Pain Perception Changes When Body Part Size Is Distorted

Summary
A study was conducted to investigate whether manipulating the visual perception of size can lead to modulation of acute pain perception in healthy volunteers. Pain threshold for each subject was measured while they viewed their own hand or a neutral object, in different sizes. The results demonstrated that pain perception depends on visual size and that increasing the size of vision enhances analgesia, while visual reduction decreases analgesia.

Introduction
Acute pain is an experience whereby noxious impulses generated from the site of injury are carried and processed in the brain. The perception of pain varies across individuals and in different situations, which indicates the subjective aspect involved in it. Studies have shown that modulating the visual appearance of the body interferes with the afferent impulses generated from the site, thereby modulating the perception of pain. “Both reported intensity of pain and neural responses to painful stimuli are reduced when participants look at their own body, compared with when they view a neutral object.” The authors studied the role of different visual contexts in the modulation of experimentally induced heat pain.

Methodology
* Healthy volunteers were subjected to thermal heat (with increasing temperature) on the dorsum of their hands till they pressed a foot-pedal to denote the perceived stimulus as painful.
* A mirror was placed in front of each subject, with their left hand behind the mirror and right hand in front of the mirror.
* Subjects were divided into two groups – one group visualized the reflection of right hand and perceived it to be their left hand (hand view condition) whereas the other group visualized the same distortion with a wooden block covering the hand (object view condition).
* The viewed size of the hand or object was manipulated into three different views: reduced image, normal image and magnified image.

Key findings
* Viewing the hand via the non-distorting mirror (hand-view condition) increased heat-pain thresholds, relative to viewing the object (objective-view condition).
* Viewing the hand by the participants (hand view condition) at its actual size produced visual pain relief.
* Visual enlargement of size increased the pain relieving effect of viewing the body, whereas visual reduction decreased the pain relieving effect.
* Manipulating view size did not have any effect on modulating pain in the object-view condition group.

Next steps/Shortcomings
The findings from the study need to be validated, as earlier studies showed contrasting results. In previous reports, viewing a stranger’s hand did not influence pain levels or the brain’s response to painful stimulation, whereas participants who viewed photographs of their partner experienced reduced heat pain. Besides, the results from a chronic pain study (complex regional pain syndrome) also reported that the pain ratings and swelling evoked by movement increased when patients viewed the affected limb enlarged and decreased when they viewed the limb in reduced size.

Conclusion
The study demonstrated that when a noxious stimulus is applied to a body part that is seen as bigger than its actual size, the stimulus needs to be greater than it otherwise would need to be, to produce pain. When a stimulus is applied to a part of the body that is perceived as smaller than its real size, the pain threshold is decreased. Cognitive therapies are currently focusing on the origin of painful signals in the form of modulating attention of the recipient towards the stimuli. However, the current study suggests that the visual impression of the body and its appearance can also have therapeutic implications in the branch of cognitive therapy for pain.

For More Information:
Visual Distortion of Body Size Modulates Pain Perception

Publication Journal: Psychological Science, February 2011
By Flavia Mancini; Matthew R Longo; University College London, University of Milano-Bicocca and University of London

*FYI Living Lab Reports Are Summaries of the Original Research.