RESPONSE OF MEDIAL MEDULLARY RETICULAR NEURONS TO OTOLITH STIMULATIONDURING BIDIRECTIONAL OFF-VERTICAL AXIS ROTATION OF THE CAT

In decerebrate cats, the extracellular activities of neurons in the me dial medullary reticular formation were studied during constant veloci ty off-vertical axis rotations (OVAR) in the clockwise (CW) and counte rclockwise (CCW) directions (at 10 degrees tilt). Spontaneously active neurons demonstrated sinusoidal position-dependent discharge modulati ons to OVAR which selectively stimulates the otoliths. Two features of neuronal responses to bidirectional OVAR were identified. Within the velocity spectrum tested (1.75-15 degrees/s), some neurons showed symm etric bidirectional response sensitivity (delta value) to CW and CCW r otations. The spread of the delta values of each of these neurons with velocity was small. This group of reticular neurons were described as exhibiting symmetric and velocity-stable bidirectional response sensi tivity. The mathematically derived gain tuning ratios of these neurons were within the range of narrowly spatiotemporal-tuned neurons. Anoth er group of reticular neurons, however, showed asymmetric bidirectiona l response sensitivity to CW and CCW rotations; a few of these neurons were responsive only to OVAR of one direction but not to both. For ea ch of this second group of neurons, the spread of the delta values wit h velocity was large. These reticular neurons were described as exhibi ting asymmetric and velocity-variable bidirectional response sensitivi ty. The gain tuning ratios of these latter neurons were found to be wi thin the range of broadly spatiotemporal-tuned neurons. Single neurons of both groups displayed orientational tuning. Both the best response orientations of neurons that showed symmetric and velocity-stable bid irectional response sensitivity and the preferred orientations of neur ons that showed asymmetric and velocity-variable bidirectional respons e sensitivity were found to point in all directions on the rotary plan e. The response dynamics of the former group of neurons was also exami ned. All showed flat response gain across the entire velocity range. S ome showed a flat response lead while others showed a progressive shif t from small response lead at low velocity to phase close to zero at h igher velocities. The functional significance of these medial medullar y reticular neurons to the direction and orientation of head tilt is d iscussed.