PROPERTIES OF OTOLITH-RELATED VESTIBULAR NUCLEAR NEURONS IN RESPONSE TO BIDIRECTIONAL OFF-VERTICAL AXIS ROTATION OF THE RAT

In decerebrate rats, the responses of tilt-sensitive neurons in the la teral and descending vestibular nuclei were studied during constant ve locity 10 degrees off-vertical axis rotations (OVAR) in the clockwise (CW) and counterclockwise (CCW) directions. Seventy three otolith-rela ted units showed sinusoidal position-dependent discharge modulation to OVAR of both directions; 20 of these showed clipped firing rates in p arts of a 360 degrees OVAR cycle. With increase in the velocity of rot ation (1.75-15 degrees/s), one group of units (n = 36) showed a stable ratio of bidirectional response sensitivity and symmetric response ma gnitudes to CW and CCW rotations. These units showed gain tuning ratio s similar to those of narrowly spatiotemporal-tuned neurons. The other group of OVAR responsive units (n = 13) exhibited velocity-variable a nd asymmetric bidirectional response sensitivities. Their gain tuning ratios were similar to those of broadly spatiotemporal-tuned neurons. For units with velocity-stable and symmetric bidirectional response se nsitivity as well as gain tuning ratio of the narrowly spatiotemporal- tuned neurons, their response gains remained stable with velocity. Som e showed stable response phase lead or lag with velocity increase whil e others showed progressive shifts from response lead of 13 degrees to response lag of - 25 degrees. The best response orientations of these units with velocity-stable and symmetric bidirectional response sensi tivity were found to point in all directions on the plane of rotation. The functional significance of these tilt- and OVAR-sensitive central otolith neurons is discussed.