2019 Immersive Virtual Reality and Virtual Embodiment for Pain Relief | Human Neuroscience

September 3, 2019, 9:27 PM

Italian researchers in Pavia  (and one from Barcelona)

in  Frontiers in Human Neurosci  21 Aug 2019.

<h3>Embodiment in VR for Pain Relief</h3>

In the context of these studies, Martini and colleagues investigated the effect of virtual body ownership on pain perception and found that looking at one’s own virtual hand also had analgesic properties, as described for the real hand (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B66">Longo et al., 2009</a>) (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#F2">Figure 2</a>). An increased experimental pain threshold was found when compared with the observation of either a real or a virtual object (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B77">Martini et al., 2014</a>). Further, they found that the feeling of ownership over the virtual arm was crucial to accomplish the analgesic effect. Regardless, the analgesic effect experienced while observing one’s own body seems to be effective even when observing an embodied virtual body if participants experienced high levels of ownership of the body. The fact that looking at one’s own “rubber hand” (after inducing the RHI) is not analgesic (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B84">Mohan et al., 2012</a>) opened up a debate regarding the extent to which looking at a surrogate body was actually analgesic. This issue was sorted out by <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B91">Nierula et al. (2017)</a>, who demonstrated the relevance of the position of the surrogate with respect to the real hand. While the rubber hand cannot be co-located with the real hand (since they both occupy physical space), the virtual hand can be co-located (or not) with the real hand. <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B91">Nierula et al. (2017)</a> demonstrated that as the distance between the real and the virtual hand increases, the analgesic effect decreases (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#F3">Figure 3A</a>). In agreement with this, previous findings by Romano and colleagues also reported reduced physiological responses to painful stimuli measured via SCRs, when participants observed a virtual body from a first-person perspective co-located with their real body compared with observing the virtual body turned 90° from the real body (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B105">Romano et al., 2015</a>). Moreover, in the same study, the authors observed that physiological responses were negatively correlated with the size of the virtual body: the bigger the virtual body, the lower the SCRs (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B105">Romano et al., 2015</a>). These results are in line with the observation of a magnified body part increased experimental heat pain thresholds (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B73">Mancini et al., 2011</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B106">Romano and Maravita, 2014</a>).

fig 2 and 3 are here.

Visual manipulations of the body modulate pain perception. One example is the study conducted by <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B76">Martini et al. (2013)</a> in which the color of a virtual arm was modified and the pain threshold was measured in healthy subjects (see <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#F3">Figure 3B</a>). Specifically, observation of a bluish “cold” virtual arm increased heat pain thresholds, whereas observation of a reddened “hot” virtual arm decreased heat pain thresholds. Co-location of the virtual body with the real one seems to be another key factor for increasing pain thresholds in healthy subjects (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B91">Nierula et al., 2017</a>).

Although evidence suggests that observing one’s own body while experiencing a painful stimulus reduces pain perception, what would happen if the painful part of the body were to fade away? To answer this question, Martini and co-workers conducted an experimental study in which the virtual body was rendered with different levels of transparency while participants were exposed to a painful heat stimulus. They found that the higher levels of transparency were inversely correlated with levels of ownership, but where the body was semi-transparent, higher levels of ownership over a see-through body resulted in an increased pain sensitivity (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B75">Martini et al., 2015</a>). Nevertheless, in clinical populations, the effect of transparency is less clear. In this regard, in a study by <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B80">Matamala-Gomez et al. (2018)</a>, two different groups of chronic arm pain patients [CRPS and peripheral nerve injury (PNI)] were immersed in VR and the virtual arm was observed by the patients at four different transparency levels (transparency test) and three different sizes (size test). In contrast to the study conducted on healthy subjects by <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B75">Martini et al. (2015)</a>, <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B80">Matamala-Gomez et al. (2018)</a>found that increasing transparency levels of the observed virtual arm decreased pain ratings in CRPS, but this did not occur in PNI. Size increase slightly increased pain ratings only in CRPS patients. Further, the authors found that patients with chronic pain can achieve levels of ownership and agency over a virtual arm similar to healthy participants. Moreover, the VR exposure to all of the conditions globally decreased the mean pain ratings by half by the end of the experiment compared to pain ratings at baseline (see <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#F4">Figure 4</a>). This study highlights the possibility that embodiment in VR decreases, at least temporarily, pain ratings in patients with chronic pain. The specific underlying mechanisms of each type of pain probably have a role in the type of strategy that is more effective for reducing pain perception in clinical populations. Further research is required to ascertain optimal dosage and duration of the effects.

fig 4 here.

Other investigations have also used embodiment in a virtual body to modulate pain perception in clinical populations. In a recent study by <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B115">Solcà et al. (2018)</a>, 24 CRPS patients were immersed in VR, embodied in a virtual body, and observed their affected virtual limb flashing in synchrony with their own detected heartbeat, or asynchronously in the control condition. Here, the authors observed reduced pain ratings and improved motor limb function while observing the synchronous heartbeat condition compared with the asynchronous control condition. Moreover, in another recent study that attempted to modulate neuropathic pain in spinal cord injury patients, the authors showed that VR exposure using multisensory stimulation is associated with mild analgesia, to suggest potential implications for spinal cord injury neurorehabilitation protocols (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B99">Pozeg et al., 2017</a>). Finally, <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B64">Llobera et al. (2013)</a> used body ownership illusions induced using immersive VR combined with a brain–computer interface (BCI) system in a single patient with dystonia of the upper limb suffering from chronic pain. The patient was embodied in a virtual body while observing a virtual hand opening either automatically or through a cognitive task assessed using a BCI that required patient effort. The evaluation was conducted also on a group of five healthy controls. The authors found that embodiment in the virtual body induced changes in electromyography and BCI tasks in the patient that were different from those observed in the controls (see <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#T1">Table 1</a> for a review).

<h2>Discussion</h2>

This review has discussed the potentialities of using an embodied virtual body in immersive VR for pain modulation. Specifically, we have discussed the use of multisensory integration applications, by means of body ownership illusions, to decrease pain perception in healthy and clinical populations.

In a systematic review conducted by <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B11">Boesch et al. (2016)</a> of non-virtual body illusions (illusory changes of size, mirror therapy, etc.) on clinical pain, they found that 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. Concerning the effects of embodiment on clinical pain, the authors discuss two studies of patients with chronic pain that showed no effect of embodiment on pain levels and suggest that a potential explanation is that embodiment and pain modulation may be separate processes. However, the review did not examine any studies that used immersive VR studies to induce embodiment. Here, we show that through an embodied virtual body, we may modulate body representation and change pain perception in healthy and clinical populations.

Regarding the importance of body representation in pain perception, it is known that many chronic pain patients have a distorted representation of the affected part of the body (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B63">Lewis et al., 2007</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B89">Moseley, 2008</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B110">Senkowski and Heinz, 2016</a>). Further, misrepresentations of the body have been associated with pain (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B69">Lotze and Moseley, 2007</a>), and several reports support structural and functional differences between people with and without pain, both at a cortical or at a subcortical level, in brain areas involved in body awareness and body perception (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B29">Flor et al., 1997</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B97">Pleger et al., 2006</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B41">Gwilym et al., 2010</a>). Distortions of body perception involving a painful part of the body (i.e., the body part feeling larger than it really is) have also been demonstrated (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B90">Moyer, 2005</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B63">Lewis et al., 2007</a>). There is some evidence that treatment directed at changing these functional brain alterations, such as graded motor imagery and sensorimotor retraining (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B88">Moseley, 2004</a>, <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B85">2006</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B97">Pleger et al., 2006</a>), reduces pain, which suggests that there is a bidirectional link between pain and body perception. In addition to this, it has been shown that pain perception is reduced with a corresponding restoration of functional cortical representation of the painful part of the body in CRPS patients (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B97">Pleger et al., 2006</a>

<h2>Future Research</h2>

These studies support a link between body perception and clinical disorders such as pain, highlighting the advantages of using embodiment through VR systems in neurorehabilitation and pain management. Nonetheless, robust and suitably powered randomized control trials are needed to further explore the full potential of body illusions and embodied technologies to modulate pain perception, especially with the use of immersive VR. Furthermore, further investigations aimed at modulating pain perception through an embodied virtual body with larger sample sizes will allow a better understanding of the contribution that the subjective feeling of ownership over an embodied virtual body has on pain perception. Moreover, future studies on this topic may make use of brain imaging techniques, which will allow better identification of the neural structures underlying the complex link between modification of body perception and pain.

Interestingly, virtual body embodiment may also allow the study empathy in pain. It is known that the mere observation of other people in pain tends to elicit empathic responses regarding pain perception in one’s body (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B60">Lamm et al., 2011</a>; <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B5">Benuzzi et al., 2018</a>). Hence, what will happen if we use embodiment to create a pain-free representation of the body? Although some authors have started to investigate how to use empathy for pain relief by using embodiment (<a href="https://www.frontiersin.org/articles/10.3389/fnhum.2019.00279/full#B32">Fusaro et al., 2016</a>), further investigations are needed to create new behavioral and cognitive training methods for modulating pain perception in clinical populations.

<h2>Conclusion</h2>

The studies commented throughout this narrative review, especially those conducted with chronic pain patients, pave the way for the design of new rehabilitation protocols with prolonged and repeated doses of embodied virtual body in immersive VR to tackle chronic pain disorders, and enable the integration of such “digital therapy” with existing conventional pain treatments.

2019 Immersive Virtual Reality and Virtual Embodiment for Pain Relief | Human Neuroscience

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