Do other motor systems obey Listing's law or Donders' law?
The head obeys Donders' law. This is because head
position is constrained to a surface.
The head does not obey Listing's law. This is because this surface is
twisted (i.e. not a plane)
The direction of this twist is consistent with the head acting like a Fick
gimbal.
A Fick gimbal has a horizontal axis (full angle
rule) mounted on a vertical axis (zero angle
rule).
Why? The possibilities are:
1) The neck is built this way.
2) Lifting the head up and down takes more effort than moving it from side
to side (note that a Fick gimbal optimizes rotations about the horizontal
axis).
The outstretched arm acts in the same way when pointing.
