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 as the head when pointing.


Copyright © 1996 Tutis Vilis and Douglas Tweed
University of Western Ontario
London Ontario Canada
Updated October 28, 2005

Comments welcome. Email to tutis.vilis@schulich.uwo.ca