SEMICIRCULAR CANAL CONTRIBUTIONS TO THE THREE-DIMENSIONAL VESTIBULO-OCULAR REFLEX: A MODEL-BASED APPROACH. Yakushin, Sergei, Mingjia Dai, Jun-Ichi Suzuki, Theodore Raphan and Bernard Cohen. Departments of Neurology and Physiology and Biophysics, Mount Sinai School of Medicine, New York, New York, 10029 USA, the Department of Otolaryngology, Teikyo University, Tokyo, Japan, the Departments of Computer and Information Science and Experimental Psychology, Brooklyn College, Brooklyn, New York 11210.
APStracts 2:0203N, 1995.
SUMMARY AND CONCLUSIONS
We studied the contribution of the individual semicircular canals to the generation of horizontal and torsional eye movements in cynomolgus monkeys. Eye movements were elicited by sinusoidal rotation about a vertical (gravitational) axis with the animals tilted in various attitudes of static forward or backward pitch. The gains of the horizontal and torsional components of the vestibulo-ocular reflex (VOR) were measured for each tilt position. The gains as a function of tilt position were fit with sinusoidal functions, and spatial gains and phases were determined. After recording control responses, the semicircular canals were plugged, animals were allowed to adapt and the test procedure was repeated. Animals were prepared with only the anterior and posterior canals intact (vertical canal ( VC ) animals), only the lateral canals intact (lateral canal ( LC ) animal) and only one anterior and the contralateral posterior canals intact (right anterior and left posterior canal ( RALP ) animals; left anterior and right posterior canal ( LARP ) animals). In normal animals, the gain of the horizontal (yaw axis) velocity of the compensatory movements decreased as they were pitched forward or backward, and a torsional velocity appeared, reversing phase at the peak of the horizontal gain. After the anterior and posterior canals were plugged ( LC animal), the horizontal component was reduced when the animal was tilted backward; the gain was zero with about -60 degrees backward tilt. The spatial phase of the torsional component had the same characteristics. This is consistent with the fact that both responses are produced by the lateral canals which from our results are tilted between 28 degrees and 39 degrees above the horizontal stereotaxic plane. After both lateral canals were plugged ( VC animals), horizontal velocity was reduced in the upright position but increased as the animals were pitched backward relative to the axis of rotation. Torsional velocities, which were zero in the upright position in the normal animal, were now 180 degrees out of phase with the horizontal velocity. The peak values of the horizontal and torsional components were significantly shifted from the normal data and were closely aligned with each other, reaching a peak value at approximately -56 degrees pitched back (-53 degrees horizontal, -58 degrees torsional). The same was true for the LARP and RALP animals; the peak values were at -59 degrees pitched back (-55 degrees horizontal, -62 degrees torsional). Likewise, in the LC animal, the peak yaw and roll gains occurred at about the same angle of forward tilt, 35 degrees (30 degrees horizontal, 39 degrees torsional). Thus, in each case, the canal plugging had transformed the VOR from a compensatory to a direction-fixed response with regard to the head. Thus, there was no adaptation of the response planes of the individual canals after plugging. The data were compared to eye velocity predictions of a model based on the geometric organization of the canals and their relation to a head coordinate frame. The model used the normal to the canal planes to form a non-orthogonal coordinate basis for representing eye velocity. An analysis of variance was used to define the goodness of fit of model predictions to the data. Model predictions and experimental data agreed closely for both normal animals and for the animals after canal lesions. Moreover, if horizontal and roll components from the LC and VC animals were combined, the summation overlay the response of the normal monkeys and the predictions of the model. In addition, a combination of the RALP and LARP animals predicted the response of the lateral canal plugged ( VC ) animals. When the operated animals were tested in light, the gains, peak values and spatial phases of horizontal and roll eye velocity returned to the preoperative values, regardless of the type of surgery performed. This indicates that vision had compensated for the lack of spatial adaptation of the response planes after plugging. It converted a non-compensatory, direction-fixed response with regard to the head to an appropriate compensatory response. These results indicate that both the vertical and lateral canals contribute a horizontal and torsional component to the vestibulo-ocular reflex (VOR) in the monkey according to the vector projection of head velocity onto the normals of the individual canals. The individual components can be predicted according to the position of the canals in the head with regard to the plane of rotation. The non-orthogonality of the canal planes must be considered when predicting canal activation of eye movements in response to rotation around any axis. 

Received 29 March 1995; accepted in final form 12 July  1995.
APS Manuscript Number J205-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.