Purpose:
Eye torsion during saccades and visual pursuit is confined to LP when the head is stationary. However, with yaw rotation the vestibulo-ocular reflex (VOR) can drive the eye into a new "dynamic" LP (DLP) with altered torsional position and orientation. We examined the effect of whole body roll on DLP.
Methods:
Using 3D search coils, head and eye positions were recorded in 10 normal humans aged 21±1 (mean±SD) yrs. With body upright, subjects fixated numerous locations on a target screen 20 cm away, from which static LP (SLP) thickness and orientation were determined. An impulse was then delivered about a mid-ocular axis to roll the whole body to a sustained angle of ±45° at accelerations of 250°/s2 and 1000°/s2, after which DLP thickness and orientation were determined for multiple fixations during 30 s.
Results:
Transient rotation induced shift in LP torsion counter to body roll, and altered LP orientation, that persisted for >30 s. Shifts were reversible with return to upright position, and opposite for opposite body roll. DLP changes varied among subjects, but not depend on acceleration (P>0.05), so we detail results for 1000°/s2. Mean SLP thickness was 1.84±0.75°, similar to DLP at 1.60±0.72°. Mean SLP orientation was 15.6° nasally and 4.2° downward. Mean torsional DLP offset was 9.8±8.5° (range 2.0-28.8), significantly different from SLP (P<0.05). The difference in torsional DLP offset for left and right rolls was 13.3±20.1° (range -0.4 to 42.1). Mean DLP tilt was 1.4±11.3° nasally and 9.5±8.6° upward for right roll, but 11.7±6.8° temporally and 1.6±5.2° upward for left roll(P<0.05).
Conclusions:
Whole body roll, stimulating both semicircular canals (SCCs) and otoliths, produces durable and occasionally large shift in torsional LP position, and alters horizontal and vertical LP orientation. Shifts are similar but larger than observed after vertical axis yaw that stimulates SCCs only. Classic OCR is a special case of more generalized SCC and otolith influence on ocular kinematics. Some apparent individual OCR variability is probably artifact of LP orientation.
Keywords: vestibulo-ocular reflex • eye movements • ocular motor control