Properties of otolith-related vestibular nuclear neurons in response to bidirectional off-vertical axis rotation of the rat

Abstract

In decerebrate rats, the responses of tilt-sensitive neurons in the lateral and descending vestibular nuclei were studied during constant velocity 10° off-vertical axis rotations (OVAR) in the clockwise (CW) and counterclockwise (CCW) directions. Seventy three otolith-related units showed sinusoidal position-dependent discharge modulation to OVAR of both directions; 20 of these showed clipped firing rates in parts of a 360° OVAR cycle. With increase in the velocity of rotation (1.75-15°/s), one group of units (n = 36) showed a stable ratio of bidirectional response sensitivity and symmetric response magnitudes to CW and CCW rotations. These units showed gain tuning ratios similar to those of narrowly spatiotemporal-tuned neurons. The other group of OVAR responsive units (n = 13) exhibited velocity-variable and 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 sensitivity as well as gain tuning ratio of the narrowly spatiotemporal-tuned neurons, their response gains remained stable with velocity. Some showed stable response phase lead or lag with velocity increase while others showed progressive shifts from response lead of 13° to response lag of 25°. The best response orientations of these units with velocity-stable and symmetric bidirectional response sensitivity 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.