DISCHARGE PROPERTIES OF BRAIN STEM NEURONS PROJECTING TO THE FLOCCULUS IN THE ALERT CAT. I. MEDIAL VESTIBULAR NUCLEUS CHERON, Guy, Miguel ESCUDERO and Emile GODAUX. Laboratory of Neurosciences, University of Mons-Hainaut, Belgium, Laboratory of Neurosciences, University of Sevilla, Spain.
APStracts 3:0090N, 1996.
SUMMARY AND CONCLUSIONS
1. The aim of this study was to characterize the signals transmitted by neurons of the medial vestibular nucleus (MVN) to the middle zone of the flocculus in alert cats. 2. Bipolar stimulating electrodes were implanted into the middle zone of each flocculus, because this zone is known to be involved in the control of horizontal eye movements. Correct implantation of the stimulating electrodes was ensured by (a) recording of Purkinje cells whose activity was related to horizontal eye movements and (b) elicitation of slow abduction of the ipsilateral eye upon electrical stimulation. 3. The rostral two thirds of the MVN were investigated by microelectrodes during stimulation of both flocculi. Antidromically activated neurons were found only in the central part of the explored area. Forty-four units were activated from the contralateral, 8 from the ipsilateral flocculus. Neurons could never be activated from both flocculi. 4. Neurons included in this study were MVN neurons that had (a) to be antidromically activated from one flocculus and (b) to modulate their firing rate during the horizontal vestibulo-ocular reflex (VOR) elicited by sinusoidal stimulation (0.1 Hz; 10, 20, 30 or 40 deg). The 39 neurons matching both criteria were classified in two groups: 22 neurons changed their firing rate during spontaneous horizontal eye movements (EM- neurons), 17 modulated their activity only during head rotation and were labeled vestibular-only neurons (VO-neurons). 5. Sufficient data were obtained from 13 EM-neurons to allow a quantitative analysis. Among those, 12 were activated from the contralateral and 1 from the ipsilateral flocculus. Their sensitivity to horizontal eye position during intersaccadic fixation was 3.54 +/- 2.75 spikes.s 1/deg. Eight EM-neurons behaved as type I neurons, 5 as type II neurons. During the slow phases of the VOR, all of these neurons combined some head-velocity sensitivity (mean +/- SD: 1.50 +/- 0.43 spikes.s-1/deg.s-1) with some horizontal eye-position sensitivity (3.61 +/- 2.45 spikes.s-1/deg). Additionally, 7 of these neurons presented a sensitivity to eye velocity (1.34 +/- 0.55 spikes.s-1/deg.s-1). The phase difference between the modulation of firing rate and eye position varied substantially between neurons. The observed phase lead with respect to eye position ranged from 2 to 110 deg (mean +/- SD: 41.9 +/- 31.8 deg). 6. Sufficient data were obtained from 10 VO- neurons to allow a quantitative analysis. Among those, 9 were activated from the contralateral and 1 from the ipsilateral flocculus. All of these neurons behaved as type I neurons. The sensitivity to head velocity was 1.64 +/- 1.07 spikes.s-1/deg.s-1. The phase lead of the modulation of spike activity with respect to head velocity ranged from 4.5 to 30.5 deg (mean +/- SD: 16.4 +/- 8.9 deg). 7. We conclude that the MVN provides the horizontal zone of the flocculus (with a strong contralateral preference) with information about head velocity (through VO-neurons and EM-neurons) and about eye velocity and position (through EM-neurons). 

Received 28 September 1994; accepted in final form 23 February 1996.
APS Manuscript Number J604-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 28 May 96