, with distinct parts that can move independently of one another. We recently developed a novel paradigm to probe the use of different body parts in simple spatial judgments, what we called the misalignment paradigm. In this study, we applied the misalignment paradigm in a perspective-taking task to investigate whether the weightings given to different body parts are shared across different spatial judgments involving different spatial axes. Participants saw birds-eye images of a person with their head rotated 45° relative to the torso. On each trial, a ball appeared and participants made judgments either of whether the ball was to the person's left or right, or whether the ball was in front of the person or behind them. By analysing the pattern of responses with respect to both head and torso, we quantified the contribution of each body part to the reference frames underlying each judgment. For both judgment types we found clear contributions of both head and torso, with more weight being given on average to the torso. Individual differences in the use of the two body parts were correlated across judgment types indicating the use of a shared set of weightings used across spatial axes and judgments. Moreover, retesting of participants several months later showed high stability of these weightings, suggesting that they are stable characteristics of people.

Self and Body Part Localization in Virtual Reality: Comparing a Headset and a Large-Screen Immersive Display.

It is currently not fully understood where people precisely locate themselves in their bodies, particularly in virtual reality. To investigate this, we asked participants to point directly at themselves and to several of their body parts with a virtual pointer, in two virtual reality (VR) setups, a VR headset and a large-screen immersive display (LSID). There was a difference in distance error in pointing to body parts depending on VR setup. Participants pointed relatively accurately to many of their body parts (i.e., eyes, nose, chin, shoulders, and waist). However, in both VR setups when pointing to the feet and the knees they pointed too low, and for the top of the head too high (to larger extents in the VR headset). Taking these distortions into account, the locations found for pointing to self were considered in terms of perceived bodies, based on where the participants had pointed to their body parts in the two VR setups. Pointing to self in terms of the perceived body was mostly to the face, the upper followed by the lower, as well as some to the torso regions. There was no significant overall effect of VR condition for pointing to self in terms of the perceived body (but there was a significant effect of VR if only the physical body (as measured) was considered). In a paper-and-pencil task outside of VR, performed by pointing on a picture of a simple body outline (body template task), participants pointed most to the upper torso. Possible explanations for the differences between pointing to self in the VR setups and the body template task are discussed. The main finding of this study is that the VR setup influences where people point to their body parts, but not to themselves, when perceived and not physical body parts are considered.

Where am I in virtual reality?

It is currently not well understood whether people experience themselves to be located in one or more specific part(s) of their body. Virtual reality (VR) is increasingly used as a tool to study aspects of bodily perception and self-consciousness, due to its strong experimental control and ease in manipulating multi-sensory aspects of bodily experience. To investigate where people self-locate in their body within virtual reality, we asked participants to point directly at themselves with a virtual pointer, in a VR headset. In previous work employing a physical pointer, participants mainly located themselves in the upper face and upper torso. In this study, using a VR headset, participants mainly located themselves in the upper face. In an additional body template task where participants pointed at themselves on a picture of a simple body outline, participants pointed most often to the upper torso, followed by the (upper) face. These results raise the question as to whether head-mounted virtual reality might alter where people locate themselves making them more "head-centred".