The influence of gravity on Donders' law for head movements.

Three-dimensional (3-D) head rotations were examined in seven human subjects when their bodies were inclined +/- 45 degrees in pitch and roll. The selection of 3-D orientations of the head while subjects looked at visual targets was similar to that seen previously for the eye: there was a small static counter pitch and counter roll of approx. 10%. Thus rotations of the head relative to the trunk are largely independent of the trunk's position relative to gravity and the head's torsional orientation is primarily dependent on its horizontal and vertical position relative to the trunk.