The function of the extraocular muscles, the theory of the coplanarity of the fixation planes.

UNLABELLED: A theory of visuomotor kinematics is derived from evidence obtained by a video method that allows the eyes to gaze freely in any direction with the head stabilized in any position in space, allows the head to move freely while the eyes are stabilized in different gaze directions, and allows the eyes to move in sync with the head. Frame by frame analysis reveals no wheel-rotation (torsion) around the visual line in any gaze direction. There is dynamic rolling around the visual line during head tilt but no static counter-rolling when the head is held steady in any position. Lateral rotations are wheel-rotations around a vertical polar axes fixed in the eyes. Vertical rotations occur around collinear axes fixed in the orbits. Oblique rotations consist of successive arcs of longitude and latitude, which project as slopes (the tangent to a point on a curve). All eye movements are commutative. Head movements enable the eyes to fixate any point at any angle in 3-D space. CONCLUSION: The extraocular muscles are programmed by the brain to maintain the fixation planes of the two eyes coplanar in all stable head positions and in all gaze directions. The retinas are in dynamic equilibrium with the brain and with each other in all head positions. The extraocular muscles function to maintain conjugacy, retinal correspondence, and retinotopy. The retinas are oriented to the brain and not to the horizon.

Evidence against mobile pulleys on the rectus muscles and inferior oblique muscle: central nervous system controls ocular kinematics.

PURPOSE: To provide evidence against the existence of orbital pulleys. METHODS: Interpretation of magnetic resonance imaging (MRI) scans; video eye tracking; ocular motor nerve stimulations; and clinical observations. RESULTS: No pulleys or planes splitting the extraocular muscles into layers were noted on MRI scans. Smooth muscle does not antagonize striate muscle. There is no physiological evidence that human rectus pulleys shift the ocular rotational axes to attain commutative behavior. In the monkey and humans, the axes of rotation are not determined by eye position. Operations on the extraocular muscles reveal no pulleys. CONCLUSIONS: The somatosensory system of the central nervous system controls the extraocular muscles. The autonomic nervous and the hormonal systems control the infrastructure of the orbit vital for the function of the extraocular muscles. The three systems are integrated and controlled by the central nervous system. Neural circuits are necessary to compensate for extraocular muscle abnormalities. There are no pulleys.

The absence of so-called compensatory ocular countertorsion: the response of the eyes to head tilt.

OBJECTIVES: To show that so-called compensatory ocular countertorsion (static ocular counterrolling) does not exist and to describe the torsional eye movements that occur while the head is tilting. METHODS: Two miniature video cameras, a fiberoptic light source, and a fixation target were suspended from a headband. The cameras, fixation target, and light source moved in synchrony with the head. One camera videorecorded iris and conjunctival landmarks, and the other recorded head movement and position. The video frames were digitized and analyzed using computer algorithms. RESULTS: The eyes showed no compensatory ocular countertorsion in any stabilized head tilt position. During head tilt, periodic torsional eye movements occurred. These movements included a tonic counterlag followed by a saccadic forward torsion that rotated the eyes prior to the head; this was followed by a saccadic countertorsion that realigned and synchronized the eyes with the head, bringing the eyes and head into equilibrium. CONCLUSIONS: Compensatory ocular countertorsion does not exist. Torsion occurs only during head tilt. The eyes are oriented to the brain and not to the horizon. When the head stabilizes in any tilted position, the retinas assume the same dynamic state of equilibrium with the brain that they assume in every other position. The main function of the oblique muscles is to stabilize the retinas in relation to the brain in all held head positions and directions of gaze. During head tilt, the oblique muscles produce involuntary torsional movements of small amplitude (up to approximately 10 degrees ) that appear to anticipate the final head position and prepare the eyes for obtaining retinal correspondence. These torsional movements may protect the retinas by dampening the effect of the head movement on this sensitive tissue.