DISCHARGE PROPERTIES OF BRAIN-STEM NEURONS PROJECTING TO THE FLOCCULUS IN THE ALERT CAT .1. MEDIAL VESTIBULAR NUCLEUS

1. The aim of this study was to characterize the signals transmitted b y neurons of the medial vestibular nucleus (MVN) to the middle zone of the flocculus in alert cats. 2. Bipolar stimulating electrodes were i mplanted into the middle zone of each flocculus, because this zone is known to be involved in the control of horizontal eye movements. Corre ct implantation of the stimulating electrodes was ensured by 1) record ing of Purkinje cells whose activity was related to horizontal eye mov ements and 2) elicitation of slow abduction of the ipsilateral eye upo n electrical stimulation. 3. The rostral two-thirds of the MVN were in vestigated by microelectrodes during stimulation of both flocculi. Ant idromically activated neurons were found only in the central part of t he explored area. Forty-four units were activated from the contralater al, eight from the ipsilateral flocculus. Neurons could never be activ ated from both flocculi. 4. Neurons included in this study were MVN ne urons that had 1) to be antidromically activated from one flocculus an d 2) to modulate their firing rate during the horizontal vestibuloocul ar reflex (VOR) elicited by sinusoidal stimulation (0.1 Hz; 10, 20, 30 , or 40 degrees). The 39 neurons matching both criteria were classifie d in 2 groups: 22 neurons changed their firing rate during spontaneous horizontal eye movements (EM-neurons), 17 modulated their activity on ly during head rotation and were labeled vestibular-only neurons (VO-n eurons). 5. Sufficient data were obtained from 13 EM-neurons to allow a quantitative analysis. Among those, 12 were activated from the contr alateral and 1 from the ipsilateral flocculus. Their sensitivity to ho rizontal eye position during intersaccadic fixation was 3.54 +/- 2.75 (SD) spikes . s(-1)/deg. Eight EM-neurons behaved as type I neurons, f ive as type II neurons. During the slow phases of the VOR, all of thes e neurons combined some head-velocity sensitivity (1.50 +/- 0.43 spike s . s(-1)/deg . s(-1)) with some horizontal eye-position sensitivity ( 3.61 +/- 2.45 spikes . s(-1)/deg). Additionally, seven of these neuron s 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 observ ed phase lead with respect to eye position ranged from 2 to 110 degree s (41.9 +/- 31.8 degrees). 6. Sufficient data were obtained from 10 VO -neurons to allow a quantitative analysis. Among those, nine were acti vated from the contralateral and one from the ipsilateral flocculus. A ll 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 ran ged from 4.5 to 30.5 degrees (16.4 +/- 8.9 degrees). 7. We conclude th at the MVN provides the horizontal zone of the flocculus (with a stron g contralateral preference) with information about head velocity (thro ugh VO-neurons and EM-neurons) and about eye velocity and position (th rough EM-neurons).