CONTROL OF SPATIAL ORIENTATION OF THE ANGULAR VESTIBULOOCULAR REFLEX BY THE NODULUS AND UVULA

Spatial orientation of the angular vestibuloocular reflex (aVOR) was s tudied in rhesus monkeys after complete and partial ablation of the no dulus and ventral uvula. Horizontal, vertical, and torsional component s of slow phases of nystagmus were analyzed to determine the axes of e ye rotation, the time constants (Tcs) of velocity storage, and its ori entation vectors. The gravito-inertial acceleration vector (GIA) was t ilted relative to the head during optokinetic afternystagmus (OKAN), c entrifugation, and reorientation of-the head during postrotatory nysta gmus. When the GIA was tilted relative to the head in normal animals, horizontal Tcs decreased, vertical and/or roll time constants (Tc-vert /roll) lengthened according to the orientation of the GIA, and vertica l and/or roll eye velocity components appeared (cross-coupling). This shifted the axis of eye rotation toward alignment with the tilted GIA. Horizontal and vertical/roll Tea varied inversely, with TChor being l ongest and Tc-vert/roll shortest when monkeys were upright, and the re verse when stimuli were around the vertical or roll axes. Vertical or roll Tcs were longest when the axes of eye rotation were aligned with the spatial vertical, respectively. After complete nodulo-uvulectomy, Tc-hor became longer, and periodic alternating nystagmus (PAN) develop ed in darkness. Tc-hor could not be shortened in any of paradigms test ed. In addition, yaw-to-vertical/roll cross-coupling was lost, and the axes of eye rotation remained fixed during nystagmus, regardless of t he tilt of the GIA with respect to the head. After central portions of the nodulus and uvula were ablated, leaving lateral portions of the n odulus intact, yaw-to-vertical/roll cross-coupling and control of Tc-v ert/roll was lost or greatly reduced. However, control of Tc-hor was m aintained, and Tc-hor continued to vary as a function of the tilted GL A. Despite this, the eye velocity vector remained aligned with the hea d during yaw axis stimulation after partial nodulo-uvulectomy, regardl ess of GIA orientation to the head. The data were related to a three-d imensional model of the aVOR, which simulated the experimental results . The model provides a basis for understanding how the nodulus and uvu la control processing within the vestibular nuclei responsible for spa tial orientation of the aVOR. We conclude that the three-dimensional d ynamics of the velocity storage system are determined in the nodulus a nd ventral uvula. We propose that the horizontal and vertical/roll Tcs are separately controlled in the nodulus and uvula with the dynamic c haracteristics of vertical/roll components modulated in central portio ns and the horizontal components laterally, presumably in a semicircul ar canal-based coordinate frame.