Research on Movement
There are many ways when human beings view an object by their eye, for instance, only with the movement of the eyes, with the movement of the head, with both head and eye movements, and so on. Humans select one
from these instantaneously and operate it. Also, when multiple body parts cooperate, hierarchical adjustments are required. For example, eyes might move at the first, adjusting the movement of the head according to the movement, and
further adjusting the movement of the shoulder. Such relationship is called “hierarchy” of the body.