From rubber hands to neuroprosthetics: Neural correlates of embodiment

© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)Our interaction with the world rests on the knowledge that we are a body in space and time, which can interact with the environment. This awareness is usually referred to as sense of embodiment. For the good part of the past 30 years, the rubber hand illusion (RHI) has been a prime tool to study embodiment in healthy and people with a variety of clinical conditions. In this paper, we provide a critical overview of this research with a focus on the RHI paradigm as a tool to study prothesis embodiment in individuals with amputation. The RHI relies on well-documented multisensory integration mechanisms based on sensory precision, where parietal areas are involved in resolving the visuo-tactile conflict, and premotor areas in updating the conscious bodily representation. This mechanism may be transferable to prosthesis ownership in amputees. We discuss how these results might transfer to technological development of sensorised prostheses, which in turn might progress the acceptability by users.Peer reviewe

Subcortical contributions to the sense of body ownership

The sense of body ownership (i.e., the feeling that our body or its parts belong to us) plays a key role in bodily self-consciousness and is believed to stem from multisensory integration. The development of experimental paradigms that allow the controlled manipulation of body ownership in laboratory settings, such as the rubber hand illusion, provide an effective tool to investigate the malleability of the sense of body ownership and the boundaries distinguishing self and other. Neuroimaging studies on body ownership converge on the involvement of several cortical regions, including the premotor cortex and posterior parietal cortex. However, relatively less attention has been paid to subcortical structures that may also contribute to body ownership perception, such as the cerebellum and putamen. Here, on the basis of neuroimaging and neuropsychological observations, we provide an overview of relevant subcortical regions and consider their potential role in generating and maintaining a sense of ownership over the body. We also suggest novel avenues for future research targeting the role of subcortical regions in making sense of the body as our own

Basic prediction mechanisms as a precursor for schizophrenia studies

Traditionally, early visual cortex (V1-3) was thought of as merely a relay centre for feedforward retinal input, providing entry to the cortical visual processing steam. However, in addition to feedforward retinal input, V1 receives a large amount of intracortical information through feedback and lateral connections. Human visual perception is constructed from combining feedforward inputs with these feedback and lateral contributions. Feedback connections allow the visual cortical response to feedforward information to be affected by expectation, knowledge, and context; even at the level of early visual cortex. In Chapter 1 we discuss the feedforward and feedback visual processing streams. We consider historical philosophical and scientific propositions about constructive vision. We introduce modern theories of constructive vision, which suggest that vision is an active process that aims to infer or predict the cause of sensory inputs. We discuss how V1 therefore represents not only retinal input but also high-level effects related to constructive predictive perception. Visual illusions are a ‘side effect’ of constructive and inferential visual perception. For the vast majority of stimulus inputs, integration with context and knowledge facilitates clearer, more veridical perception. In illusion these constructive mechanisms produce incorrect percepts. Illusory effects can be observed in early visual cortex, even when there is no change in the feedforward visual input. We suggest that illusions therefore provide us with a tool to probe feedforward and feedback integration, as they exploit the difference between retinal stimulation and resulting perception. Thus, illusions allow us to see the changes in activation and perception induced only by feedback without changes in feedforward input. We discuss a few specific examples of illusion generation through feedback and the accompanying effects on V1 processing. In Schizophrenia, the integration of feedback and feedforward information is thought to be dysfunctional, with unbalanced contributions of the two sources. This is evidenced by disrupted contextual binding in visual perception and corresponding deficits in contextual illusion perception. We propose that illusions can provide a window into constructive and inferential visual perception in Schizophrenia. Use of illusion paradigms could help elucidate the deficits existing within feedback and feedforward integration. If we can establish clear effects of illusory feedback to V1 in a typical population, we can apply this knowledge to clinical subjects to observe the differences in feedback and feedforward information. Chapter 2 describes a behavioural study of the rubber hand illusion. We probe how multimodal illusory experience arises under varying reliabilities of visuotactile feedforward input. We recorded Likert ratings of illusion experience from subjects, after their hidden hand was stimulated either synchronously or asynchronously with a visible rubber hand (200, 300, 400, or 600ms visuotactile asynchronicity). We used two groups, assessed by a questionnaire measuring a subject’s risk of developing Schizophrenia - moderate/high scorers and a control group of zero-scorers. We therefore consider how schizotypal symptoms contribute to rubber hand illusory experience and interact with visuotactile reliability. Our results reveal that the impact of feedforward information on higher level illusory body schema is modulated by its reliability. Less reliable feedforward inputs (increasing asynchronicity) reduce illusion perception. Our data suggests that some illusions may not be affected on a spectrum of schizotypal traits but only in the full schizophrenic disorder, as we found no effect of group on illusion perception. In Chapter 3 we present an fMRI investigation of the rubber hand illusion in typical participants. Cortical feedback allows information about other modalities and about cognitive states to be represented at the level of V1. Using a multimodal illusion, we investigated whether crossmodal and illusory states could be represented in early visual cortex in the absence of differential visual input. We found increased BOLD activity in motion area V5 and global V1 when the feedforward tactile information and the illusory outcome were incoherent (for example when the subject was experiencing the illusion during asynchronous stimulation). This is suggestive of increased predictive error, supporting predictive coding models of cognitive function. Additionally, we reveal that early visual cortex contains pattern representations specific to the illusory state, irrespective of tactile stimulation and under identical feedforward visual input. In Chapter 4 we use the motion-induced blindness illusion to demonstrate that feedback modulates stimulus representations in V1 during illusory disappearance. We recorded fMRI data from subjects viewing a 2D cross array rotating around a central axis, passing over an oriented Gabor patch target (45°/ 135°). We attempted to decode the target orientation from V1 when the target was either visible or invisible to subjects. Target information could be decoded during target visibility but not during motion-induced blindness. This demonstrates that the target representation in V1 is distorted or destroyed when the target is perceptually invisible. This illusion therefore has effects not only at higher cortical levels, as previously shown, but also in early sensory areas. The representation of the stimulus in V1 is related to perceptual awareness. Importantly, Chapter 4 demonstrated that intracortical processing can disturb constant feedforward information and overwrite feedforward representations. We suggest that the distortion observed occurs through feedback from V5 about the cross array in motion, overwriting feedforward orientation information. The flashed face distortion illusion is a relatively newly discovered illusion in which quickly presented faces become monstrously distorted. The neural underpinnings of the illusion remain unclear; however it has been hypothesised to be a face-specific effect. In Chapter 5 we challenged this account by exploiting two hallmarks of face-specific processing - the other-race effect and left visual field superiority. In two experiments, two ethnic groups of subjects viewed faces presented bilaterally in the visual periphery. We varied the race of the faces presented (same or different than subject), the visual field that the faces were presented in, and the duration of successive presentations (250, 500, 750 or 1000ms per face before replacement). We found that perceived distortion was not affected by stimulus race, visual field, or duration of successive presentations (measured by forced choice in experiment 1 and Likert scale in experiment 2). We therefore provide convincing evidence that FFD is not face-specific and instead suggest that it is an object-general effect created by comparisons between successive stimuli. These comparisons are underlined by a fed back higher level model which dictates that objects cannot immediately replace one another in the same retinotopic space without movement. In Chapter 6 we unify these findings. We discuss how our data show fed back effects on perception to produce visual illusion; effects which cannot be explained through purely feedforward activity processing. We deliberate how lateral connections and attention effects may contribute to our results. We describe known neural mechanisms which allow for the integration of feedback and feedforward information. We discuss how this integration allows V1 to represent the content of visual awareness, including during some of the illusions presented in this thesis. We suggest that a unifying theory of brain computation, Predictive Coding, may explain why feedback exerts top-down effects on feedforward processing. Lastly we discuss how our findings, and others that demonstrate feedback and prediction effects, could help develop the study and understanding of schizophrenia, including our understanding of the underlying neurological pathologies

Touching on elements for a non-invasive sensory feedback system for use in a prosthetic hand

Hand amputation results in the loss of motor and sensory functions, impacting activities of daily life and quality of life. Commercially available prosthetic hands restore the motor function but lack sensory feedback, which is crucial to receive information about the prosthesis state in real-time when interacting with the external environment. As a supplement to the missing sensory feedback, the amputee needs to rely on visual and audio cues to operate the prosthetic hand, which can be mentally demanding. This thesis revolves around finding potential solutions to contribute to an intuitive non-invasive sensory feedback system that could be cognitively less burdensome and enhance the sense of embodiment (the feeling that an artificial limb belongs to one’s own body), increasing acceptance of wearing a prosthesis.A sensory feedback system contains sensors to detect signals applied to the prosthetics. The signals are encoded via signal processing to resemble the detected sensation delivered by actuators on the skin. There is a challenge in implementing commercial sensors in a prosthetic finger. Due to the prosthetic finger’s curvature and the fact that some prosthetic hands use a covering rubber glove, the sensor response would be inaccurate. This thesis shows that a pneumatic touch sensor integrated into a rubber glove eliminates these errors. This sensor provides a consistent reading independent of the incident angle of stimulus, has a sensitivity of 0.82 kPa/N, a hysteresis error of 2.39±0.17%, and a linearity error of 2.95±0.40%.For intuitive tactile stimulation, it has been suggested that the feedback stimulus should be modality-matched with the intention to provide a sensation that can be easily associated with the real touch on the prosthetic hand, e.g., pressure on the prosthetic finger should provide pressure on the residual limb. A stimulus should also be spatially matched (e.g., position, size, and shape). Electrotactile stimulation has the ability to provide various sensations due to it having several adjustable parameters. Therefore, this type of stimulus is a good candidate for discrimination of textures. A microphone can detect texture-elicited vibrations to be processed, and by varying, e.g., the median frequency of the electrical stimulation, the signal can be presented on the skin. Participants in a study using electrotactile feedback showed a median accuracy of 85% in differentiating between four textures.During active exploration, electrotactile and vibrotactile feedback provide spatially matched modality stimulations, providing continuous feedback and providing a displaced sensation or a sensation dispatched on a larger area. Evaluating commonly used stimulation modalities using the Rubber Hand Illusion, modalities which resemble the intended sensation provide a more vivid illusion of ownership for the rubber hand.For a potentially more intuitive sensory feedback, the stimulation can be somatotopically matched, where the stimulus is experienced as being applied on a site corresponding to their missing hand. This is possible for amputees who experience referred sensation on their residual stump. However, not all amputees experience referred sensations. Nonetheless, after a structured training period, it is possible to learn to associate touch with specific fingers, and the effect persisted after two weeks. This effect was evaluated on participants with intact limbs, so it remains to evaluate this effect for amputees.In conclusion, this thesis proposes suggestions on sensory feedback systems that could be helpful in future prosthetic hands to (1) reduce their complexity and (2) enhance the sense of body ownership to enhance the overall sense of embodiment as an addition to an intuitive control system

The embodied user : corporeal awareness & media technology

Human beings are proficient users of tools and technology. At times, our interactions with a technological artifact appear so effortless, that the distinction between the artifact and the body starts to fade. When operating anthropomorphically designed teleoperation systems, for example, some people develop the vivid experience that they are physically there at the remote site (i.e., telepresence). Others might even come to sense the slave robot’s arms and hands as their own. The process in which the central nervous system categorizes an object as a part of the body, and in which a discrimination is made between what is contained within and outside the bodily boundaries, is called self-attribution. The aim of this thesis is twofold: (a) To determine the personal factors (e.g., the characteristics of an individual’s psychological makeup) and situational factors (e.g., the appearance of objects) that constrain or facilitate self-attribution, and (b) to determine the degree to which these factors affect people’s experiences with media technology. In Chapter 2, we describe the theoretical framework of our research which is centered on a conception of the user of technology as an embodied agent. In this chapter we distinguish two important, but often confused aspects of embodiment: the body schema, and the body image. The body schema is defined as a dynamic distributed network of procedures aimed at guiding behavior. In contrast, we defined the body image as a part of the process of consciousness and, thus, as consisting of those higher-order discriminations (or qualia) that pertain to the body, and one’s self-perception thereof. To investigate the individual and situational factors that constrain or facilitate selfattribution (i.e., incorporation into the body image), we employ the experimental paradigm of the rubber-hand illusion (Botvinick & Cohen, 1998). In this illusion, which is induced by stroking a person’s concealed hand together with a visible fake one, some people start to sense the fake hand as an actual part of their body. In Chapter 3, we investigate the rubberhand illusion under two mediated conditions: (1) a virtual reality condition, where both the fake hand and its stimulation were projected on the table in front of the participant, and (2) a mixed reality condition, where the fake hand was projected, but its stimulation was unmediated. Our experiment reveals that people can develop the rubber-hand illusion under mediated conditions, but the resulting illusion may, depending on the technology used, be less vivid than in the traditional unmediated setup. In Chapter 4, we investigate the extent to which visual discrepancies between the foreign object and a human hand affect people in developing a vivid rubber-hand illusion. We found that people experience a more vivid illusion when the foreign object resembles the human hand in terms of both shape and texture. Taken together, the experiments in Chapters 3 and 4 support the view that the rubber-hand illusion is not merely governed by a bottom-up process (i.e., based on visuotactile integration), but is affected, top-down, by a cognitive representation of what the human body is like (e.g., Tsakiris and Haggard, 2005). In the rubber-hand illusion, people commonly misperceive the location of their concealed hand toward the direction of the fake hand (Tsakiris & Haggard, 2005). As such, this so-called proprioceptive drift is often used as an alternative to self-reports in assessing the vividness of the illusion (e.g., Tsakiris & Haggard, 2005). In Chapter 5, we investigate the extent to which the observed shift in felt position of the concealed hand can be attributed to experiencing the illusion. For this purpose, we test how various features of the experimental setup of the rubber-hand illusion, which in themselves are not sufficient to elicit the illusion, affect proprioceptive drift. We corroborate existing research which demonstrates that looking at a fake hand or a tabletop for five minutes, in absence of visuotactile stimulation, is sufficient to induce a change in the felt position of an unseen hand (e.g., Gross et al., 1974). Moreover, our experiments indicate that the use of proprioceptive drift as a measure for the strength of the rubber-hand illusion yields different conclusions than an assessment by means of self-reports. Based on these results, we question the validity of proprioceptive drift as an alternative measure of the vividness of the rubber-hand illusion. In Chapter 6, we propose and test a model of the vividness of the rubber-hand illusion. In two experiments, we successfully modeled people’s self-reported experiences related to the illusion (e.g., "the fake hand felt as my own") based on three estimates: (a) a person’s susceptibility for the rubber-hand illusion, (b) the processing demand that is required for a particular experience, and (c) the suppression/constraints imposed by the situation. We demonstrate that the impressions related to the rubber-hand illusion, and by inference the processes behind them, are comparable for different persons. This is a non-trivial finding as such invariance is required for an objective scaling of individual susceptibility and situational impediment on the basis of self-reported experiences. Regarding the validity of our vividness model, we confirm that asynchrony (e.g., Botvinick & Cohen, 1998) and information-poor stimulation (e.g., Armel & Ramachandran, 2003) constrain the development of a vivid rubber-hand illusion. Moreover, we demonstrate that the correlation between a person’s susceptibility for the rubber-hand illusion and the extent of his of her proprioceptive drift is fairly moderate, thereby confirming our conclusions from Chapter 5 regarding the limited validity of proprioceptive drift as a measure of the vividness of the rubber-hand illusion. In Chapter 7, we investigate the extent to which the large individual differences in people’s susceptibility for the illusion can be explained by body image instability, and the ability to engage in motor imagery of the hand (i.e., in mental own hand transformations). In addition, we investigate whether the vividness of the illusion is dependent on the anatomical implausibility of the fake hand’s orientation. With respect to body image instability, we corroborate a small, but significant, correlation between susceptibility and body image aberration scores: As expected, people with a more unstable body image are also more susceptible to the rubber-hand illusion (cf. Burrack & Brugger, 2005). With respect to the position and orientation of the fake hand on the table, we demonstrate that people experience a less vivid rubber-hand illusion when the fake hand is orientated in an anatomically impossible, as compared to an anatomically possible manner. This finding suggests that the attribution of foreign objects to the self is constrained by the morphological capabilities of the human body. With respect to motor imagery, our results indicate a small, but significant, correlation between susceptibility and response times to a speeded left and right hands identification task. In other words, people who are more attuned to engage in mental own hand transformations are also better equipped to develop vivid rubber-hand illusions. In Chapter 8, we examine the role of self-attribution in the experience of telepresence. For this purpose, we introduce the technological domain of mediated social touch (i.e., interpersonal touching over a distance). We anticipated that, compared to a morphologically incongruent input medium, a morphologically congruent medium would be more easily attributed to the self. As a result, we expected our participants to develop a stronger sense of telepresence when they could see their interaction partner performing the touches on a sensor-equipped mannequin as opposed to a touch screen. Our participants, as expected, reported higher levels of telepresence, and demonstrated more physiological arousal with the mannequin input medium. At the same time, our experiment revealed that these effects might not have resulted from self-attribution, and thus that other psychological mechanisms of identification might play a role in telepresence experiences. In Chapter 9, the epilogue, we discuss the main contributions and limitations of this thesis, while taking a broader perspective on the field of research on media technologies and corporeal awareness

Limb ownership and voluntary action: human behavioral and neuroimaging studies

To be able to interact with our surroundings in a goal directed manner, we need to have sense what our body is made up of as well as a sense of being able to control our body. These two experiences, the sense of body ownership and the sense of agency, respectively, are fundamental to our self-perception but have historically not received any notable attention from the scientific community. This lack of interest probably stems from the fact that these experiences are phenomenologically thin in our everyday lives and that we cannot voluntarily turn them off, they are constantly there. However, for patients suffering from disturbances in the processes underlying these experiences, their importance becomes exceedingly clear. Lesions in the frontal, temporal or parietal lobe can lead to patients losing the sense of ownership of their limb (asomatognosia), and sometimes even attributing the limb to someone else (somatoparaphrenia). Similarly, patients suffering from lesions in the frontal lobe, parietal lobe or corpus callosum can experience a lack of control over their own hand (anarchic hand syndrome), while patients suffering from schizophrenia display difficulties in distinguishing self-generated from externally generated actions, implicating disturbances in the processes underlying the sense of agency. With the discovery of body illusions, combined with functional neuroimaging, it became possible to study the perceptual and neural mechanisms of the sense of body ownership in healthy volunteers. Studies using these illusions have elucidated the perceptual rules of body ownership as well as its neural correlates and has given rise to a number of different philosophical, neurocognitive and computational models of the sense of body ownership. Meanwhile, the sense of agency has mostly been studied disconnected from the sense of body ownership, focusing on agency over self-generated external sensory effects such as auditory tones. This thesis sought to bring these two experiences together and advance our knowledge of the perceptual and neural mechanisms underlying the sense of body ownership and the sense of agency as well how these two experiences interact. Studies I & II investigate certain aspects of the sense of body ownership, and in particular its relation to the visuo-proprioceptive recalibration of limb position often seen in bodily illusions. Study III investigated how this visuo-proprioceptive recalibration is related to voluntary, but unconscious movements. Study IV investigated the neural correlates of the sense of body ownership and agency as well as their interaction. In Study I, we present empirical evidence in favor of models where the subjective sense of limb ownership is not reliant on a visuo-proprioceptive recalibration of perceived limb position. In Study II, we show that the subjective sense of limb ownership and the visuo-proprioceptive recalibration of limb position have similar temporal decay curves, suggestive of a causal relationship between them. In Study III, we show that the increase in the recalibration of limb position seen in active movements is not dependent on conscious intention, action awareness or salient error signals, indicative of an unconscious efference copy-based mechanism. Finally, in Study IV, we identify brain regions in the frontal and parietal lobe which are associated with the sense of body ownership, while brain regions in the frontal and temporal lobe are associated with the sense of agency. We show that the sense of agency in the presence of a sense of body ownership (i.e., agency of bodily actions) is associated with increased activity in the primary sensory cortex, whereas the sense of agency in the absence of ownership (i.e., agency of external events) is associated with increased activity in the visual association cortex. Together, these findings shed light on the perceptual and neural mechanisms underlying the sense of body ownership and agency as well as their interaction

Body schema plasticity after stroke: Subjective and neurophysiological correlates of the rubber hand illusion

[EN] Stroke can lead to motor impairments that can affect the body structure and restraint mobility. We hypothesize that brain lesions and their motor sequelae can distort the body schema, a sensorimotor map of body parts and elements in the peripersonal space through which human beings embody the reachable space and ready the body for forthcoming movements. Two main constructs have been identified in the embodiment mechanism: body-ownership, the sense that the body that one inhabits is his/her own, and agency, the sense that one can move and control his/her body. To test this, the present study simultaneously investigated different embodiment subcomponents (body-ownership, localization, and agency) and different neurophysiological measures (galvanic skin response, skin temperature, and surface electromyographic activity), and the interaction between them, in clinically-controlled hemiparetic individuals with stroke and in healthy subjects after the rubber hand illusion. Individuals with stroke reported significantly stronger body-ownership and agency and reduced increase of galvanic skin response, skin temperature, and muscular activity in the stimulated hand. We suggest that differences in embodiment could have been motivated by increased plasticity of the body schema and pathological predominance of the visual input over proprioception. We also suggest that differences in neurophysiological responses could have been promoted by a suppression of the reflex activity of the sympathetic nervous system and by the involvement of the premotor cortex in the reconfiguration of the body schema. These results could evidence a body schema plasticity promoted by the brain lesion and a main role of the premotor cortex in this mechanism.This work was supported by Ministerio de Economia y Competitividad of Spain (Project NeuroVR, TIN2013-44741-R, Project REACT, TIN2014-61975-EXP, and Grant BES-2014-068218), and by Universitat Politecnica de Valencia (Grant PAID-10-14).Llorens Rodríguez, R.; Borrego, A.; Palomo, P.; Cebolla, A.; Noé-Sebastián, E.; Bermúdez I Badia, S.; Baños Rivera, RM. (2017). Body schema plasticity after stroke: Subjective and neurophysiological correlates of the rubber hand illusion. Neuropsychologia. 96:61-69. https://doi.org/10.1016/j.neuropsychologia.2017.01.00761699

The Neural Correlates of Bodily Self-Consciousness in Virtual Worlds

Bodily Self-Consciousness (BSC) is the cumulative integration of multiple sensory modalities that contribute to our sense of self. Sensory modalities, which include proprioception, vestibulation, vision, and touch are updated dynamically to map the specific, local representation of ourselves in space. BSC is closely associated with bottom-up and top-down aspects of consciousness. Recently, virtual- and augmented-reality technology have been used to explore perceptions of BSC. These recent achievements are partly attributed to advances in modern technology, and partly due to the rise of virtual and augmented reality markets. Virtual reality head-mounted displays can alter aspects of perception and consciousness unlike ever before. Consequently, many strides have been made regarding BSC research. Previous research suggests that BSC results from the perceptions of embodiment (i.e., the feeling of ownership towards a real or virtual extremity) and presence (i.e., feeling physically located in a real or virtual space). Though physiological mechanisms serving embodiment and presence in the real world have been proposed by others, how these perceptual experiences interact and whether they can be dissociated is still poorly understood. Additionally, less is known about the physiological mechanisms underlying the perception of presence and embodiment in virtual environments. Therefore, five experiments were conducted to examine the perceptions of embodiment and presence in virtual environments to determine which physiological mechanisms support these perceptions. These studies compared performance between normal or altered embodiment/presence conditions. Results from a novel experimental paradigm using virtual reality (Experiment 4) are consistent with studies in the literature that reported synchronous sensorimotor feedback corresponded with greater strength of the embodiment illusion. In Experiment 4, participants recorded significantly faster reaction times and better accuracy in correlated feedback conditions compared to asynchronous feedback conditions. Reaction times were also significantly faster, and accuracy was higher for conditions where participants experienced the game from a first- versus third-person perspective. Functional magnetic resonance imaging (fMRI) data from Experiment 5 revealed that many frontoparietal networks contribute to the perception of embodiment, which include premotor cortex (PMC) and intraparietal sulcus (IPS). fMRI data revealed that activity in temporoparietal networks, including the temporoparietal junction and right precuneus, corresponded with manipulations thought to affect the perception of presence. Furthermore, data suggest that networks associated with embodiment and presence overlap, and brain areas that support perception may be predicated upon those that support embodiment. The results of these experiments offer further clues into the psychophysiological mechanisms underlying BSC

Perception and Emotion in Virtual Reality: The Role of the Body and the Contribution of Presence

This thesis reports four studies in the context of virtual reality (VR), feelings of presence, emotion, and perception. Previous research established the existence of cross-dimensional perceptual interrelations such as the interconnection between experienced motion and subjective time. This is thought to result from a common perceptual system. However, the specifics of this system are a matter of ongoing research. An important binding factor between perceptual dimensions is the bodily self, which was described as a reference for perception. In Study I, manipulations of the size of a virtual self-representation were shown to affect the spatial judgment of objects. In Study II, the degree of self-motion in an immersive virtual environment (IVE) influenced the subjective perception of time, corroborating previous findings about the common perceptual system. Besides the virtual self-representation, there is another important variable in VR experiments: Presence is described as the feeling of being in a mediated environment. Presence was not associated with improved performance in the spatial and temporal judgments of Studies I and II. However, in Study III, presence in a gaming activity was linked to improved mood after an experimental stress-induction. This especially applied to VR gaming, where impressions about the subjective realism of the IVE might have been crucial for mood repair. As outlined in Study IV, it is important to distinguish between presence as an attentional allocation to the mediated world and as an individual judgment about its realism. Taken together, the results from all studies corroborate the idea of the self as a fundamental perceptual reference, confirm results about the psychological connection between space and time, emphasize the benefits of VR gaming in improving mood, and elucidate the role of perceived realism in assessing presence in IVEs