Neural basis for eye velocity generation in the vestibular nuclei of alert monkeys during off-vertical axis rotation

Activity of "vestibular only" (VO) and "vestibular plus saccade" (VPS) units was recorded in the rostral part of the medial vestibular nucleus and caudal part of the superior vestibular nucleus of alert rhesus monkeys. By estimating the "null axes" of recorded units (n = 79), the optimal plane of activation was approximately the mean plane of reciprocal semicircular canals, i.e., lateral canals, left anterior-right posterior (LARP) canals or right anterior-left posterior (RALP) canals. All units were excited by rotation in a direction that excited a corresponding ipsilateral semicircular canal. Thus, they all displayed a "type I" response. With the animal upright, there were rapid changes in firing rates of both VO and VPS units in response to steps of angular velocity about a vertical axis. The units were bidirectionally activated during vestibular nystagmus (VN), horizontal optokinetic nystagmus (OKN), optokinetic afternystagmus (OKAN) and off-vertical axis rotation (OVAR). The rising and falling time constants of the responses to rotation indicated that they were closely linked to velocity storage. There were differences between VPS and VO neurons in that activity of VO units followed the expected time course in response to a stimulus even during periods of drowsiness, when eye volocity was reduced. Firing rates of VPS units, on the other hand, were significantly reduced in the drowsy state. Lateral canal-related units had average firing rates that were linearly related to the bias or steady state level of horizontal eye velocity during OVAR over a range of ±60 deg/s. These units could be further divided into two classes according to whether they were modulated during OVAR. Non-modulated units (n = 5) were VO types and all modulated units (n = 5) were VPS types. There was no significant difference between the bias level sensitivities relative to eye velocity of the units with and without modulation (P>0.05). The modulated units had no sustained change in firing rate in response to static head tilts and their phases relative to head position varied from unit to unit. The phase did not appear to be linked to the modulation of horizontal eye velocity during OVAR. The sensitivities of unit activity to eye velocity were similar during all stimulus modalities despite the different gains of eye velocity vs stimulus velocity during VN, OKN and OVAR. Therefore, VO and VPS units are likely to carry an eye velocity signal related to velocity storage. For example, when unit sensitivities were related to head or surround velocity, sensitivity relative to OVAR was less than for VN or OKN. Firing rates of both vertical canal-related VO and VPS units (n= 19) were strongly modulated during OVAR, although they did not show changes in discharge rate during static head tilts relative to the spatial vertical up to a maximal 25 deg. In some cases the amplitude of the modulation increased with increases in head velocity and eye velocity. Average activity of vertical canal-related units was linearly related to steady state horizontal eye velocity in the ipsilateral direction during OVAR. The mean sensitivities of RALP units were not significantly different from those of LARP neurons (P>0.05). Together, their mean sensitivity during OVAR about a subject yaw axis was 0.34 (imp/s)/(deg/s) relative to horizontal eye velocity. This could be explained as a contribution of the vertical canals to horizontal eye velocity due to their orientation in the head. During OVAR to the ipsilateral side, the bias level of neuronal activity decreased and saturated. For steps of rotation about a vertical axis with the animal upright, the firing rates of RALP and LARP units were linearly related to stimulus velocity and eye velocity. Contralateral rotation excited the units reflecting the orientation of the semicircular canals relative to the yaw axis of rotation. RALP and LARP units also responded during horizontal optokinetic stimulation producing both OKN and OKAN. All the "vertical canal" units had dynamic characteristics closely related to velocity storage. Their response characteristics were consistent with the model that they contribute to horizontal slow phase velocity as part of a three-dimensional system based on a semicircular canal frame of reference. Otolith-related units (n= 5) in the vestibular nuclei showed no evidence of velocity storage and were modulated in accordance with head position during OVAR. Mean amplitude of the modulation of activity during OVAR at a 20 deg tilt and 60 deg/s rotational velocity was 24 imp/s. The data indicate that the vestibular nuclei contain the requisite signals to generate horizontal eye velocity during OVAR. VO and VPS units probably contribute to the "bias" or velocity storage component while otolith units mainly contribute to the oscillations in eye velocity by generating gravity dependent eye position changes during OVAR. In addition to the velocity storage component of horizontal eye velocity, the vertical VO neurons also have oscillations in their discharge patterns probably related to the vertical component of eye movements generated by the velocity storage integrator.