Thursday, October 08, 2009

Visual analgesia: Seeing the body reduces pain : Neurophilosophy

Vision is now well known to modulate the senses of touch and pain. For example, various studies have shown that looking at oneself being touched has the opposite effect - it can enhance tactile acuity, so that one can discriminate between two pinpoints which would otherwise feel like a single sensation. And last year, researchers from the University of Oxford showed that making a limb look larger or smaller than it actually using binoculars can respectively enhance and diminish painful sensations.

These phenomena occur because the brain fuses stimuli from different sensory systems to generate a coherent experience of bodily sensations. The precise mechanisms are unknown, and it is also unclear whether these effects depend upon specific visual stimuli. But according to a new study from University College London, the general "context" of vison is enough to modulate pain. In the current issue of the Journal of Neuroscience, they report that  merely looking at one's hand can affect the perception of laser-induced pain, and how it is processed in the cerebral cortex. Together with earlier work, these findings point to a simple method for managing acute pain. 

To investigate whether vision modulates pain, Matthew Longo and his colleagues carried out three simple experiments. In the first, 30 participants looked into a mirror which was placed on the table in front of them, and positioned just to the right of their body midline (below), so that when they turned to the right to look into the mirror, they saw what appeared to be their right hand, but was actually the reflection of their left hand. In another condition, the participants placed their left hand under the table, and a book was put in its place on the table, so that they saw its reflection when they looked into the mirror.

The remaining experiments were variations of this. In one, the mirror was removed from the table, and the participants were asked to look directly at either their right hand or at the book. The third experiment was similar to experiment one, but included another condition, in which one of the experimenters placed his left hand on the table, so that the participants saw the reflection of their own left hand, that of the experimenter's, or the reflection of the book, when they looked into the mirror.

Each experiment began with a 1 minute "induction period, during which the participants looked passively into the mirror. 30 short laser pulses, each lasting between 2-4 thousandths of a second, were then applied to surface of the right hand. This activates two types of nerve fibres to induce the sensation of pain without touch. The duration and intensity of the laser was adjusted for each participant, to the minimal value which induces a painful sensation. At the same time, electroencephalography (EEG) was used to record activity associated with pain perception, from parts of the brain which process sensory information. And afterwards, subjective reports of the felt pain were obtained from each participant, by means of a questionnaire. 

The participants therefore saw one of three things, depending on the experimental conditions. In experiment one, they saw either the reflection of their left hand as it was burnt by the laser, or that of the book. In the second experiment, they looked directly at their right hand or at the book while the laser was applied to their left hand, which was resting in their lap, out of sight. And in the third, they saw either the reflection of their left hand, or that of the experimenter's left hand, or of the book. 

The participants reported significantly less pain and unpleasantness when they were able to see their left hand as the laser pulses were applied. And the EEG readings provided objective confirmation that they were indeed experiencing less pain: the signals from electrodes lying over the pain processing regions of the brain - which were activated only during laser application - were reduced in size. This effect was specific to seeing one's own hand - the pain was not reduced in the conditions in which the participants looked at the reflection of the experimenter's hand, even if it looked very similar to their own. 

This study shows that viewing the body can reliably attenuate the perception of pain. How might such an effect occur? One possibility is that vision conflicts with proprioceptive inputs related to the position of the body within space, so that . However, this is unlikely, as viewing the hand had the same effect in the experiments in which the mirror was removed, and in which the visual and proprioceptive inputs from the hand are perfectly matched. The authors instead suggest that the analgesic effect occurs because it enhances the sense of body ownership, the sense that one's body belongs to oneself. Viewing one's hand in pain may therefore increase one's sense of ownership over the hand, and in turn increase bodily control of it. 

Exactly how vision diminshes pain - and has the opposite effect on touch - is unclear, but it is somehow related to multisensory integration, the process by which information from different sensory modalities is combined. One possibile explanation, put forward by the authors, is that visual inputs modulate inhibitory interneurons in the somatosensory cortex, which can be activated by different types of sensory inputs, and which use the neurotransmitter GABA to dampen down the activity of nearby cells. Thus, visual inputs may activate these cells, leading to inhibition of other neurons that are involved in pain processing, or may inhibit them, so that activity in neurons processing touch-related information is increased.      

It is already known that the so-called mirror box apparatus used here can reduce phantom pain in amputees. This was thought to be effective because it promotes reorganization of the somatosensory cortex, or because it repairs the distorted body image by recalibrating proprioception through vision. However, the participants in this new study were not amputees, but healthy volunteers, so these findings suggest that the analgesic effect vision is broader than previously thought.  They also point to a simple method for managing acute pain with a well-defined cause, such as a burn, or a broken bone (as opposed to chronic, long-lasting pain,  whose cause is often unclear).

For more about this study, see this post by The Neurocritic

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