Nonlinearity and asymmetry in the human vestibulo-ocular reflex

The horizontal vestibulo-ocular reflex was studied in normal and unilaterally vestibulopathic human subjects; the latter before and after surgical vestibular ablation. Subjects were oscillated at 0.025-4 Hz, 50 o/s peak head velocity, and for 0.025 and 0.25 Hz, at 50-300 o/s peak head velocity. Gain (peak eye/head velocities), phase (delay in degrees between peak eye and head velocities), and asymmetry (including d.c. bias; the average eye velocity over the cycle) were assessed for each rotation. In patients preoperatively, gain was subnormal while phase lead was greater than normal only at lowest frequencies. As head velocity increased at 0.025 and 0.25 Hz, gain dropped in patients, but not in normals. Furthermore, d.c. bias at 0.25 Hz tended to shift abnormally toward the lesioned side in most patients. After surgery, gain dropped and phase lead rose for all stimuli, but showed varied recovery over 4 months. d.c. bias also rose after surgery, and showed an enhancement with increasing head velocity at 0.25 Hz. d.c. bias remained unchanged even after 4 months for 300 o/s rotations, but disappeared at low head velocity. d.c. bias (and other asymmetry measures) represents a lateralizing sign which can be improved by utilizing high head velocities at moderate frequencies.