DISCHARGE PROPERTIES OF BRAIN-STEM NEURONS PROJECTING TO THE FLOCCULUS IN THE ALERT CAT .2. PREPOSITUS HYPOGLOSSAL NUCLEUS

1. The aim of this study was to characterize the signals transmitted b y the neurons of the nucleus prepositus hypoglossi (NPH) to the middle zone of the flocculus of the cat. The methods, the behavioral testing , and the animals used in this study were the same as those used in th e accompanying paper on medial vestibular nucleus neurons. 2. The rost ral two-thirds of the NPH was explored in alert animals with microelec trodes during stimulation of the middle zone of both flocculi. Dischar ges of neurons were analyzed during spontaneous eye movements (head fi xed) and during horizontal vestibuloocular reflex (VOR) activity elici ted by sinusoidal stimulation (10, 20, 30, or 40 degrees at 0.1 Hz). F orty neurons were found to be antidromically activated from only one o r the other of the two flocculi (latency: 0.99 +/- 0.17 ms, mean +/- S D): 37 from the contralateral flocculus and 3 from the ipsilateral one . None of the neurons could be activated antidromically from both floc culi. Floccular stimulation never resulted in direct inhibition of the se NPH neurons. 3. Of the 37 units antidromically activated from the c ontralateral flocculus, 26 were recorded sufficiently long to allow fu ll quantitative analysis. Most of these (20 neurons) were classified a s burst-tonic (BT) neurons. The BT neurons exhibited during each sacca de made in one direction (the ON direction) a burst of spikes, and dur ing postsaccadic fixation a tonic activity that increased with gaze di splacement in the ON direction. The mean sensitivity of the neurons to eye velocity during the ''ON'' saccades was 3.3 +/- 1.6 spikes . s(-1 ). deg(-1). s(-1). During intersaccadic fixation, the mean sensitivity to eye position was 3.6 +/- 2.5 spikes . s(-1). deg(-1). During the V OR, the majority showed modulation in relation to both eye position an d eye velocity. The mean sensitivity to eye position during the VOR wa s 3.4 +/- 2.6 spikes . s(-1). deg(-1) (range: 0.2-8.1 spikes . s(-1). deg(-1)). The mean sensitivity to eye velocity during the VOR was 2.1 +/- 1.3 spikes . s(-1). deg(-1). s(-1). The mean phase lead of with re spect to eye position was 16.4 +/- 6.8 degrees (range: 6.0-28.9 degree s). Eighty percent of the BT neurons behaved as type I neurons. Forty- seven percent of the BT neurons also presented some head velocity sens itivity (1.48 +/- 0.6 spikes . s(-1). deg(-1). s(-1), mean +/- SD). 4. Other NPH cells antidromically activated from the contralateral flocc ulus were classified in two groups: bidirectional burst (BE) neurons ( n = 4) and burst-driving (ED) neurons (n = 2). The BE neurons were cha racterized by a burst discharge during every horizontal saccade or VOR quick phase, irrespective of the direction. The mean sensitivity of t he BE neurons to eye velocity during saccades was 3.3 +/- 7.8 (SD) spi kes . s(-1). deg(-1). s(-1). Both ED neurons increased their firing ra te during the slow VOR phases induced by an ipsilateral rotation (type I neurons) and exhibited high-frequency bursts in association with ip silaterally directed quick phases. 5. The results indicate that the ma in projection of the NPH onto the middle zone of the flocculus comes f rom contralaterally located type I BT neurons. Signals transmitted in this path associate a high sensitivity for eye velocity with a high se nsitivity for eye position. This type of input is consistent with the suggestion that the main function of the flocculus is to control the g ain of downstream reflexes and to perform a fine adjustment of the gaz e holding command.