ROTATIONAL KINEMATICS OF THE HUMAN VESTIBULOOCULAR REFLEX .2. VELOCITY STEPS

1. Gain matrices were used to quantify the three-dimensional vestibulo ocular reflex (VOR) in five human subjects who were accelerated over 1 s and then spun at a constant 150 degrees/s for 29 s in darkness. Rot ations were torsional, vertical and horizontal, about earth-vertical a nd earth-horizontal axes. 2. Elements on the main diagonal of the gain matrices were much smaller than the optimal value of -1, and torsiona l gain was weaker than vertical or horizontal. Off-diagonal elements, indicating cross talk, were minimal except for a small but consistent horizontal response to torsional head rotation. 3. Downward slow phase s were more than twice as fast as upward at the start of rotation abou t both earth-vertical and earth-horizontal axes, but the asymmetry van ished later in the rotation. 4. During earth-vertical-axis rotation, a ll matrix elements decayed to zero. The main-diagonal torsional and ve rtical gains waned with time constants close to that of the cupula (6. 7 and 7.3 s). Velocity storage prolonged the horizontal response to ho rizontal head rotation (time constant 14.2 s) but not the horizontal r esponse to torsion (7.7 s). A simple explanation is that velocity stor age acts on a central estimate of head motion that accurately distingu ishes horizontal from torsional and that the inappropriate horizontal eye velocity response to torsion occurs because of cross talk downstre am from velocity storage. 5. During earth-horizontal-axis rotation, th e torsional, vertical, and horizontal main-diagonal elements declined, with time constants of 7.6, 8.2, and 7.9 s, to maintained nonzero val ues, all equal to about -0.1. Off-diagonal elements, including the hor izontal response to torsion, decayed to zero, so that the otolith-driv en reflex, late in the rotation, was equally strong in all dimensions and almost free of detectable cross talk. 6. The difference between ga in curves over the course of earth-vertical- and earth-horizontal-axis rotations was not constant but increased with time, suggesting that t he VOR response to earth-horizontal-axis rotation is not a simple sum of canal and otolith reflexes.