Ocular torsion and primary retinal meridians.

I measured ocular torsion during and after head- and body-tilt, ocular convergence, and conjugate gaze. Extensive examination on 15 normal human subjects and additional observations on 400 consecutive patients without extraocular motor defects failed to show static ocular tension. During dynamic head-tilt, intermittent, small-amplitude, saccadic, wheel-like torsional eye movements were recorded as moving faster than the head so that the eyes preceded the head. Between these intermittent saccades, and at the end of the head-tilt, there was no residual ocular torsion. The absence of static ocular torsion allows for formulation of a generalization linking the oculomotor to the visual sensory system. The primary retinal meridians become aligned with each other in the normal individual during binocular fixation, regardless of the position of the head in space or the point of ocular fixation. The vestibular and oculomotor systems function to inhibit rather than produce ocular torsion; the eyes are oriented to a plane in the brain rather than to the horizon. In particular, the oblique muscles function mainly to maintain the alignment of the primary retinal meridians and, therefore, to inhibit torsion in the normal state.