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Comment by FeepingCreature | Hacker News Reader

> this neural representation of the body is sliced into three sections, one for the feet, another for the hands and a third for the mouth

This smells like Inverse Kinematics to me.

If correct, that'd mean we don't generally move joints manually. Rather, at the planning level the brain just says "put the hand at relative position X" and the motor cortex works backwards how to make it happen.

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I took a few classes taught by Graziano, one of the motor researchers cited in this article. I remember he was convinced that the motor cortex does much higher level planning and orchestration than other people believed, and that it stored entire movement templates across different areas as described here. One of the main ones was picking up something and bringing it to your mouth to eat.

I honestly would not be surprised to learn that a substantial amount of the specific content of spoken (or indeed keyboarded) utterances originates in movements of mouth and hands planned by the motor cortex. The reason being that sometimes the speed of production precludes the involvement of ‘higher’ cortical functions, which proceed at a slower pace.

Yes. Body movement is very strong inverse kinematics. This is why personal trainers exist: "lift thing" is intuitive, "lift thing with specific muscles along a specific path" is not.

It could be much weirder than that; the way movement works could be the brain very strongly predicting that a limb will be at a certain position and the rest of the body scrambling to prove that prediction true. See https://slatestarcodex.com/2017/09/06/predictive-processing-...

Wow, very interesting, thanks for sharing!

I'm also wondering where the inertial measurement unit (IMU) is in there. People often forget about the inner ear a sensory organ. Moving body parts is one thing, but moving the body from here to there requires tight coupling of this this additional feedback. We clearly have a 3-axis gyro in each ear ;-)

Apart from the inner ear, there are also other feedback paths worth considering: muscular sensing, skin stretch sensing, even acoustic transmission along ligaments.

Yeah, proprioception in general is amazing. I always find it fun to do stuff like close my eyes, then quickly spread my arms apart, and just as quickly bring them back together and try to touch specific finger tips together; seems to be generally easiest with index and middle fingers for me. I also enjoy being able to extend that proprioception to tools - being able to feel the engagement of a screwdriver with a screw head, etc.

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