Effects of horizontal-vertical eye positions on the 3D on-direction of extraocular muscles and extraocular motoneurons.

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1. Three head-stabilized monkeys (Macaca fuscata) were engaged in this study. A dual scleral search coil method was used for three-dimensional eye movement recordings. Trochlear nerve fibers were identified with extracellular single unit recordings. Thereafter, we stimulated the trochlear nerve fibers at the sites near to the trochlear decussation every 2 seconds during spontaneous eye movements while holding each monkey in different static roll positions (upright, 40ﾟ right ear down and 40ﾟ left ear down).2. Stimulation trains consisted of 0.25 ms negative rectangular pulses. Stimulus frequency was 500 Hz and current strength, and train duration were varied over the ranges of 10 -50 muA and 25-200msec.3. Electrical stimulation moved the eye away from Listing's plane during stimulation periods. Afterwards, drifts back to Listing's plane started 6-8 ms after stimulus end points, which was the same latency observed between stimulus onset and eye movement onset. Evoked eye movements were consisted of large intorsional and large downward components and small abduction components. The evoked direction was similar to the anatomically reported pulling direction of the superior oblique muscle in rhesus monkeys. The axis of rotation of evoked eye movements was independent of the electrical stimulation intensity and duration. On average, the axis of rotation of evoked eye movements in the coronal (torsional-vertical) plane was independent of the horizontal component of initial eye positions.4. We conclude that, in the alert monkeys, the axis of eye rotation evoked by electrical stimulation of the trochlear nerve is stabilized to the orbit in coronal plane which is almost in parallel with the plane formed by the superior oblique muscle path and the imaginary center of eye ball. Soft pulley that is reported in the superior, inferior, medial and lateral rectus muscles does not work effectively in the superior oblique muscle.