Reversing the rubber hand illusion with demand characteristics

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.

Affective touch modulates the rubber hand illusion

Our sense of body ownership results from the ongoing integration of perceptual information coming from the different senses (i.e., multisensory integration). The Rubber Hand Illusion (RHI) has been extensively studied to investigate the malleability of body ownership through contrasting multisensory information. Indeed, during the RHI, stroking a visible rubber hand synchronously to participants’ hand hidden from sight generates the illusion of ownership of the rubber hand (embodiment) and the mis-location of participants’ hand as closer to the rubber hand (proprioceptive drift). It is well known that the RHI is optimally evoked by a pleasant stroking (affective) touch, but what of an unpleasant (painful) stroking touch – does hedonic valence matter? To this aim, participants repeated the RHI while receiving different types of touch: pleasant, painful, and neutral. Results showed, for the first time, that the subjective intensity of the tactile stimulation experienced across the different conditions modulates the strength of the proprioceptive drift. Notably, participants reported a stronger RHI (mis-placed body ownership) from stimulation rated as more intense and involving an interoceptive activation (pain and pleasantness vs. neutral). We propose that interoceptive information, regardless of the valence of the stimuli (positive or negative), are perceived as more intense and enhance, through the activation of the limbic system, multisensory integration. In the context of the RHI, this translates to a stronger illusion in terms of proprioceptive drift.

Parkinson's disease alters multisensory perception: Insights from the Rubber Hand Illusion

The rubber hand illusion is one reliable way to experimentally manipulate the experience of body ownership. However, debate continues about the necessary and sufficient conditions eliciting the illusion. We measured proprioceptive drift and the subjective experience (via questionnaire) while manipulating two variables that have been suggested to affect the intensity of the illusion. First, the rubber hand was positioned either in a posturally congruent position, or rotated by 180°. Second, either the anatomically same rubber hand was used, or an anatomically incongruent one. We found in two independent experiments that a rubber hand rotated by 180° leads to increased proprioceptive drift during synchronous visuo-tactile stroking, although it does not lead to feelings of ownership (as measured by questionnaire). This dissociation between drift and ownership suggests that proprioceptive drift is not necessarily a valid proxy for the illusion when using hands rotated by 180°.

Empirical evidence suggests that body ownership and the sense of agency operate as an interactive system correlated with the level of consciousness during tasks involving modifications in body representation. This study sought to elucidate the nature of this association by documenting the verbal manifestations of this interaction. Specifically, the study aimed to reveal the role of a sense of agency in the individual sensitivity to the Rubber Hand Illusion (RHI). The sense of agency was measured through a post hoc interview wherein participants could verbally express their rubber hand ownership and real hand disownership experiences following the RHI examination. The RHI was induced in 49 healthy, right-handed college volunteers, including 28 males (mean age 28.6) and 21 females (mean age 26.6). Three main scores - ownership, disownership, and proprioceptive drift - were defined to measure individual sensitivity to Rubber Hand Illusion. Verbal reports related to the RHI were analyzed utilizing an automated narrative content analysis toolkit which explored the deeper content of words and stories to identify situation-driven cognitive processes, specifically focusing on the rate of sense of agency and other cognitive variables. The findings indicated that a greater disruption in the sense of agency predicts increased sensitivity to the Rubber Hand Illusion. Therefore, individuals with a lower rate sense of agency exhibit increased malleability in body representation when a rubber hand illusion is induced.

In the rubber hand illusion (RHI), participants see a fake hand touched synchronously with their real hand, which is hidden from view. The three-way interaction between vision, touch, and proprioception induces the sensation that the dummy hand belongs to oneself (i.e., subjective embodiment) and the illusory displacement of the real hand towards the fake one (i.e., proprioceptive drift). In the literature, there are mixed results (some positive and some null) regarding the existence of a relationship between subjective embodiment and proprioceptive drift. We conducted a Bayesian meta-analysis to tackle this issue quantitatively. Evidence strongly favours the presence of a correlation between subjective embodiment and proprioceptive drift, supporting the model proposed by Botvinick and Cohen in 1998. However, the correlation is around .35, a value suggesting that the two indices capture different facets of the RHI. This result clarifies the association between the illusory effects produced by the RHI and may be helpful for designing studies having appropriate statistical power.

Patients with dystonia display a number of disturbances in the cognitive processing of movements, such as movement simulation and prediction, but whether these deficits point to a deeper rooted disturbance of perceptual bodily representations remains unknown. A useful way to investigate the sense of body ownership is the rubber hand paradigm, in which an illusion of ownership is established by synchronous stroking of the participants' real unseen hand and a visible fake hand, whereas similar asynchronous stroking does not bring about the illusion. This paradigm allows testing of both the subjective experience of feeling ownership over the rubber hand and the proprioceptive relocation of the real unseen hand towards the viewed rubber hand. Previous studies have mapped these different aspects onto two anatomically distinct neuronal substrates, with the ventral premotor cortex processing the illusory feeling of ownership and the inferior parietal lobule and cerebellum processing proprioceptive drift. We applied the rubber hand illusion task to healthy subjects and to patients affected by two different types of focal dystonia-one specifically affecting the hand (focal hand dystonia) and one not affecting the hand (torticollis and blepharospasm). Results showed that in patients with focal hand dystonia, the proprioceptive drift was selectively disrupted on the dystonic hand while the subjective experience of the illusion was retained. In the non-dystonic hand and in the other two groups (non-hand dystonia and healthy subjects), the rubber hand illusion resembled the typical pattern with synchronous stroking eliciting the illusion. These findings provide support for the contention that the mechanisms underlying the presence of the illusory feeling of ownership and the proprioceptive drift are different. Selective impairment of the limb recalibration on the dystonic hand points to underlying deficits in integrating the visual-tactile input with the proprioceptive information in order to update the current body position and may support a model linking dystonia to dysfunctions in a network comprising the inferior parietal cortex and the cerebellum.

Correlations between within-subject variability of pain intensity reports and rubber hand illusion proprioceptive drift

Subjective, behavioral, and physiological responses to the rubber hand illusion do not vary with age in the adult phase.

The rubber hand illusion in action

Rubber hand illusion is an epoch-making phenomenon on the study about the body ownership. The neural correlates of this illusory phenomenon has been revealed by many researchers, while the information flow in the cortex related this illusion was not clear. We recorded EEG during simultaneous presentation of visuotactile stimuli using a rubber hand with a setting inducing rubber hand illusion, and also obtain the subjective reports for illusory feeling. Using renormalized partial directed coherence (RPDC) as an index of a cortical causality, we evaluated the information flow inducing the illusory experiences. As a result, at the low frequency band, we found the significant increase of neural connectivity between the parietal area and the other cortical areas. By correlation analysis, we found that some causalities are correlated with the rubber hand illusion and proprioceptive drift.

Bodily self-awareness relies on a constant integration of visual, tactile, proprioceptive, and motor signals. In the 'rubber hand illusion' (RHI), conflicting visuo-tactile stimuli lead to changes in self-awareness. It remains unclear whether other, somatic signals could compensate for the alterations in self-awareness caused by visual information about the body. Here, we used the RHI in combination with robot-mediated self-touch to systematically investigate the role of tactile, proprioceptive and motor signals in maintaining and restoring bodily self-awareness. Participants moved the handle of a leader robot with their right hand and simultaneously received corresponding tactile feedback on their left hand from a follower robot. This self-touch stimulation was performed either before or after the induction of a classical RHI. Across three experiments, active self-touch delivered after-but not before-the RHI, significantly reduced the proprioceptive drift caused by RHI, supporting a restorative role of active self-touch on bodily self-awareness. The effect was not present during involuntary self-touch. Unimodal control conditions confirmed that both tactile and motor components of self-touch were necessary to restore bodily self-awareness. We hypothesize that active self-touch transiently boosts the precision of proprioceptive representation of the touched body part, thus counteracting the visual capture effects that underlie the RHI.

The sense of the body is deeply rooted in humans, and it can be experimentally manipulated by inducing illusions in at least two aspects: a subjective feeling of ownership and a proprioceptive sense of limb position. Previous studies mapped these different aspects onto anatomically distinct neuronal regions, with the ventral premotor cortex processing subjective experience of ownership and the inferior parietal lobule processing proprioceptive calibration. Lines of evidence suggest an involvement also of the cerebellum, but its precise role is not clear yet. To investigate the contribution of the cerebellum in the sense of body ownership, we applied the rubber-hand illusion paradigm in 28 patients affected by neurodegenerative cerebellar ataxia, selectively involving the cerebellum, and in 26 age-matched control participants. The rubber hand illusion is established by synchronous stroking of the participants' real unseen hand and a visible fake hand. Short asynchronous stroking does not bring about the illusion. We tested the subjective experience of the illusion, evaluated through a questionnaire and the proprioceptive drift of the real unseen hand toward the viewed rubber hand. In patients with cerebellar ataxia, we observed reduced sense of the subjective illusory experience specifically after synchronous stroking. In contrast, the proprioceptive drift was enhanced after synchronous and after asynchronous stimulation. These findings support the contention that the mechanisms underlying the presence of the illusion and the proprioceptive drift may be differently affected in different conditions. Impairment of the subjective sense of the illusion in cerebellar patients might hint at an involvement of cerebellar-premotor networks, whereas the proprioceptive drift typically associated with synchronous stroking appears to rely on other circuits, likely involving the cerebellum and the parietal regions.

The sense of body ownership, a feeling that one’s body belongs to the self, is an essential aspect of self-consciousness. Studies have focused on emotions and bodily states that could influence multisensory integration for the sense of body ownership. Based on the Facial Feedback Hypothesis, the purpose of this study was to examine whether displaying specific facial expressions affects the rubber hand illusion. We hypothesized that the expression of a smiling face changes the emotional experience and facilitates the formation of a sense of body ownership. In the experiment, participants (n = 30) were asked to hold a wooden chopstick in their mouths to simulate smile, neutral, and disgusted facial expressions during the induction of the rubber hand illusion. The results did not support the hypothesis, showing that proprioceptive drift (an index of illusory experience) was enhanced when the subjects displayed a disgusted facial expression, while the subjective reports of the illusion were not affected. These results, together with the previous studies regarding the effect of positive emotions, suggest that bodily affective information, regardless of its valence, facilitates multisensory integration and could influence the conscious representation of the bodily self.