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R. S. Jampel; The Eye Shares Kinematic Principles With the Head; Eye and Head Movements Are Commutative. Invest. Ophthalmol. Vis. Sci. 2008;49(13):758. doi: https://doi.org/.
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Understanding head movements is essential for understanding eye movements. The principles governing the kinematics of the head are deducible from observation alone. Eye movements required instruments for study because the eyes rotate in a confined space, are conjugated, have no boney joints, are obscured by the lids and the orbital rim, and respond to the complex visual sense. Despite these differences I will provide evidence that the eyes share kinematics principles with the head.
Head movements are described by observations that anyone can make. For eye movements I designed a video apparatus that allows: (1) the eye to gaze freely in any direction with the head stabilized in any position in space and (2) that allows the head to move freely while the eye is stabilized in different gaze directions. These experiments are made possible by suspending two miniature video cameras, a moveable fixation target, and a light source from a headband. One camera videos eye movements at 12X magnification, and the other head movements. Ocular orientation is determined by superimposing stabl eye surface markings from sequential magnified video frames.
The head: Yaw movements of the head occur around the odontoid axis that is fixed in the head. The axes for pitch and tilt of the head are defused throughout the cervical spine. Successive rotations around the yaw and pitch axes of the head produce ellipses of varying magnitudes (circumduction). There is no torsion (wheel-rotation) of the head. The eye: Yaw of the eye takes place around a fixed vertical axis in the eye. Pitch takes place around an axis fixed in the orbit. Successive rotation around these axes produces circumduction. With the head stabilized there is no axial rotation of the retina (torsion). During head tilt there is some transient rolling of the eyes, which does not enter consciousness.
Both the head and the eyes have fixed vertical (polar) axes in their substances that rotate with them and horizontal axes that are located and fixed outside their substances. When stabilized they are limited to 2D of rotational freedom. The head is incapable of torsion. Circumduction and oblique head or gaze movements produce slopes (gradients). During rapid head tilts the eyes show transient 1D forward rolling movements. For every position of the eye or the head there is only one orientation, which is independent of the order of rotation. It follows that eye and head movements are commutative, which are necessary conditions for retinal correspondence.
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