Unexpected role of the oblique muscles in the human vertical fusional reflex

1. If a weak vertically oriented prism is inserted before one eye, binocular single vision is restored by vertically divergent eye movements (one eye turning upward, the other downward); and it is usually assumed that the vertical rectus muscles mediate that fusional reflex. 2. When vertically divergent eye movements occur, both eyes also systematically rotate in parallel around their lines of sight (conjugate cyclotorsion). The direction of these unexpected eye movements demonstrates that they must be due to the oblique muscles, not the vertical recti. 3. The magnitude of these conjugate torsional movements is large enough to imply that the oblique muscles, in producing such torsion, would simultaneously effect all the divergent vertical re-orientation of the eyes required by the targets. 4. The cyclotorsion is accompanied by systematic translation of the eye along a nasal-temporal axis; the direction and extent of that non-rotational displacement indicate that the eye movements of the fusional reflex may well be mediated exclusively by the superior oblique muscles, acting against fixed tone in the inferior oblique muscles. 5. This revised understanding of the oculomotor co-ordination involved in the vertical fusional reflex has significant implications for both neurophysiology and oculomotor surgery.