By Todd Hargrove


I'm an author, bodyworker and movement therapist. I write about coordination, pain, complexity, play, the nervous system, body/mind issues and more.

The Thermal Grill Illusion

William R. Holmes, as a Magician
Body Map Error

On this blog I frequently discuss the idea that pain can sometimes be caused by the brain’s faulty perception of the body as opposed to actual tissue damage to the body. Put another way, errors in the way the brain maps the body can cause threat which causes pain, even when there is no damage at the site of pain.

The phenomenon of sensory motor mismatch illustrates this concept perfectly. Sensory motor mismatch means that the brain receives sensory information that is inconsistent with its motor commands. It is not surprising that such inconsistency would cause confusion, but I find it fascinating that it can actually cause pain.

A sensory motor mismatch can be created in the following way. Place your arms on either side of a mirror and look at the reflection of the stationary front arm, while moving the arm that is hidden behind the mirror. This creates confusion because the brain knows that it is moving the left arm, but it “sees” the left arm as stationary because of the illusion created by the mirror. It’s not surprising that this would feel weird or perhaps be a fun thing to do while engaging in recreational drug use, but the interesting thing for purposes of this blog is that it actually causes pain to many people.

In one study, twenty six of twenty nine fibromyalgia patients reported feeling a transient increase in pain or other symptoms indicating a flare up of their condition. Based on these facts, the researchers speculated that some form of sensory motor mismatch may be at the root cause of fibromyalgia. I don’t know about that, but it definitely provides strong evidence for one of the main claims I make on this blog, namely that errors in the brain maps can result in pain.

The next issue is whether such mapping errors and resulting pain can be reduced with movement or other sensory stimulation. Evidence supporting this point can be found in recent research involving the “thermal grill illusion” (“TGI”).  The thermal grill illusion is widely used by researchers because it is an easy way to study pain without actually doing any tissue damage to the participants in the study. The illusion is created by placing the index and ring fingers in warm water and the middle finger in cold water. This unusual sensory input apparently confuses the brain into thinking the middle finger is in boiling water, because it somehow results in a feeling that the middle finger is painfully hot.

In a recent study, researchers induced pain through the TGI and asked the subjects to press their fingers together.  This cut the pain levels by 64%. However, they were unable to reduce the pain by doing several other forms of touching, such as touching the hands of other people, or by pressing their hands together in an overlapping fashion.

The researchers noted that: "TGI was reduced only when thermosensory and tactile information from all three fingers was fully integrated. That is, TGI reduction required a highly coherent somatosensory pattern, including coherence between tactile and thermal patterns and coherence of stimuli between the two hands." In other words, the pain didn’t go away until the brain received sufficient sensory information to correct the distortions in the body maps.

One of the scientists stated that: “our work suggests that therapies aimed at strengthening the multisensory representation of the body may be effective in reducing pain."

Yes indeed. As frequent readers of this blog know, the Feldenkrais Method and Z-Health are two excellent therapies aimed at improving the body maps. Use them to cure any potential illusions that your brain might have about your body.

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