Valeria Bellan

The effect of bodily illusions on clinical pain: a systematic review and meta-analysis.

Abstract This systematic review and meta-analysis critically examined the evidence for bodily illusions to modulate pain. Six databases were searched; 2 independent reviewers completed study inclusion, risk of bias assessment, and data extraction. Included studies evaluated the effect of a bodily illusion on pain, comparing results with a control group/condition. Of the 2213 studies identified, 20 studies (21 experiments) were included. Risk of bias was high due to selection bias and lack of blinding. Consistent evidence of pain decrease was found for illusions of the existence of a body part (myoelectric/Sauerbruch prosthesis vs cosmetic/no prosthesis; standardized mean differences = −1.84, 95% CI = −2.67 to −1.00) and 4 to 6 weeks of mirror therapy (standardized mean differences = −1.11, 95% CI = −1.66 to −0.56). Bodily resizing illusions had consistent evidence of pain modulation (in the direction hypothesized). Pooled data found no effect on pain for 1 session of mirror therapy or for incongruent movement illusions (except for comparisons with congruent mirrored movements: incongruent movement illusion significantly increased the odds of experiencing pain). Conflicting results were found for virtual walking illusions (both active and inactive control comparisons). Single studies suggest no effect of resizing illusions on pain evoked by noxious stimuli, no effect of embodiment illusions, but a significant pain decrease with synchronous mirrored stroking in nonresponders to traditional mirror therapy. There is limited evidence to suggest that bodily illusions can alter pain, but some illusions, namely mirror therapy, bodily resizing, and use of functional prostheses show therapeutic promise.

Untangling visual and proprioceptive contributions to hand localisation over time.

Abstract Previous studies showed that self-localisation ability involves both vision and proprioception, integrated into a single percept, with the tendency to rely more heavily on visual than proprioceptive cues. Despite the increasing evidence for the importance of vision in localising the hands, the time course of the interaction between vision and proprioception during visual occlusion remains unclear. In particular, we investigated how the brain weighs visual and proprioceptive information in hand localisation over time when the visual cues do not reflect the real position of the hand. We tested three hypotheses: Self-localisations are less accurate when vision and proprioception are incongruent; under the same conditions of incongruence, people first rely on vision and gradually revert to proprioception; if vision is removed immediately prior to hand localisation, accuracy increases. Sixteen participants viewed a video of their hands, under three conditions each undertaken with eyes open or closed: Incongruent conditions (right hand movement seen: inward, right hand real movement: outward), Congruent conditions (movement seen congruent to real movement). The right hand was then hidden from view and participants performed a localisation task whereby a moving vertical arrow was stopped when aligned with the felt position of their middle finger. A second experiment used identical methodology, but with the direction of the arrow switched. Our data showed that, in the Incongruent conditions (both with eyes open and closed), participants perceived their right hand close to its last seen position. Over time, the perceived position of the hand shifted towards the physical position. Closing the eyes before the localisation task increased the accuracy in the Incongruent condition. Crucially, Experiment 2 confirmed the findings and showed that the direction of arrow movement had no effect on hand localisation. Our hypotheses were supported: When vision and proprioception were incongruent, participants were less accurate and initially relied on vision and then proprioception over time. When vision was removed, this shift occurred more quickly. Our findings are relevant in understanding the normal and pathological processes underpinning self-localisation.

Pain by Association? Experimental Modulation of Human Pain Thresholds Using Classical Conditioning

UNLABELLED A classical conditioning framework is often used for clinical reasoning about pain that persists after tissue healing. However, experimental studies demonstrating classically conditioned pain in humans are lacking. The current study tested whether non-nociceptive somatosensory stimuli can come to modulate pain thresholds after being paired with painful nociceptive stimuli in healthy humans. We used a differential simultaneous conditioning paradigm in which one nonpainful vibrotactile conditioned stimulus (CS(+)) was simultaneously paired with an unconditioned painful laser stimulus, and another vibrotactile stimulus (CS(-)) was paired with a nonpainful laser stimulus. After acquisition, at-pain-threshold laser stimuli were delivered simultaneously with a CS(+) or CS(-) vibrotactile stimulus. The primary outcome was the percentage of at-threshold laser stimuli that were reported as painful. The results were as expected: after conditioning, at-threshold laser trials paired with the CS(+) were reported as painful more often, as more intense, and as more unpleasant than those paired with the CS(-). This study provides new evidence that pain thresholds can be modulated via classical conditioning, even when the stimulus used to test the threshold cannot be anticipated. As such, it lays a critical foundation for further investigations of classical conditioning as a possible driver of persistent pain. PERSPECTIVE This study provides new evidence that human pain thresholds can be influenced by non-nociceptive somatosensory stimuli, via a classical conditioning effect. As such, it lays a critical foundation for further investigations of classical conditioning as a possible driver of persistent pain.

Integrating Self-Localization, Proprioception, Pain, and Performance.

The ability to know where our own body and body parts are in space is often taken for granted, yet it is of fundamental importance for the majority of our everyday activities, let alone high performance activities such as dancing. This review focuses on the concept of self-localization, the monitoring of the space surrounding ones body, and the disruptions that occur in the presence of pain. A conceptual model is presented of the cortical body matrix with which to consider self-localization; also provided are its historical context, underlying assumptions, and current limitations. Issues described include the neurophysiological and behavioral background to the cortical body matrix model, its application to pain and performance, and the rapidly growing use of bodily illusions to investigate how it is that we know where we are, that we exist in a given location, and that we can interact with the space that surrounds us. Recent insights are drawn on from behavioral, clinical, neuroimaging, and physiological research. Spatial performance is discussed in people with and without pain and its relevance for prevention of injuries, the role of pain during performance, and pain education for dancers and their teachers.

Exploring the roles of body ownership, vision and virtual reality on heat pain threshold.

In this issue, you will find a paper entitled, ‘Modulation of pain threshold by virtual embodiment’ (Martini et al., 2014). The authors aimed to investigate the impact of virtual body ownership on pain threshold using a virtual environment in which participants viewed a first-person perspective of a virtual body that replaced their own. Thirty-two participants undertook four conditions. Three conditions involved being ‘in’ the virtual environment, and viewing a virtual limb with (1) synchronous or (2) asynchronous movements of their real and virtual fingers, or (3) viewing a virtual object rather than a virtual hand. The final condition involved being outside the virtual environment and viewing a fixation point on a screen that occluded their hand from vision. The authors reported that heat pain threshold was significantly higher in the virtual synchronous condition than it was in viewing a virtual object condition and in viewing the fixation point condition. They concluded that ownership of a virtual arm increases thermal pain threshold. The question of what makes the body feel like one’s own has long been explored by philosophers and more recently by psychologists and neuroscientists. A stable representation of what makes up our own body has vital evolutionary advantages, yet there are some neurological and psychological disorders in which this sense of ‘body ownership’ is disrupted, with patients reporting a sense that a body part does not belong to them or even that it belongs to somebody else. The parameters involved in body ownership have been widely investigated in healthy participants using bodily illusions such as the rubber hand illusion (RHI). The RHI uses congruent visual and tactile stimuli to a dummy hand and the participant’s hidden real hand to evoke the perception that the stimulus comes from the rubber hand. The majority of participants also develop a sense of ownership over the dummy hand. Interestingly, two well-powered experiments undertaken by independent groups showed no effect of the RHI on experimental pain levels in the real limb (Mohan et al., 2012). This study by Martini et al. (2014) raises some intriguing questions into the mechanisms behind the analgesia observed inside the virtual environment. The distraction of being inside an immersive virtual reality can in itself decrease pain during painful medical procedures (e.g., Hoffman et al., 2011), and simply looking at one’s own body in itself has an analgesic effect in healthy participants (Longo et al., 2009). Martini et al. proposed that their study extends these earlier findings by Longo et al. by showing that looking at a fake body may also be analgesic as long as it is perceived as one’s own. That there was no difference in pain threshold between the asynchronous and synchronous conditions, even though perceived ownership was (predictably) greater in the synchronous condition, leaves us unconvinced. That said, pain thresholds in the asynchronous virtual condition were not significantly different from the virtual object condition, or when the arm was occluded outside virtual reality, which suggests that the study may have been underpowered to detect the difference. Therefore, we think the most likely explanation for the reported results is that the observed analgesia is produced by a combination of vision and ownership of a virtual body inside a virtual environment. We do not think the current study design allows one to disentangle the relative involvement of these factors. What about the role of vision? The authors found that viewing the virtual limb with synchronous movements led to higher pain thresholds than when the participant’s real limb was hidden outside the virtual environment, yet here the role of vision cannot be disentangled from the effects of merely being inside the virtual environment. The virtual synchronous condition also led to higher pain thresholds than viewing a virtual object, yet vision cannot be disentangled from the effects of ownership, as confirmed by the much lower ratings of ownership in the object condition. Thus, it seems that neither vision nor ownership is sufficient to explain the observed analgesia. The study does, however, suggest that the combination of vision

The parietal cortex and pain perception: a body protection system

Pain is a very relevant function for our survival and its alteration leads to important consequences for peoples lives. In the last decades, researchers have started to investigate pain from a neurocognitive and neuropsychologic perspective, showing some important similarities and differences with other cognitive and perceptual functions. The complexity of pain perception, due to its multicomponential nature, has led to the need to interpret pain within a multisensory frame of reference. That is, this function and its neural foundation cannot be fully understood without taking into consideration the processing of other information that is concurrently presented together with noxious stimuli. The extant research in this field has shown that the parietal cortex plays a major role in pain perception, due to its involvement in the integration of information coming from different sources, to the support of body ownership, and to its role in sustaining spatial attention and awareness. It is likely that this neurocognitive knowledge will lead to better treatments of chronic and acute pain in the future.

No Telescoping Effect with Dual Tendon Vibration.

The tendon vibration illusion has been extensively used to manipulate the perceived position of one’s own body part. However, findings from previous research do not seem conclusive sregarding the perceptual effect of the concurrent stimulation of both agonist and antagonist tendons over one joint. On the basis of recent data, it has been suggested that this paired stimulation generates an inconsistent signal about the limb position, which leads to a perceived shrinkage of the limb. However, this interesting effect has never been replicated. The aim of the present study was to clarify the effect of a simultaneous and equal vibration of the biceps and triceps tendons on the perceived location of the hand. Experiment 1 replicated and extended the previous findings. We compared a dual tendon stimulation condition with single tendon stimulation conditions and with a control condition (no vibration) on both ‘upward-downward’ and ‘towards-away from the elbow’ planes. Our results show a mislocalisation towards the elbow of the position of the vibrated arm during dual vibration, in line with previous results; however, this did not clarify whether the effect was due to arm representation contraction (i.e., a ‘telescoping’ effect). Therefore, in Experiment 2 we investigated explicitly and implicitly the perceived arm length during the same conditions. Our results clearly suggest that in all the vibration conditions there was a mislocalisation of the entire arm (including the elbow), but no evidence of a contraction of the perceived arm length.

Evidence for ‘visual enhancement of touch' mediated by visual displays and its relationship with body ownership

Several studies have shown that watching ones own body part improves tactile acuity and discrimination abilities for stimuli presented on that location. In our experiment we asked the participants to localize tactile stimuli presented on the left or right arm. During the task the participants were not allowed to watch their body, but they could see another persons left arm via a LCD display. This arm could be touched or not during the presentation of the tactile stimuli. We found that when the participants saw a finger touching the arm on the screen, their responses to the tactile stimuli presented on the left and on the right arm were faster than when the arm on the screen was approached but not touched. Critically, we did not find any illusion of ownership related to the hand seen on the screen. We concluded that the effects found might be mediated by higher order multisensory mechanisms related to the allocation of attentional resources to the body.