SITE OF INTERACTION BETWEEN SACCADE SIGNALS AND VESTIBULAR SIGNALS INDUCED BY HEAD ROTATION IN THE ALERT CAT - FUNCTIONAL-PROPERTIES AND AFFERENT ORGANIZATION OF BURSTER-DRIVING NEURONS

1. Extracellular spikes of burster-driving neurons (BDNs) were recorde d within and immediately below the prepositus hypoglossi nucleus in th e alert cat. BDNs were characterized by short-latency activation after stimulation of the contralateral vestibular nerve (latency: 1.4-2.7 m s) and the ipsilateral superior colliculus (latency: 1.7-3.5 ms). Conv ergence of vestibular and collicular inputs was found in all of 85 BDN s tested. Firing of BDNs increased during contralateral horizontal hea d rotation and decreased during ipsilateral rotation. A burst of spike s was induced in association with contralateral saccades and quick pha ses of nystagmus. 2. BDNs showed irregular tonic discharges during fix ation. There was no significant correlation between the firing rate du ring fixation and horizontal or vertical eye position in most BDNs. Du ring horizontal sinusoidal head rotation, the change in firing rate wa s approximately proportional to and in phase with contralateral head v elocity. The phase lag of the response relative to head angular veloci ty was 13.8 +/- 20.1 degrees (mean +/- SD) at 0.5 Hz and 7.2 +/- 13.5 degrees at 0.2 Hz on the average. The gain was 0.88 +/- 0.25 (spikes/s )/(degrees/s) at 0.5 Hz and 1.19 +/- 0.49 (spikes/s)/(degrees s) at 0. 2 Hz. 3. Quantitative analysis of burst activity associated with sacca des or quick phases indicated that the ON direction of BDNs was contra lateral horizontal. The number of spikes in the burst was linearly rel ated to the amplitude of the contralateral component of rapid eye move ments. The slope of regression line was, on the average, 1.14 +/- 0.48 spikes/deg. There was no significant difference between the mean slop es for saccades and quick phases. The number of spikes depended on the difference between initial and final horizontal eye positions and not on the absolute eye position in the orbit. The mean burst firing rate was proportional to the mean velocity of the contralateral component of rapid eye movements. The slope of the regression line was 0.82 +/- 0.34 (spikes/s)/(degrees/s). Significant correlation was also found be tween intraburst instantaneous firing rate and instantaneous component eye velocity. 4. Objects presented in the contralateral visual field elicited a brief burst of spikes in BDNs independent of any eye moveme nt. Contralateral saccades to the target were preceded by an early res ponse to the visual stimulus and subsequent response associated with e ye movement. 5. Excitation of BDNs produced by stimulation of the ipsi lateral superior colliculus was facilitated by contralateral horizonta l head rotation, Therefore saccadic signals from the superior collicul us to BDNs may be augmented by vestibular signals during head rotation . We suggest that BDN signals regulate premotor burst neurons to facil itate saccadic eye movements in eye-head coordination.