Comment on Neuroscience basics for LessWrongians

Fri, 27 Jul 2012 10:01:45 GMT

Good catch!

There are several potential explanations:

1) That particular experiment was repeated several times for TV recording purposes, until they got a good take. JW (the patient) may have pointed with the left hand at first, but knowing what was happening would switch to his right hand since he is, in fact, right handed.

1b) Similarly, that particular experiment has generally been done before, priming the patient. Note how in the subsequent experiment they say "this is being done for the first time", implying that the previous experiments are long established repeat experiments for the camera.

2) A severed corpus callosum does not mean that all connections between the two hemispheres are cut. Here's a paper saying that for eight adult humans born with complete agenesis of the corpus callosum, fMRIs still show that both hemispheres seem equally synchronized despite having no corpus callosum (!).

Never underestimate the neuroplasticity of the human brain, even when the damage happened as an adult, especially in a case like JW's in which a significant amount of time has passed since the surgery.

In other words, it's more of a trend of the hemispheres working separately without a corpus callosum, not an absolute rule.

3) In so far as we know anything about brain lateralisation, we know that the left motor cortex does in fact only control the right side of the body (not counting cranial nerves). Decades of stroke victims have made that case.

However, there are many steps involved in "controlling your right hand". The motor axons that go down to your right hand do in fact all originate in your left hemisphere, but they just execute the command, they don't come up with it.

The planning, intention, high level computing that goes on that eventually yields that command is not solely located in that hemisphere. Simplified, you can have an emotional reaction in your right hemisphere that eventually translates to your left motor cortex pulling the trigger and depolarising/firing the neurons. This transferring, as described above, can still happen via other bundles that get reappropriated over time. Providing residual function, if you will, similar to the phenomenon of blindsight.

So the professor's statement was true, dependent on his referring to the whole sequence of steps.

Hope this helps!

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