Prosthetic embodiment: systematic review on definitions, measures, and experimental paradigms

The term embodiment has become omnipresent within prosthetics research and is often used as a metric of the progress made in prosthetic technologies, as well as a hallmark for user acceptance. However, despite the frequent use of the term, the concept of prosthetic embodiment is often left undefined or described incongruently, sometimes even within the same article. This terminological ambiguity complicates the comparison of studies using embodiment as a metric of success, which in turn hinders the advancement of prosthetics research. To resolve these terminological ambiguities, we systematically reviewed the used definitions of embodiment in the prosthetics literature. We performed a thematic analysis of the definitions and found that embodiment is often conceptualized in either of two frameworks based on body representations or experimental phenomenology. We concluded that treating prosthetic embodiment within an experimental phenomenological framework as the combination of ownership and agency allows for embodiment to be a quantifiable metric for use in translational research. To provide a common reference and guidance on how to best assess ownership and agency, we conducted a second systematic review, analyzing experiments and measures involving ownership and agency. Together, we highlight a pragmatic definition of prosthetic embodiment as the combination of ownership and agency, and in an accompanying article, we provide a perspective on a multi-dimensional framework for prosthetic embodiment. Here, we concluded by providing recommendations on metrics that allow for outcome comparisons between studies, thereby creating a common reference for further discussions within prosthetics research

The neuroscience of body memory: Recent findings and conceptual advances

The body is a very special object, as it corresponds to the physical component of the self and it is the medium through which we interact with the world. Our body awareness includes the mental representation of the body that happens to be our own, and traditionally has been defined in terms of body schema and body image. Starting from the distinction between these two types of representations, the present paper tries to reconcile the literature around body representations under the common framework of body memory. The body memory develops ontogenetically from birth and across all the life span and is directly linked to the development of the self. Therefore, our sense of self and identity is fundamentally based on multisensory knowledge accumulated in body memory, so that the sensations collected by our body, stored as implicit memory, can unfold in the future, under suitable circumstances. Indeed, these sets of bodily information had been proposed as possible key factors underpinning several mental health illnesses. Following this perspective, the Embodied Medicine approach put forward the use of advanced technologies to alter the dysfunctional body memory to enhance people’s well-being. In the last sections, recent experimental pieces of evidence will be illustrated that targeted specifically bodily information for increasing health and wellbeing, by means of two strategies: interoceptive feedback and bodily illusions

Balancing Interoception and Exteroception: Vestibular and Spatial Contributions to the Bodily Self

Experiencing the body as a coherent, stable, entity involves the dynamic integration of information from several internal (i.e. interoceptive) and external (i.e. exteroceptive) sensory sources, to produce a feeling that the body is mine (sense of body ownership), that I am in control (sense of agency) and I am aware of its movements (motor awareness). However, the exact contribution of these different sensory sources to self-consciousness, as well as the context in which we experience them, is still a matter of debate. This thesis aimed to investigate the neurocognitive mechanisms of body ownership, agency and motor awareness, including interoceptive (via affective touch), proprioceptive, exteroceptive (visuo-spatial) and vestibular contributions to body representation, in both healthy subjects and brain damaged patients. To examine the role of the vestibular and interoceptive systems in body ownership, a series of studies in healthy subjects was devised, using multisensory illusions (i.e. the rubber hand illusion; RHI), that involve the integration of interoceptive and exteroceptive sensory sources, and using electrical stimulation of the vestibular system (i.e. Galvanic Vestibular Stimulation; GVS). To investigate ownership, agency and motor awareness in neuropsychological patients with disorders of ownership and/or unawareness of motor deficits, behavioural manipulation of body ownership (via a rubber hand) and visual perspective (via a mirror) were tested. Finally, to explore underlying mechanisms of awareness of one’s own performance (i.e. meta-cognition), two studies were carried out in healthy subjects using behavioural manipulations of spatial reference frames (either centred on the subject, i.e. egocentric, or world-centred, i.e. allocentric). The results of these studies indicate that the vestibular system balances vision and proprioception according to contextual relevance: when there is no tactile stimulation, visual cues are stronger than proprioceptive ones (i.e. proprioceptive drifts are greater); when touch is delivered synchronously, this effect is enhanced (even more when touch is affective rather than neutral). However, when touch is only felt but not seen, the vestibular system downregulates vision in favour of proprioception (i.e. proprioceptive drifts are smaller), whilst the opposite happens when touch is only vicariously perceived via vision. Nevertheless, when the rubber hand is positioned in a non-biomechanically possible fashion, there appears to be no difference in proprioceptive drifts in comparison with anatomically plausible positions, suggesting that such rebalancing may be more related to basic multisensory integration processes underlying body representation. In patients with disorders of the self, visual cues seem to dominate over proprioceptive ones, leading to strong feelings of ownership of a rubber hand following mere exposure to it; however, the same is not true for agency, which seems to be more susceptible to changes in the environment (i.e. presence or absence of a visual feedback following attempted movement). Moreover, manipulating visual perspective using a mirror (from 1st to 3rd) seem to lead to a temporary remission of dis-ownership but not motor unawareness, suggesting that awareness may not be influenced by online changes in visual perspectives. Finally, when judging their own performance in a visuo-proprioceptive task from an egocentric rather than an allocentric perspective, healthy subjects appear less objective prospectively rather than during the task (i.e. their belief updating is biased when judging their ability to complete a task egocentrically). In sum, the work described above adds to the evidence that the sense of self derives from a complex integration of several sensory modalities, flexibly adjusting to the environment. Following brain damage, such flexibility may be impaired, even though it can be influenced by spatial perspective. Similarly, the point of reference from which we perceive stimuli affects the way we judge our own perceptual choices. Hence, the way we represent our bodily self is a dynamic process, constantly updated by exteroceptive and interoceptive incoming stimuli, regulated by the vestibular system. These findings could provide new avenues in rehabilitating disorders of the self (such as unawareness and dis-ownership)

Interpersonal motor interactions shape multisensory representations of the peripersonal space

This perspective review focuses on the proposal that predictive multisensory integration occurring in one’s peripersonal space (PPS) supports individuals’ ability to efficiently interact with others, and that integrating sensorimotor signals from the interacting partners leads to the emergence of a shared representation of the PPS. To support this proposal, we first introduce the features of body and PPS representations that are relevant for interpersonal motor interactions. Then, we highlight the role of action planning and execution on the dynamic expansion of the PPS. We continue by presenting evidence of PPS modulations after tool use and review studies suggesting that PPS expansions may be accounted for by Bayesian sensory filtering through predictive coding. In the central section, we describe how this conceptual framework can be used to explain the mechanisms through which the PPS may be modulated by the actions of our interaction partner, in order to facilitate interpersonal coordination. Last, we discuss how this proposal may support recent evidence concerning PPS rigidity in Autism Spectrum Disorder (ASD) and its possible relationship with ASD individuals’ difficulties during interpersonal coordination. Future studies will need to clarify the mechanisms and neural underpinning of these dynamic, interpersonal modulations of the PPS

Touching is believing: creating illusions and feeling of embodiment with mid-air haptic technology

Over the last two decades, the sense of touch has received new attention from the scientific community.Several haptic devices have been developed to address the complexity of the sense of touch, the latest addition being mid-air (contactless) haptic technology. An interesting series of previous research has suggested an easier way to tackle the complexity of designing convincing tactile sensations by exploiting tactile illusions. Tactile illusions rely on perceptual shortcuts based on the psychophysics of the tactile receptors. Currently, studies exploring the perceptual space of mid-air haptics and its applicability in the tactile illusions field are still limited in number. This thesis aims to contribute to the field of Human-Computer Interaction (HCI) by investigating the perceptual design space of ultrasonic mid-air haptics technology. Specifically, in a first set of three studies, we investigate the absolute thresholds (minimal amount of a property of astimulus that a user can detect) for control points (CP) at different frequencies on the hand and arm (Study 1). Then we investigate the optimal sampling rate needed to drive the device in an optimal fashion and its relationship with shape size (Study 2). Next, we apply a new technique to increase users’ performance in a shape discrimination task (Study 3). In Study 4, we start the exploration of a tactile illusion of movement using contact touch and later, we apply a similar procedure to investigate the feasibility of creating a tactile illusion of movement between the two non-interconnected hands by using mid-air touch (Study 5). Finally, in Study 6, we explore our sense of touch in VR, while providing an illusion of rain drops through mid-air haptics, to recreate a virtual hand illusion (VHI) to explore the boundaries of our sense of embodiment. Therefore, the contribution of this work is threefold: a) we contribute by adding new knowledge on the psychophysical space for mid-air haptics, b) we test the potential to create realistic tactile sensations by exploiting tactile illusions with mid-air haptic technology, and c) we demonstrate how tactile illusions mediated by mid-air haptics can convey a sense of embodiment in VR environments

Vestibular Contributions to the Sense of Body, Self, and Others

There is increasing evidence that vestibular signals and the vestibular cortex are not only involved in oculomotor and postural control, but also contribute to higher-level cognition. Yet, despite the effort that has recently been made in the field, the exact location of the human vestibular cortex and its implications in various perceptional, emotional, and cognitive processes remain debated. Here, we argue for a vestibular contribution to what is thought to fundamentally underlie human consciousness, i.e., the bodily self. We will present empirical evidence from various research fields to support our hypothesis of a vestibular contribution to aspects of the bodily self, such as basic multisensory integration, body schema, body ownership, agency, and self-location. We will argue that the vestibular system is especially important for global aspects of the self, most crucially for implicit and explicit spatiotemporal self-location. Furthermore, we propose a novel model on how vestibular signals could not only underlie the perception of the self but also the perception of others, thereby playing an important role in embodied social cognition

The Rocketbox Library and the Utility of Freely Available Rigged Avatars

As part of the open sourcing of the Microsoft Rocketbox avatar library for research and academic purposes, here we discuss the importance of rigged avatars for the Virtual and Augmented Reality (VR, AR) research community. Avatars, virtual representations of humans, are widely used in VR applications. Furthermore many research areas ranging from crowd simulation to neuroscience, psychology, or sociology have used avatars to investigate new theories or to demonstrate how they influence human performance and interactions. We divide this paper in two main parts: the first one gives an overview of the different methods available to create and animate avatars. We cover the current main alternatives for face and body animation as well introduce upcoming capture methods. The second part presents the scientific evidence of the utility of using rigged avatars for embodiment but also for applications such as crowd simulation and entertainment. All in all this paper attempts to convey why rigged avatars will be key to the future of VR and its wide adoption

Vestibular contributions to the sense of body, self, and others

There is increasing evidence that vestibular signals and the vestibular cortex are not only involved in oculomotor and postural control, but also contribute to higher-level cognition. Yet, despite the effort that has recently been made in the field, the exact location of the human vestibular cortex and its implications in various perceptional, emotional, and cognitive processes remain debated. Here, we argue for a vestibular contribution to what is thought to fundamentally underlie human consciousness, i.e., the bodily self. We will present empirical evidence from various research fields to support our hypothesis of a vestibular contribution to aspects of the bodily self, such as basic multisensory integration, body schema, body ownership, agency, and self-location. We will argue that the vestibular system is especially important for global aspects of the self, most crucially for implicit and explicit spatiotemporal self-location. Furthermore, we propose a novel model on how vestibular signals could not only underlie the perception of the self but also the perception of others, thereby playing an important role in embodied social cognition