Three-dimensional organization of otolith-ocular reflexes in rhesus monkeys I. Linear acceleration responses during off-vertical axis rotation. ANGELAKI, DORA E. AND BERNHARD J. M. HESS. Dept. of Surgery (Otolaryngology), University of Mississippi Medical Center, Jackson MS, Dept. of Neurology, University Hospital Zürich, CH-8091, Switzerland.
APStracts 2:0007N, 1996.
SUMMARY AND CONCLUSIONS
1. The dynamic properties of otolith-ocular reflexes elicited by sinusoidal linear acceleration along the three cardinal head axes were studied during off-vertical axis rotations in rhesus monkeys. As the head rotates in space at constant velocity about an off-vertical axis, otolith-ocular reflexes are elicited in response to the sinusoidally varying linear acceleration (gravity) components along the interaural, nasooccipital or vertical head axis. Since the frequency of these sinusoidal stimuli is proportional to the velocity of rotation, rotation at low and moderately fast speeds allows the study of the mid and low frequency dynamics of these otolith-ocular reflexes. 2. Animals were rotated in complete darkness in the yaw, pitch and roll planes at velocities ranging between 7.4(/s and 184(/s. Accordingly, otolith-ocular reflexes (manifested as sinusoidal modulations in eye position and/or slow phase eye velocity) were quantitatively studied for stimulus frequencies ranging between 0.02 Hz and 0.51 Hz. During yaw and roll rotation, torsional, vertical and horizontal slow phase eye velocity was sinusoidally modulated as a function of head position. The amplitudes of these responses were symmetric for rotations in opposite directions. In contrast, mainly vertical slow phase eye velocity was modulated during pitch rotation. This modulation was asymmetric for opposite direction rotations. 3. Each of these response components in a given rotation plane could be associated with an otolith- ocular response vector whose sensitivity, temporal phase and spatial orientation were estimated based on the amplitude and phase of sinusoidal modulations during both directions of rotation. Based on this analysis which was performed either for slow phase eye velocity alone or for total eye excursion (including both slow and fast eye movements), two distinct response patterns were observed: (a) Response vectors with pronounced dynamics and spatial/temporal properties that could be characterized as the low frequency range of "translational" otolith-ocular reflexes. (b) Response vectors associated with an eye position modulation in phase with head position ("tilt" otolith-ocular reflexes). 4. The responses associated with two otolith-ocular vectors with pronounced dynamics consisted of horizontal eye movements evoked as a function of gravity along the interaural axis and vertical eye movements elicited as a function of gravity along the vertical head axis. Both responses were characterized by a slow phase eye velocity sensitivity that increased three-to five-fold and large phase changes of approximately 100-180( between 0.02 Hz and 0.51 Hz. These dynamic properties could suggest non-traditional temporal processing in utriculo-ocular and sacculo-ocular pathways, possibly involving spatio-temporal otolith-ocular interactions. 5. The two otolith- ocular vectors associated with eye position responses in phase with head position ("tilt" otolith-ocular reflexes) consisted of torsional eye movements in response to gravity along the interaural axis, and vertical eye movements in response to gravity along the nasooccipital head axis. These otolith-ocular responses did not result from an otolithic effect on slow eye movements alone. Particularly at high frequencies (i.e., high speed rotations), saccades were responsible for most of the modulation of torsional and vertical eye position which was relatively large (on average ( 8-10(/g) and remained independent of frequency. This reflex dynamics can be simulated by a direct coupling of primary otolith afferent inputs to the oculomotor plant. Despite the large modulation of torsional and vertical eye position due to both fast and slow eye movements, modulation of slow phase eye velocity was small for all frequencies (on average ( 4-10 (/s/g). 6. These results suggest that torsional and vertical responses (previously described as "counter-rolling" and "counter-pitching" or "tilt" otolith-ocular reflexes) represent an otolithic effect on both fast and slow eye movement components of the primate oculomotor system. The combined otolithic effect on both fast and slow eye movements possibly reflects a dynamic dependence of oculomotor (i.e., Listing's) coordinates on head orientation in space, suggesting a function that is probably related to spatial orientation and motor coordination rather than gaze stabilization. 

Received 4 April 1995; accepted in final form 28 December 1995.
APS Manuscript Number J301-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 22 January 96