Exploring the roles of body ownership, vision and virtual reality on heat pain threshold
Helen R. Gilpin, Valeria Bellan, Alberto Gallace, G. Lorimer Moseley · European Journal of Pain · 2014
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 and ownership of a virtual arm might increase heat pain thresholds. Might other factors also be at play here? Could the virtual synchronous condition, by activating multisensory neural networks (or via a super-additivity effect), just be more distracting than the other conditions? Would a similar pattern be observed if participants viewed someone else's limb inside the virtual environment rather than their own, or viewed their own arm outside the virtual environment? Relevant to this is the finding that viewing a mannequin can increase pain thresholds even when an illusion of ownership is not induced (Hänsel et al., 2011). Notwithstanding these open questions, the study by Martini et al. provides valuable insight into how experimentally induced pain can be modulated by manipulating multisensory input and raises some fascinating new possibilities of using virtual reality to modulate clinical pain. None declared.