Neurophilosophy

Visual analgesia: Seeing the body reduces pain

VISION is now well known to modulate the senses of touch and pain. Various studies have shown that looking at oneself being touched 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 using binoculars to make a limb look larger or smaller than it actually is 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. 

mirror box.JPG

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, or that of the book. In the second experiment, they looked directly at their right hand while the laser was applied to it, or at the book. 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 right hand, or the reflection of the 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. 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.

Related:


Longo, M., et al (2009). Visually Induced Analgesia: Seeing the Body Reduces Pain. J. Neurosci. 29: 12125-12130. DOI: 10.1523/JNEUROSCI.3072-09.2009.

Comments

  1. #1 anon
    October 8, 2009

    This seems interesting but it might be more selecrtive Anyone ripping their fingernail back knows looking at it is only going to make it worse.. you just ignore it and go on to something else.. if your working at something.. and just do your best not to mess with it.. perhaps this study suggests ananlyzing it in a mirror.. but sounds a bit weird there.

  2. #2 Petter
    October 8, 2009

    Body Pains are generally ignored by us, thinking that these are very common and silly incident and not worth

    of or serious enough to pay for a doctor.Pain signals are processed in the brain – so it has to be involved

    somehow linked with anxiety and depression to back pain. It is great to help you for getting relief from the

    body pain by my site Body Pain for your wellfare.The key to

    improving your back symptoms and your emotional state lies in your ability to re-kindle your sense of control

    and personal power visit our site Body Pain

  3. #3 Louise Mowder
    October 8, 2009

    Theoretically, what would the effect of seeing be upon Central Pain Syndrome, where a continual, unrelenting sensation of extreme burning cold is felt as the result of some physical damage to the Central Nervous System? In that condition, you have the perception of pain in a peripheral location, such as an arm or a leg, without any actual injury to the limb. Yet Ramachandran’s “mirror box” has no effect upon pain sensations in that case, even though the pain is generated purely within the CNS.

    (The “mirror box” treatment of amputees was “used” with miraculous success in this past week’s episode of “House, M.D.”)

  4. #4 E Wolke
    October 8, 2009

    I wonder if noticeably asymmetrical folks would be fooled by this particular methods — say, people with birthmarks or (like myself) musical instrument players who have one hand that is callused and more muscular and developed than the other. I’m sure they screened for more obvious things like amputations and such.

    I am also interested in what this might have to do with a central nervous system pain syndrome, having Fibromyalgia and associated issues myself. Visual feedback seems to have something to do with it for me, because the pain is much worse in the morning before I have opened my eyes. Go figure. Perhaps this will be useful.

    As always, something to make you think. Thank you!

  5. #5 John D
    October 8, 2009

    Two requests for clarification:

    (i) Was the pain applied to the right hand or the left hand? The right hand would make more sense given the conclusions, but you seem to say it was applied to both right and left at different points.

    (ii) One of your sentences on the reasons for this phenomenon seems to trail off without conclusion.

  6. #6 John Smith
    October 8, 2009

    So…when I go in for my flu shot, am I supposed to look at my arm as the shot is administered? I always look away.

  7. #7 Mo
    October 9, 2009

    anon: As the Neurocritic points out in his comments thread, this effect might be specific fore certain types of injury.

    Petter: Oops! Your links appear to be broken, and I’m not going to fix them, because I don’t like comments which are posted solely for promotional purposes.

    Louise and E Wolke: The pain reduction reported here appears to be dependent on multisensory integration, so I’d suggest that it wouldn’t work with Central Pain Syndrome, because in this condition there is no visual or propriopective input corresponding to the felt pain.

    E Wolke: I just read a paper which discusses the characteristics of visual inputs with regards to the rubber hand illusion, fro which the same principles apply. I can’t remember which paper it was, or what conclusion the authors reached, but I think that in both cases, the visual inputs only need to be vaguely similar to the real limb. This is supported by the new study which shows that subjects can be fooled into thinking that a computer-generated limb, which does not look very realistic, is actually a part of their body (see yesterday’s post).

    John D: Pain was applied to the right hand in all cases. If you tell me which sentence you’re referring to, I can clarify that too.

    John Smith: Yes, this suggests that you should look at your arm while you’re being given the jab.

  8. #8 John D
    October 9, 2009

    Second sentence in the 8th paragraph reads:

    One possibility is that vision conflicts with proprioceptive inputs related to the position of the body within space, so that…

  9. #9 Mo
    October 9, 2009

    Oh, I see. That was an editing error. Thanks for pointing it out – I’ve fixed it now.

  10. #10 IanW
    October 9, 2009

    Mo – In your other blog: “The virtual body illusion and immersive Second Life avatars” there’s something on the page which crashes Internet Explorer every time I try to copy the text (I usually copy the text of sci-blog artcles and paste it into notepad to read offline).

    This didn’t occur when I did the same thing in this blog. Do you have any idea what it is? I highlight the text, starting at the end of the blog and moving upwards, and every time I get to the top of the text, IE crashes! It’s amazing! It strikes me as an excellent way to protect online text from being copied if we can figure out what is which does this!

    (I’m posting this here rather than in the actual blog where this happens because I already posted a comment there and Sci-Blogs refuses to let you post more than once in succession – for spam prevention, presumably).

  11. #11 Mo
    October 9, 2009

    Ian: I have no idea why that’s happening, or why you’re using Internet Explorer. Have you tried Firefox, Chrome or Safari? I’d recommend any of those over IE.

  12. #12 just another one
    October 10, 2009

    ok, so how about this situation?.. you accidentally cut yourself and you know you have cut yourself but you haven’t seen the cut and therefore it does not hurt that much, the moment you take the time to see the cut and realize it was some what a big cut your pain suddenly becomes stronger… is there any relationship between what’s said on the article and this kind of situation?

  13. #13 TengXB
    October 21, 2009

    Maybe the vision reduces the fear, so the subjects feel less pain.
    Maybe it is not about various neuron, but about simple pure psychological phenomenon, for example since we see it, we know it hurt less.

The site is currently under maintenance and will be back shortly. New comments have been disabled during this time, please check back soon.