Skeletal geometry underlying head movements.

TL;DR: The stereotyped resting posture adopted by seven of nine studied vertebrate species is introduced and the number of degrees of freedom of the articulations of the cervical vertebral column is reduced, contributing to the fact that orienting movements when studied by cineradiography are in fact stereotyped and confined to certain planes of space, corroborating Bernstein' hypothesis.

Abstract: The relationship between eye and head movements has been studied in recent years utilizing a number of methodological approaches. Behavioral studies and electromyographic recordings have revealed a tonic eye-head coupling in the rabbit,' the and the m0nkey.4.~ However, little attention has been devoted to the general question of the posture underlying eye-head coordination. The complexity of the vertebral arrangement in the cervical vertebral column and the necessity of assuming that-at least in the vertical plane-the elements of the head-neck system move around several frequency-dependent centers of rotation6 prompted us to reevaluate the skeletal geometry underlying head movements. As a general hypothesis, Bernstein' suggested that certain preferred axes for motor activity may have become prevalent in order to coordinate multiarticular systems, each of them having several degrees of freedom. This hypothesis formed the basis of an X-ray cinematography study on head movements during orienting behavior in different vertebrates. In the first section of this paper, we shall introduce the stereotyped resting posture adopted by seven of nine studied vertebrate species. These results have already been published elsewhere* and will be summarized here briefly to the extent necessary to follow the subsequent two sections. In the second section we shall propose that the number of degrees of freedom of the articulations of the cervical vertebral column is reduced as a result of the geometry of the skeletal system described in the first section. This reduction probably contributes to the fact that orienting movements when studied by cineradiography are in fact stereotyped and confined to certain planes of space, corroborating Bernstein's' hypothesis. Finally, in the third section, we shall try to demonstrate that these results are of consequence in understanding the pathology of postural control. We shall take as an example the postural syndrome observed in guinea pigs following hemilabyrinthectomy.