Yaw direction neurons in the cat inferior olive

1. Single units that responded to yaw rotation were recorded extracellularly in the caudal inferior olive (IO) of barbiturate-anesthetized cats. Of 276 neurons, 55 responded reliably to yaw, and extensive quantitative data were recorded from 25. 2. No yaw-sensitive IO neuron responded to somatosensory or auditory stimuli but two responded, though unreliably, to flash. 3. Yaw-sensitive IO cells fired at low (1-4 spikes/s), irregular rates during one direction of rotation. Though cells responded reliably during yaw, firing rates varied considerably from cycle to cycle. Rotation speed and acceleration were not represented in any cell's firing rate. 4. Eighty five percent (47/55) of yaw-sensitive cells fired during contralateral rotation, 9% (5/55) during ipsilateral rotation, and 6% (3/55) fired from late in the ipsilateral phase of a sinusoidal oscillation to the middle of the contralateral phase. 5. Responses were tested to 0.1-Hz sinusoidal yaw oscillations with a range of peak angular velocities (1-200 degrees/s). Thresholds were not sharp because of the cycle to cycle variability in response rates but were estimated using averaged responses. The peak rate of the most sensitive cell was driven to criterion (2 SD above spontaneous rate) by an oscillation with a peak velocity of 1 degrees/s. Other cells reached criterion between 5 and 50 degrees/s. 6. Sinusoidal oscillation at all frequencies tested (0.01-0.5 Hz) elicited approximately the same firing rates. Even at 0.01 Hz cells responded well. Responses lagged acceleration by approximately 25 degrees at 0.01 Hz and shifted to later parts of the cycle as frequency increased so that firing lagged acceleration by approximately 200 degrees at 0.5 Hz. 7. Histological reconstruction showed that yaw-sensitive neurons were recorded in olivary subnucleus beta (N beta), the dorsal cap of Kooy (DC), the posterior medial region of the medial accessory division of the inferior olive (MAO), and in the medial-lateral center of the caudal MAO. 8. Yaw-sensitive neurons in the inferior olive provide a signal to the cerebellum that indicates the direction of passive rotation over a wide range of velocity and acceleration. The signal from individual neurons does not reliably encode either rotation velocity or acceleration. Yaw-sensitive IO neurons are therefore unlike other central vestibular neurons but are similar to somatosensory IO cells which signal the presence, but not the intensity of a stimulus.