Discharge characteristics of vestibular and saccade neurons in the rostral midbrain of alert cats.

1. The interstitial nucleus of Cajal (INC) is involved in generation of vertical eye movements. To further explore the neural substrates of INC involvement in eye movements, we examined the discharge characteristics of neurons in the rostral midbrain that discharge for vestibular stimulation and saccades in alert cats. We analyzed 75 vestibular and saccade neurons (VSNs) that showed characteristic discharge during pitch rotation. Of these, 50 exhibited gradually increasing activity during upward slow phases and burst of action potentials during downward fast phases induced by downward pitch. The remaining 25 cells showed the opposite response pattern: gradually increasing activity during downward slow phases and a burst during upward fast phases induced by upward pitch. We classified these cells as downward VSNs and upward VSNs, respectively. The effects of electrical stimulation of the contralateral vestibular nerve were tested for 13 downward VSNs and 2 upward VSNs; all of them were activated at short latencies. 2. All vertical VSNs also showed burst activity associated with spontaneous saccades. The preferred direction for the burst was always the same for saccades and vestibular fast phases. The onset of burst activity preceded the onset of fast eye movement by an average of 35 +/- 14 ms (mean +/- SD). During fast eye movement in other directions, many of these cells also showed an inconsistent burst and the burst often began after the onset of saccades. Half of the upward VSNs examined showed a pause in activity during downward fast eye movement. 3. Burst parameters in the preferred directions were correlated with saccade parameters in half of the vertical VSNs examined, although the correlation coefficients were typically 0.5-0.6. 4. All vertical VSNs had irregular resting activity. Half of the downward VSNs examined showed eye-position sensitivity toward the direction opposite to the preferred direction for the burst activity but only if the period of analysis was restricted to discharge shortly before and after saccades. 5. All downward VSNs examined during sinusoidal rotation in several vertical planes received strong excitatory input from the contralateral anterior canal. Three of four upward VSNs examined received contralateral posterior canal excitation; the exception received ipsilateral posterior canal excitation.(ABSTRACT TRUNCATED AT 400 WORDS)