Psychophysiological effects of threatening a rubber hand that is perceptually embodied in healthy human subjects
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Laura Petrini
In this issue of Scandinavian Journal of Pain, a study by Johnson and colleagues investigated the roles of arousal and threatening in the peripersonal space during a rubber hand illusion [1]. The study is designed to evaluate whether or not arousal is selectively associated with threatening stimuli entering into the peripersonal space. The study uses the paradigm of the rubber hand illusion, where participants are induced to behave as if a rubber hand is part of their own body. The illusion is usually induced by brushing a person’s hand, hidden from view, while synchronously brushing a visible rubber hand. In this study, the authors used brushing to simulate a non-threatening condition and a needle to modulate a threatening condition.
This paper outlines one of the most intriguing aspects: how self-body representation influences perception and cognition. Recent theories are suggesting that perception and cognition are influenced by internal representations of the body. This hypothesis is corroborated by research evidence showing for example that vision of the body increases spatial acuity of touch.
Both experimental and clinical studies also highlight the importance of body representation in the modulation of pain experience using cross-modal illusions. These studies show that body self-representation and pain perception are related.
1 The use of cross-modal illusion (rubber hand illusion) in pain research
In recent years, an increasing number of studies on cross- modal illusions (i.e.: rubber hand illusion, mirror box illusion) have investigated how multisensory interactions shape human body representation and in turn modulate pain perception.
The most widely known example of a cross-modal illusion is the rubber hand illusion (RHI) [2]. RHI is a suitable paradigm to uncover mechanisms of body ownership, plasticity of body representations, and in particular disentangle the influence of alterations in selfrepresentation on sensory processing.
It has been suggested that one of the mechanisms through which RHI arises is due to the resolution of a multisensory conflict involving visual, tactile and proprioceptive information, culminating in a re-calibration of the location of the touch and the felt position of the hand with the result that touch appears to be felt by the rubber hand. Nevertheless, alternative explanations have also been proposed, i.e. Bayesian perceptual learning. RHI involves a number of multisensory areas of the brain including premotor, parietal and cerebellar structures.
RHI has been widely investigated in experimental and clinical settings, although its applicability in rehabilitation is still in an early stage of study. In pain research, RHI has produced alterations in sensory processing, in particular resulting in powerful analgesic effects. These analgesic effects are explained with an increase of ownership over a fake “pain-free” hand, while a concomitant reduction in awareness (or dis-ownership) of one’s own affected body part is experienced.
This is fascinating, but not all the experimental results have reported a reduction of pain perception. In fact, well-powered experiments have shown no effect on RHI on experimental pain perception over the real hand [3,4]. Recently, modifications of the classical rubber hand with an “injured/wounded” rubber hand have generated a reduction of thermal pain threshold [5] and tolerance threshold as well as an increase in pain intensity and unpleasantness [6].
2 Plasticity of body representation
We often have the impression that our body representation is stable and relative fixed over time. Experiments using the RHI have shown, instead, that our body representation is more fragile and malleable than it seems. Body representation requires indeed some degree of plasticity so that changes in the actual physical body can be mirrored by corresponding changes in the brain’s maps and in the conscious body image [7]. More bizarre variations of the RHI, such as having the illusion of an elongated arm or owning a third arm, have produced changes in the corresponding somatosensory maps [8]. Interestingly, body maps topography seems to reflect the perceived rather than the physical aspects of the peripheral stimulus.
Growing evidence shows that people with chronic pain have a distorted body image, defined as a conscious representation of the body, the way one's body feels to be its own [9]. The most obvious example is related to phantom limb pain, where amputees report that their missing limb feels heavy, swollen, floating, stuck in a particular position, missing digits or entire segments (“telescoping”). Patients affected by Complex Regional Pain Syndrome (CRPS) report similar evidence, often describing their painful limb as swollen and depicted as lager than it really is. Distortions in shape are also observed such as too short or with a missing segment [9].
Often, however, the symptoms resemble the ones seen in patients with hemi-spatial neglect, therefore these symptoms are referred to as “neglect-like”. In these cases, patients reported that the affected limb seemed foreign or did not seem to belong to them. Sometime they have difficulties in identifying which finger is touched, or they cannot move their fingers unless they look at them.
3 The role of peripersonal space in perception
Johnson et al. recorded skin conductance response (SCR), in order to provide a more objective measure of the psychological and autonomic arousal caused by the rubber hand illusion [1]. It is in fact preferable to combine objective and subjective methods to avoid ambiguity especially when subjects answer to self-report questionnaires. These answers, in fact, can be easily biased by the task demand or by the metaphorical descriptions of the experience. This is even more true in the case of perceptual illusions.
Johnson et al. reported no differences in SCR between nonthreatening and threatening stimuli when entering into the peripersonal space (space around the body) [1]. This is perse an interesting aspect since it adds extra value to the discussion of the role of the peripersonal space in the construction of the body perception. Probably, any type of stimulus approaching the rubber hand is able to evoke a protective response. Thus, it seems that we not only own our body parts but also the space around them as well. Similar examples are also seen in pain perception. Pain can be precipitated by viewing an object approaching the limb and pain increases as the object gets closer.
4 Phantom-like sensations in healthy individuals
In the study of Johnson et al. some participants reported sensations, such as tingling and numbness, as they were generated from the rubber hand, although there was a high inter-individual variability in the generation of these experiences [1]. These projected sensations resemble phantom-like sensations observed in amputees, and are also reported by other authors [10]. Recently, Lewis et al. [11] using a RHI with a missing finger reported similar sensations, such as tingling and numbness together with the vivid presence of a finger that they could not see.
5 Conclusion and implications
Cross-modal illusions, such as RHI, are essential experimental paradigm for pain research. Further research is necessary to understand the complex relationship between body self-representation and pain perception.
DOI of refers to article: http://dx.doi.org/10.1016/j.sjpain.2015.10.004.
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Conflicts of interest: None has been declared.
References
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© 2015 Scandinavian Association for the Study of Pain
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- Editorial comment
- Psychophysiological effects of threatening a rubber hand that is perceptually embodied in healthy human subjects
- Original experimental
- A preliminary investigation into psychophysiological effects of threatening a perceptually embodied rubber hand in healthy human participants
- Editorial comment
- Analysis of C-reactive protein (CRP) levels in pain patients – Can biomarker studies lead to better understanding of the pathophysiology of pain?
- Clinical pain research
- Serum C-reactive protein levels predict regional brain responses to noxious cold stimulation of the hand in chronic whiplash associated disorders
- Editorial comment
- Importance of early diagnosis of complex regional pain syndrome (CRPS-1 and CRPS-2): Delayed diagnosis of CRPS is a major problem
- Clinical pain research
- Delayed diagnosis and worsening of pain following orthopedic surgery in patients with complex regional pain syndrome (CRPS)
- Editorial comment
- Associative learning mechanisms may trigger increased burden of chronic pain; unlearning and extinguishing learned maladaptive responses should help chronic pain patients
- Original experimental
- When touch predicts pain: predictive tactile cues modulate perceived intensity of painful stimulation independent of expectancy
- Editorial comment
- Low back pain among nurses: Common cause of lost days at work and contributing to the worldwide shortage of nurses
- Observational study
- Pain-related factors associated with lost work days in nurses with low back pain: A cross-sectional study
- Editorial comment
- Assessment of persistent pelvic pain after hysterectomy: Neuropathic or nociceptive?
- Clinical pain research
- Characterization of persistent pain after hysterectomy based on gynaecological and sensory examination
- Editorial comment
- Transmucosal fentanyl for severe cancer pain: Nasal mucosa superior to oral mucosa?
- Original experimental
- Facilitation of accurate and effective radiation therapy using fentanyl pectin nasal spray (FPNS) to reduce incidental breakthrough pain due to procedure positioning
- Editorial comment
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- Clinical pain research
- Peripheral morphine reduces acute pain in inflamed tissue after third molar extraction: A double-blind, randomized, active-controlled clinical trial
- Editorial comment
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- Observational study
- The Survey of Pain Attitudes: A revised version of its pediatric form
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- Observational study
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- The role of catastrophizing in the pain–depression relationship
- Clinical pain research
- The mediating role of catastrophizing in the relationship between pain intensity and depressed mood in older adults with persistent pain: A longitudinal analysis
- Announcement
- May 26-27, 2016 Scandinavian Association for the Study of Pain, Reykjavik, Iceland May 25, 2016 PhD course
