Wednesday, June 3, 2009

It's all in your head

I think most of us are pretty willing to accept that the "will" or "urge" to move originates in the brain, and that the nerve stimulus that initiates the movement also originates in the brain.

What you might not know (I didn't) is that those two impulses--wanting to move, and initiating the movement--may actually happen in different parts of the brain.

I suppose it's not really surprising that this should be the case; the brain is, after all, a pretty big place (from a neuron's perspective), and obviously everything doesn't happen all in one spot. But in the May 8 issue of Science, Desmurget et al give pretty good evidence that the area that starts your body moving is distinct from the area that actually generates the urge to move.

The researchers studied seven human patients undergoing brain surgery for tumors. All seven were conscious during the surgery (possible because the brain, although the largest concentration of nervous tissue in the body, has no actual pain receptors on its surface), so they were able to answer questions. (Although it's not made clear in the article, presumably the patients were on several medications to relax them, but they were still conscious.)

In brain surgeries like this, doctors sometimes stimulate areas of the brain near the tumor to identify what parts of the body (or personality) may be affected by the surgery. In this case, the researchers used a similar technique to learn more about how the brain works.

During each surgery, several different regions of the patient's brain were stimulated with a small electrical probe. The shocks varied in intensity and duration. The researchers repeated the stimulations up to four times for each location, to check for reproducibility.

What they found out strikes me as pretty interesting. It turns out that, for several of the patients, when parts of the inferior posterior parietal cortex were stimulated, the patients felt an urge to move one or more body parts (arm, lips, chest, etc). If the stimulation was repeated with a higher intensity, the patients thought that they had actually moved that body part, even though no movement actually occurred. (The researchers report that one patient even said "I moved my mouth, I talked, what did I say?", although no mouth movement or speech was observed.)

Additionally, when portions of the premotor cortex were stimulated, the patients did actually move some of their body parts. When the stimulation was increased, the movement became more pronounced. However, and this was the part that I thought was kind of neat, the patients were completely unaware that they had moved at all. In fact, when they were specifically asked, the patients denied that they had moved, even when the movement was quite significant (e.g., raising an arm, or making a fist).

During the procedures, the researchers monitored the electrical signals in the patients' muscles as well. They saw no evidence of muscle movement when the parietal cortex was stimulated, even when patients were sure they had moved.

As an interesting side note, Desmurget et al report that stimulation of the right inferior parietal cortex caused patients to want to move their left limbs--hands, arms, feet, etc. However, stimulation of the left inferior parietal cortex seemed to prompt a desire to move the lips, or to talk.

Desmurget, M., et al, 2009. "Movement Intention After Parietal Cortex Stimulation in Humans." Science 324: 811-813. doi 10.1126/science.1169896
Haggard, P., 2009. "The Sources of Human Volition." Science 324: 731-733. doi 10.1126/science.1173827

UPDATE: This post appears in the June 15 Scientia Pro Publica at Mauka to Makai.

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